Stone Slate Roofing: Technical advice note

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HistoricEngland.org.uk/advice 08268_Cover_10Black 4/5/05 9:30 am Page 1

2005

Stone Slate Roofing Technical advice note 08268 26/4/05 10:33 pm Page 2

Contents Technical advice note This technical advice note gives guidance for architects, surveyors, and building owners, on the repair and 1 Introduction ...... 2 reroofing of traditional stone slate roofs in England. It deals specifically with stone slates from sedimentary rocks (mainly limestone and sandstone) rather than with real slates, which are from 2 Stone as a roofing metamorphic rock.As the material and methods of stone slate roofing vary from region to region, it material ...... 2 concentrates on general principles. For guidance on the specific local roofing types and methods, advice 2.1 Terminology ...... 2 should be sought from the local planning authority. Some local authorities provide leaflets on regional roofing practice. Further advice will be available in “Stone Roof Conservation: a best practice guide” (see 2.2 Stone types...... 4 References). 2.3 The production of stone slates...... 4 1 Introduction to be reviewed as more information comes 3 The principles of stone to hand and experience grows. Research slate roofing...... 4 If any regard is to be had to the general beauty into the construction of stone slate roofs is of the landscape, the natural material of the a neglected field. Future research will 4 Planning and historic special countryside should be used instead of reveal more information on traditional buildings legislation ...... 5 imported material. (William Morris, On the practice and help us to understand better external covering of roofs, 1890) the regional variations of stone slate 5 Inspection and maintenance ...... 6 roofing. 1.1 Sandstone and limestone slating is a 6 Investigation and highly regionalised roofing form, recording ...... 6 fundamental to the distinctive local 2 Stone as a roofing material character of buildings in many parts of the 7 The repair and reroofing country. However, local character and 2.1 Terminology of stone slate roofs ...... 7 distinctiveness are under threat and stone 7.1 Materials for repair and slate roofing is becoming increasingly rare. 2.1.1 Stone slates are known in different reroofing...... 7 As locally produced stone slates come to parts of the country as grey slates, flags, 7.1.4 Stone slates ...... 7 the end of their natural lives, declining flagstones, thackstones, slats, flatstones, 7.1.5 Ridges ...... 8 supplies of new ones have led to the use of stone tiles, and tilestones. Geologists 7.1.6 Fixings...... 8 imported or artificial substitutes which prefer the term tilestone as these 7.1.7 Torching, roofing felts and have little to do with local building limestone and sandstone products are not, vapour barriers...... 10 traditions. If the special qualities of our geologically, slates. That is to say that they 7.1.8 Bedding and pointing ...... 10 built heritage are to be conserved, any are made from sedimentary, rather than repair or reroofing should be based on the from metamorphic rocks, and 7.1.9 Roof timbers ...... 11 principle of replacing ‘like with like’. Stone consequently split along bedding rather 7.2 Construction and slate roofs are no exception. Keeping the than cleavage planes. Stone slate is workmanship ...... 11 local stone slate roof of an historic building however the most commonly used term 7.2.1 Specification ...... 11 helps to conserve its significant character. and has been adopted in this leaflet. 7.2.2 Stripping, sizing and stacking . . 11 7.2.3 Reroofing ...... 11 1.2 The gradual decline of the stone slate 2.1.2 The word ‘delph’ (plural ‘delves’) 7.2.4 Detailing ...... 14 roofing industry has led to a loss of has been used in preference to ‘quarry’ expertise of both roofers and specifiers, and ‘quarrying’ because of the unpopular 7.2.5 Insulation and ventilation . . . . 15 and there is little written information image the latter have in the public’s 7.3 Minor and localised repairs . . 16 available on best practice methods for perception. Other names are used locally, 7.4 Choosing a slater ...... 16 reroofing. The revival of traditional including ‘quar’ in Dorset and ‘pit’ over a methods and the adaptation of historic wide area of the country. 8 Costs ...... 17 details so that roofs meet modern 8.1 Life-cycle costs ...... 17 performance standards has created 2.2 Stone types 8.2 Grants...... 17 confusion and resulted in the unnecessary failure of a number of roofs. This leaflet 2.2.1 Stone slates have been used for Appendix 1 Stone slate roofs: attempts to clarify these issues and roofing in Britain since Roman times. checklist for recording construction provides information on best practice as Wherever a rock could be split to form a details ...... 18 we understand it today. reasonably thin slab, it has been exploited for roofing and examples exist for almost Appendix 2 Stone slate roofs: 1.3 This technical advice note has been every geological period and rock type. checklist for recording materials . . . . 19 developed from several documents written Until the second half of this century, stone Glossary ...... 20 when stone roofing was a live art; from slates were quarried or mined extensively practical experience, and from advice throughout Britain and although most References ...... 21 given by stone slaters, manufacturers, quarries were small and served a small conservation officers, architects and locality, collectively they formed a Further information ...... 22 surveyors. Some of the issues raised are significant industry. The table identifies matters of conservation principle rather the regions in which stone slates have Sourcing stone slates . . . . back cover than technical fact, and others may need been manufactured and used, their

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Figure 1 Geological formations important for stone slates in England, and some historical locations for production and use

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Table Stone slates: local usage and sources before selecting, splitting, and dressing are done by hand. Limestones, including sandy limestones Dorset JURASSIC: 2.3.2 Stone slates were mined at Purbeck, Forest Marble Purbeck in Dorset, at Collyweston and Somerset JURASSIC: Duston in Northamptonshire, at (South and East) Lias, Forest Marble Stonesfield and elsewhere in the Cotswolds, in Yorkshire and occasionally Cotswolds JURASSIC: in Derbyshire. Access to the fissile rock East Gloucestershire, Purbeck, Corallian, Forest Marble, North Oxfordshire, Hampen Marly Beds,Trougham and Taynton was by adits or shafts with the rock and North Wiltshire Stone (Stonesfield Slate), Fullers Earth, worked along galleries. At Collyweston Chipping Norton Limestone and Stonesfield, the splitting was carried out by frost action. The raw block was East Midlands JURASSIC: Northamptonshire, Blisworth Limestone, Upper Estuarine either stored underground or taken to the Lincolnshire, Limestone, Chipping Norton Limestone surface where it was wetted and covered Cambridgeshire, Lincolnshire Limestone, (Collyweston Slate), in earth until the frosts came. The frost and Leicestershire Northampton Sand, Lias then swelled the natural moisture within the stone and split it into slates. Frost- Yorkshire JURASSIC: (North-east) Scarborough Limestone split slates may be thinner and therefore lighter than those split by hand (Fig 2). Nottingham to South Shields, East PERMIAN: Derbyshire (Whitwell) Magnesian limestone 2.3.3 However, most stone slates were Sandstones, including calcareous sandstones surface delved rather than mined. Where Surrey, Sussex,West Kent CRETACEOUS: the rock has been weathered, it is Horsham stone weakened along the bedding planes, West Gloucestershire, Bristol CARBONIFEROUS: allowing it to be split into thin layers of a Upper Westphalian/Coal Measures suitable thickness for roofing. In some – Pennant Measures areas, the weathering actually completes the splitting process and stone slates are Welsh Marches DEVONIAN – OLD RED SANDSTONE Gloucestershire, and SILURIAN – PRIDOLI: found at the surface or turned up by Herefordshire, Dittonian, Downtonian,Tilestones ploughing. Traditionally the delves tended Worcestershire, to be small in scale, supplying one village and Shropshire ORDOVICIAN: or even a single farmstead (Fig 3), Hoar Edge Grit, Cheney Longville Flags, although some developed into large Hope Shale (Corndon Hill) operations supplying industrial towns. South Pennines CARBONIFEROUS: Even so, their markets were always local. North Staffordshire, Lower Westphalian/Coal Measures Today the remaining delves often supply North Derbyshire, Namurian/Millstone Grit slates far beyond their traditional markets. and East Cheshire

Lancashire,Yorkshire, CARBONIFEROUS: 2.3.4 Once the rock had been split to Durham, Northumberland Lower Westphalian/Coal Measures, the required thickness, it was dressed to Namurian/Millstone Grit size and shape. In some cases the faces of Cumbria PERMIAN: the slates were also tooled to provide a New Red Sandstone flatter surface. The slates were holed – Penrith Sandstone using a pointed tool, from one or both sides. This provided a tapered or CARBONIFEROUS: Namurian/Millstone Grit hourglass hole which gripped the timber peg commonly used to hold the slate in Note: for more detail see Hughes in Wood (2003) place on the roof (Figs 4 and 5).

geological period and formation names, overburden, the block for stone slate 3 The principles of stone and is a reminder of how extensively they production has traditionally been obtained slate roofing were once used (Fig 3). by open quarrying or delving (exploiting the near-surface weathered rock), or 3.1 Local stone slating traditions alternatively by mining (mainly to extract evolved from the slater’s response to the 2.3 The production of stone slates unweathered rock). Sometimes delving particular geology and climatic conditions 2.3.1 Roofing stones are produced from began with the extraction of the weathered of an area – it is a highly regionalised either weathered rock (subjected to glacial rock and progressed to mining. In both activity. Local practice varies from region and post-glacial weathering) or from cases it was (and remains) a manual to region, with the production of the deeper, unweathered rock. Depending on process although, today, small excavators stone slates, design of the roof, roofing the lie of the ground and the amount of are sometimes used to loosen the rock methods and detailing all contributing to

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Figure 4 The rock is split along the bedding planes into a suitable thickness for roofing.

Figure 2 Collyweston, Northamptonshire: stone laid out for splitting by winter frost (Photograph: Daniel Martin)

the special qualities of an area’s built regularly up a roof slope and represent, as heritage. far as possible, the range of sizes typically used in the region (see front cover image). 3.2 Stone slates are randomly sized (see Glossary) and laid in diminishing courses. 3.3 In many parts of the country stone The principle of random roofing is that slates were used at the eaves courses in the range of stone slate sizes reflected combination with thatch or tiles for a what was available at that time from the variety of reasons. Eaves slating has sound Figure 5 Once split to a suitable thickness, the stone is dressed roughly square and a fixing hole quarry and maximised the usable rock. practical advantages: it evolved as a means made. Unfortunately, some manufacturers today of protecting the eaves of thatched buildings have limited the range of sizes they from harm by animals, but also provides a earlier stone roof which has been gradually produce and this alters the appearance of hard edge for resting ladders and a long lost because of lack of make-up slates a roof. The selection of replacement overhang for water runoff. In most cases, during successive re-slating.The appearance materials should reflect the original mix of however, where stone slates are present only is now considered to be traditional and sizes. Stone slates should diminish in size at the eaves, they are the remnants of an should be conserved (Fig 13).

3.4 There is no British Standard for specifying either materials or construction methods for stone slate roofing, although many of the details and recommendations made in BS 5534: 2003 Code of practice for slating and tiling are applicable to stone slates. However, it should be recognised that this British Standard does not deal in detail with random roofing. Many of the traditional details typical of stone slate roofing, which have performed satisfactorily for centuries, conflict with its recommendations. However, departure from British Standard practice should be based on sound, proven knowledge of local traditional practice.

4 Planning and historic buildings legislation

Figure 3 The duck pond of this West Sussex farmhouse may originally have been the quarry for the Horsham 4.1 The controls for listed buildings, and slates on the roof. buildings in conservation areas, are

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covered by the Planning (Listed Building out annually by the building owner and there are known technical defects in the and Conservation Areas) Act 1990, and (for example) quinquennially by an existing details. Thus it is essential to Planning policy guidance: planning and the architect or building surveyor, will enable record the details of the roof coverings historic environment (known as PPG 15 – minor problems to be identified and dealt and the supporting structure, both before see References). Consent is needed for with before they become serious. it is disturbed and during stripping. The alterations to a listed building if these Inspections should include an internal and materials or products, their dimensions, would affect its character as a building of external examination, checking for slipped shape, composition, colour, and texture architectural or historic interest. slates, loose ridges, and defective are all important, as is the way they have Obviously, stone slate roofs are very rainwater goods including flashings and been assembled. The checklists in important features whose loss will almost gutters. Signs of decay may be indicated Appendices 1 and 2 should be completed always be resisted by the local planning internally by moisture staining of the roof as far as is possible and appropriate, and authority. For all but the most minor timbers, ceilings and walls, presence of should be supported with photographs repairs it is prudent to speak to the local mould or fungal growth, and by timber and drawings. When photographing slating planning authority to ascertain whether or decay and loss of torching. Leaks can be a always try to include views straight on to not Listed Building Consent will be symptom of more serious design or the surface, including a scale to provide a required for the works. construction problems, so it may be record of the size mix (markings on tape necessary to inspect the overall structure. measures rarely show up on photographs: 4.2 Unlisted buildings in conservation If, for example, the lap of the slates is a metre length subdivided into 25mm areas do not have the same protection. inadequate, or the pitch of a roof is too bars, painted alternately black and white, Planning permission is required for any shallow, wind-driven rain will penetrate. is suitable for most purposes). If a change which would represent a material comprehensive investigation cannot be alteration to the external appearance of a 5.2 Regular maintenance is part of the undertaken before the work begins, the building if it is not in residential use. day-to-day responsibility of all owners and checklists should be used as stripping Permission is not usually required if a occupiers, and a well maintained stone takes place and the reslating methods building is in residential use. However, it slate roof can last for many generations. reviewed as relevant. is always advisable to check in case the Excessive plant growth may affect the local planning authority has any additional performance of a roof but some rare 6.3 The type and size of the various powers under an Article 4 Direction (of mosses live on stone roofs and may be materials should be recorded as well as the 1990 Act), which could require protected. Before removing a moss their condition and, if they have permission to be obtained to remove or building owners should check its status deteriorated, whether they need to be alter a stone slate roof. with the local nature conservation replaced or can be repaired. The condition organisation. With care it is possible to of the roof timbers and the slates will be 4.3 An application for a new building (or remove unprotected plants without difficult to assess until the roof has been alterations to an existing building) in a damaging a protected species. Ensuring stripped and the slates can be examined conservation area will be subject to local that the roof is free of excess moss (which on the ground or on a wide, load-bearing planning policies, so an early consultation can retain moisture and accelerate scaffold. with the local planning authority’s Design delamination) and creepers such as ivy or Conservation Officer is recommended. (which can dislodge slates) will save on 6.4 It is important to record as much Development proposals are required to future repair bills. Leaves, accumulated information as possible about the preserve or enhance the character or silt, and debris should be regularly materials and the detailing of the roof. For appearance of the area so, where stone removed from the valleys, gutters, gullies, the stone slates, information about their slate roofs are common, new buildings rainwater pipes, drains, and channels. colour, texture, edge treatments (square or may be required to incorporate the same bevelled, dressed or sawn), and sizes may type of roof covering. This is especially 6 Investigation and recording all be helpful in identifying their origin. likely if the relevant appraisal confirms Often building owners or local slaters will their positive contribution to the overall 6.1 If the roof covering requires repair, be able to identify the type of slates and character. specialist advice from someone familiar even the source quarry. It is also useful to with stone slate roofing in the area (either record any vegetation growing on slates, 4.4 Detailed recommendations for the an experienced architect/surveyor, or a its type (lichen, moss etc), and its colour – application of planning legislation to stone specialist stone slating consultant or which may be indicative of the slates’ roofs is available in Stone Roof contractor) should be sought to ensure mineral composition and origin (Fig 16). Conservation: a Best Practice Guide at that the causes of failures are correctly www.stoneroof.org.uk/best.html identified. It is particularly important to 6.5 A careful distinction needs to be distinguish between leaks due to made between apparent defects and inadequate construction (gaps etc) and original subtleties of the construction that 5 Inspection and long-term deterioration of, for example, contribute to the character and technical maintenance slates, nails or pegs. performance of the roof. For example, it is easy to assume that the roof has sagged or 5.1 Planned and regular inspection and 6.2 The roof coverings should be moved. Apart from the visual implications maintenance are the best ways of ensuring reinstated in their existing form except of correcting this (the charm of many the continued satisfactory performance of where inappropriate and unsympathetic stone roofs is their unevenness), they may a roof. Inspections, which can be carried earlier changes have occurred, or where originally have been constructed as they

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appear today. If, for example, the rafters stone roofs. The local nature conservation insulation may well require the roof are installed with a slight concavity it aids organisation should be consulted before a surface to be elevated, leading to problems the laying of the slates. Elsewhere, roofs roof is cleaned. at eaves, verges and abutments. A fully with gable copings may have been thought-out solution, which will probably constructed with the rafters at a slightly need specialist advice, may lead to the steeper pitch than the copings to allow for 7 The repair and reroofing of conclusion that traditional details are best the reducing thickness of the slates up the stone slate roofs for traditional materials. slope, thus making the roof surface and the copings parallel. In some regions it 7.1 Materials for repair and 7.1.4 Stone slates was common practice to upturn each reroofing 7.1.4.1 Stone slates are remarkably course of slates slightly at the abutment in durable. It is usually possible to salvage a order to direct water away from this 7.1.1 Stone slates are reusable and can large proportion for reuse, making them vulnerable junction. This was often done last for hundreds of years; a stone slate among the most sustainable of all roofing by raising the end rafter and using roof, providing it is well maintained, can materials. As long as the slates are sound, narrower slates to accommodate the tilt. last for at least a century and possibly with no cracks or delamination (see much longer. The need to repair a roof Glossary) through their whole length, they 6.6 Where there is a need to record the will most frequently arise from the will be suitable for reuse. Where they have construction of the existing roof the deterioration of the nails or pegs used to softened or delaminated at their heads, or important dimensions are the batten fix the slates to the laths or battens, the where they have been damaged along their gauge (see Fig 9) and the slate length decay of the laths or battens or of the edges, they should be redressed, using (below the fixing hole) at each batten. rafters, or the delamination or cracking of appropriate hand tools, rather than sawn. The headlaps at each slate length and the the slates, especially at their top edges. This means that salvaged slates are often minimum sidelap should also be recorded, Sometimes the problems will have been smaller than the originals. Slates should although because of movement in the roof compounded by the misguided application be discarded if delamination is extensive these may not be as originally installed. If of bitumen or mortar patches or covering and severe. Where the fixing hole has these measurements are to be useful, a to the entire roof. broken or enlarged, a new hole can be record should also be made and related to made at the same end of the slate. the above measurements, of any water 7.1.2 In order to retain historic integrity penetration or timber rotting. without compromising performance, as 7.1.4.2 Wherever possible, new stone much as possible of the original roof slates rather than second-hand should be 6.7 The interpretation of the covering and supporting structure should used. Reclaimed slates should be used constructional details of stone roofs is a be retained during repair and reroofing. If only on the building or group of buildings complex exercise which is best carried out the defects are extensive enough, reroofing from which they were removed. The trade by specialists with an understanding of may be necessary and Listed Building in second-hand slates inevitably leads to how roof slating works and experience of Consent may be required (see 4 Planning the unnecessary destruction of roofs and local variations. and historic buildings legislation). It is undermines the viability of quarries almost impossible to determine the producing new slates; it also often results 6.8 While the appearance of a roof may precise cost of reroofing until the roof has in inappropriate regional exchanges and be of interest, photographs of the exterior been stripped, the materials examined the use of non-local slate. Substitute will be most unlikely to supply useful more closely, and the extent of damage to materials, such as artificial slates made of information about the way the roof was the roof timbers and the condition of the fibre resin, concrete tiles, ‘reconstituted slated. stone slates determined. However, an stone’ and so on, are inappropriate experienced stone slater should be able to alternatives to real stone slates and are not 6.9 Bats Before the roof covering is give a reasonable cost estimate before suitable for use on historic buildings. disturbed, the roof should be inspected stripping the roof although it should be internally for evidence of bats. If there is recognised that it may need to be revised 7.1.4.3 New stone slates should match any possibility that bats may be present, or when the roof has been opened up. the existing ones as closely as possible in any evidence of occupation is found, terms of geological type, colour, texture, English Nature must be consulted before 7.1.3 The introduction of man-made size, thickness and edge dressing. It is work begins. Because bats return to the roofing products, new performance important to recognise the slates same place year after year a bat roost is criteria, and the consequent changes in particular to an area and know that even protected even if bats are not always roof detailing have resulted in a within the same region that a variety of present. All British bats and their roosts standardisation of roof construction. If slates was sometimes used. If difficulties are protected and it is illegal for anyone unsatisfactory results or even roof failures are encountered in achieving a good without a licence intentionally to disturb, are to be avoided, the adaptation of match locally, it may be acceptable to injure or kill a wild bat or obstruct access traditional details to accommodate source slates from outside the region to any place used as a roost. Some timber- modern materials must be carefully provided that they are geologically and preservative chemicals are toxic to bats thought through – will the felt be pierced visually similar. However, it is important and must not be used. by the pegs, will the introduction of to ensure that they have suitable insulation and ventilation affect the lie of weathering and durability characteristics, 6.10 Plants Some protected plants, for the slates and result in leaks? Roof and are finished in the local tradition. example a moss Grimmia ovalis, grow on innovations such as the introduction of Sandstone and limestone slates and

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ridges, including sandy limestones, should 7.1.4.8 Cleft laths are split from introduced. Modern preservative-treated never be substituted for each other, nor hardwood and whilst they can be nailed to softwood sawn battens are more should they be used together. rafters, they are unsuitable to receive commonly used today, and the slates are slating nails. Battens, on the other hand, frequently nailed rather than pegged. 7.1.4.4 Some manufacturers are offering are sawn from softwood, being nailed to Whether modern methods are acceptable stone slates sawn to thickness. These can rafters and able to receive nails which or not will depend on the specific be technically and aesthetically retain slates or tiles in place. Decisions on conservation objectives and the unacceptable and, if they fail to meet the retention or replacement of existing importance of the building; the local these criteria, their use on historic laths may be based on many issues: Conservation Officer should be consulted buildings should be resisted. If traditional (Fig 7). split slates are available they should always • the age of the roof be used in preference. If they are not • the importance and condition of the 7.1.6.2 Pegs were most commonly of currently available but might be specially roof oak, although other woods and, in some manufactured in a traditional way, this • the listing grade of the building parts of the country, animal bones were option should be investigated. If fissile • the rarity of riven lath in a particular used. New oak pegs and treated softwood stone is no longer available a local and area versions are available in a variety of building specific decision will have to be • whether or not the appearance of the dimensions. Green pegs should not be made whether to use sawn slates, or underside of the roof is important used as they are likely to shrink, and slip similar imported slates, or some other • whether or not the laths are part of the from the peg holes. Pegs should be substitute. If sawn slates are used they ceiling construction roughly square sectioned and dried must always be sawn along the naturally • if the use of slate nails is desirable or before use so that after fixing absorbed occurring bedding planes or they will acceptable moisture makes them swell, locking them weather differentially and may eventually • the extent to which other components into the holes. The fixing holes for break. of the roof structure are authentic wooden pegs were traditionally tapered • if insulation and/or an underlay are to or hourglass shaped: this also helps to 7.1.4.5 Slates should preferably be be incorporated grip the pegs and is still good practice. dressed rather than sawn to size. Sawing • if the roof is to have batten space reduces the likelihood of producing the ventilation installed. 7.1.6.3 Large headed non-corroding natural mix of random sizes, and is metal, preferably copper, pegs 38–63mm aesthetically unpleasing, leaving a straight, The basic principle which should be long are also used to hang stone slates. blunt edge. Sawn edges can be dressed applied is to change as little as possible Aluminium pegs should be avoided subsequently – albeit at an additional cost. unless there are sound legal or technical where they may be in contact with reasons for more extensive changes. alkaline materials such as limestone slates 7.1.4.6 Slates should be reused with or mortar. If the existing peg holes are their original orientation (top face up, top 7.1.5 Ridges large, pegs with large heads will lock in edge up). This is because the underside 7.1.5.1 Ridges were traditionally made position and prevent the slates slipping. and the head of the slate deteriorate most from stone, sawn or chiselled from a Where the stone slates are to be drilled rapidly and may not be as weather block, and should be reused if they are or re-drilled, for nailing or metal peg resistant as the originally exposed surface. sound. It is possible to obtain from hanging, the holes should be parallel masonry quarries new ridges selected to sided at a slightly larger diameter than 7.1.4.7 Laths and battens The issue of match the original type, colour, and the peg shank and countersunk to receive like-for-like repair or replacement of laths texture of the stone. The exterior faces of the head. Electric drills without hammer or battens will depend on the particular a new ridge should be dressed or tooled in action are suitable and each slate, situation. Wherever possible and practical, the manner traditionally employed in the properly supported, should be drilled the retention of historic detail should be locality. In some regions clay ridge tiles individually. Attempting to drill through achieved in all elements of re-roofing. are traditional. a pile of slates will inevitably damage Vernacular details such as cleft laths form them. intrinsic parts of historic roofs and year by 7.1.5.2 Ridges are back-bedded in year they become rarer. mortar so that, as far as the undulations in 7.1.6.4 Nails for fixing slates should be the slating permit, the mortar does not large-headed, large-gauge copper. Nails

Bedding mortar show at the long edges. They are also for fixing battens, laths, or boards may be hidden behind haunched at the butt joints. stainless steel for maximum durability, ridge provided that they are strong enough to Vertical edge 7.1.6 Fixings drive into hard timber, otherwise 7.1.6.1 Traditionally, stone slates were galvanised or sheradised nails are fixed with pegs hung over laths, frequently adequate. Length, gauge, and type should head bedded in lime mortar, and the be appropriate to the batten size and underside was often torched. Laths were wind uplift requirements in the particular Figure 6 The edges of the ridge should be formed split from oak, fir, or more rarely sweet location (BS 5534, 2003). as shown to avoid large and unsightly mortar fillets. chestnut, and are again readily available. Similarly ridges should not be edge pointed, rather the bedding should be neatly struck off as work From the nineteenth century, sawn 7.1.6.5 All metal fixings should be progresses. softwood battens and nails began to be checked for compatibility with the

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7i Traditional fixing method: 7iv Insulation at ceiling level and verges pegs and single torching felt supported to allow ventilation and to avoid ponding eaves to ridge ventilation

possible use of proprietary eaves vapour barrier vents

ventilation

7v a Insulation below the rafters

eaves to ridge ventilation

vapour barrier

7ii Traditional fixing methods: pegs and full torching

7v b Insulation between the rafters

vapour barrier

eaves to ridge ventilation path min 50mm counter batten

7iii Nail fixed stone slates 7v c Insulation above the rafter (which changes the roof line)

felt alternative positions for vapour barrier

stop batten to prevent insulation sliding down the roof slope under eaves to ridge ventilation path minload 50mm counter batten

Figure 7 Fixing methods and the use of different types of torching and roofing felts for stone slating

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materials they come in contact with. Very 7.1.7.3 If the slates are pegged over felt, 7.1.8 Bedding and pointing large, wide slates may require double fixing. thicker battens, or counter battens of an 7.1.8.1 Ideally, mortar (other than appropriate thickness will be required to torching) should not be used in stone 7.1.6.6 Where methods of fixing are prevent the felt being pierced (Fig 7iii). If slating, as there is always a danger of altered, the appearance of the roof may be the slates are pegged but not torched, large- drawing moisture between the stones and inadvertently changed. For example, the headed metal pegs may be used to stop the into the roof. However, the use of mortar horizontal alignment of the tails of slates pegs tilting. In recent years, double bedding and pointing seems to be will be much more even and regular when battening has been adopted to prevent pegs regionally specific and local practice the stone slates are drilled and nailed than tilting but for conservation purposes the should prevail. It is preferable and cheaper where they are pegged and hung on laths. mortar methods are more appropriate. to increase the lap rather than to rely on This can be avoided by drilling the fixing these methods to make a roof water-tight, holes freehand rather than in a jig or 7.1.7.4 Underslating felt should conform and the use of mortar for bedding and against a stop. to BS 747 type 1F or 5U, and be fixed to pointing should be carefully considered. the rafters with large-headed clout nails, Spot bedding in lime mortar is used in 7.1.7 Torching, roofing felts, vapour with a minimum vertical lap of 150mm some parts of the country, but only as a permeable membranes and vapour below 35 degrees pitch and 100mm at 35 means of helping the stone slates to lie flat barriers degrees and above, and a horizontal lap of without rocking. This is also a method 7.1.7.1 Pegged roofs were traditionally at least one rafter space. It should be sometimes employed in minor repairs to torched to render them windproof. nailed to each rafter. The use of roofing isolated patches on roofs. It should be ‘Mossing’ (driving moss into the joints of felt is no substitute for good used very sparingly and the spots of the slates with a mossing iron) was workmanship, which by itself will result in mortar kept away from the tails of the another means of achieving the same a weather-tight roof (Fig 7, iii-v). slates and the perpendicular joints to result. These methods have now been (NB BS 747 is to be withdrawn.The avoid drawing moisture into the roof. In largely superseded by the use of roofing specification for 1F and 5U felts is some very exposed areas, the heads of felts. Torching, or tiering as it was also given in BS 5534:2003 Annex A.) slates are bedded in a thin band of lime known, was traditionally a mix of lime and mortar. aggregate (sand and/or crushed stone), 7.1.7.5 If insulation is introduced into usually with animal hair added, and was the roof, increased ventilation may be 7.1.8.2 Pointing applied at the tail of the applied to the underside of the roof either required and it may be necessary to use a slate and at the perpendicular joint is a as half or full torching. Half, or single VPM and a water-vapour barrier. There is method employed in some regions and torching, was applied at the upper edge of a British Standard specification for VPMs can be carefully reinstated where it the laths, and full torching filled the space – BS 4016 – and their selection and use represents traditional practice. As with between the laths. The torching had to be should be based upon calculations using spot bedding, it has the potential to draw applied carefully avoiding forcing mortar realistic (extreme not average) parameters moisture into the roof structure. It also down between the joints, which would for the internal and external environments. dries out very slowly after rain, with the draw in moisture. Torching also served to Vapour barriers may be formed by closed- consequence that the roof structure can hold the pegs in place and stop them cell insulation with taped joints, or by use be wet for long periods. Furthermore, tilting and falling out (Fig 7i and ii). of 500 gauge polythene or similar material. pointing can exacerbate frost damage to Traditional roofs and roof spaces were BS 5250: 2002 and BRE 2002 (see the backs of the slates. In some regions, very well ventilated. Any change in the References) give further guidance on pointing is held back from the edges of construction, especially introduction of performance requirements for such the slates, or raked out, so it is not visible; felts or vapour permeable membranes materials (Fig 7iv and v). elsewhere, it is exposed – this is visually (VPM) with or without insulation, may undesirable. If it is applied, pointing must well reduce this and lead to deterioration 7.1.7.6 Ventilation can be introduced penetrate upwards or sideways between of the fabric. Careful consideration should into gables, at the eaves, at the ridge, or the slates as little as possible to minimise be given to all the aspects of such within the roof slope. Proprietary eaves the extent of resultant wetting. If the roof innovations. If the roof can be wind- vents are usually unobtrusive but may is laid to the correct pitch and sufficient proofed without the use of felt or VPM require adjustment of the eaves tilt to head and side laps are applied, pointing this may well be the best option. ensure that the slates lie properly. will not be necessary to keep the roof Proprietary ridge vents do not match weather tight. 7.1.7.2 It is preferable to retain all stone slates, thus the use of gable historic details, including torching. This is ventilation is the preferred solution where 7.1.8.3 Pointing and bedding should still a successful and appropriate this is an option. Where clay ridges are never be combined with torching. It is technique if carried out correctly and, traditional vent ridges may be acceptable. almost impossible to prevent the two properly applied, allows the battens to dry Individually designed ridge ventilation coming into contact and forming a wick out even after occasional wetting by solutions such as open joints, raising of which continuously draws in water. This driving rain. A mix of 1:3 mature lime ridges and so on, should be designed with can result in total failure of a roof within putty to aggregate, with approximately one care, as they may not provide adequate 25 years of reslating. handful of clean hair, well teased out, ventilation and may leak. There are added to one bucket of the mortar, and currently no slate-vents suitable for stone 7.1.8.4 Where bedding and pointing are evenly mixed, will produce a satisfactory roofs, so in-slope ventilation is not a used careful consideration should be given torching mortar. practical option. to the use of underslating felt. Where they

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are used together the humidity in the batten space may be high for extended periods resulting in the deterioration of the slates and battens.

7.1.8.5 If the water vapour passing through VPMs is not vented from their upper surface they will not be effective. For this reason they should never be used with bedding and pointing unless effective through-ventilation of the batten space is provided by some other means.

7.1.9 Roof timbers 7.1.9.1 To avoid unnecessary loss of existing material, timbers should be cleaned down without defrassing (see Glossary) and inspected for signs of active decay or infestation. Replacement Figure 8 Stone slates removed from the roof and sorted according to size.The roofer will now be able of roofing timbers should be restricted to calculate the amount of new material required for reroofing. to those which threaten the structural soundness of the roof. Repairs to roof 7.2 Construction and Where new stone slates are needed, the timbers should be carried out as far as workmanship specification should include source possible in the same timber, using 7.2.1 Specification (geological/geographical/delph), sizes, appropriate repair methods which 7.2.1.1 The specification is relied on by thickness, edge dressing style and, if minimise loss of existing timbers, and the roofer to provide a full description of required, surface dressing. do not regularise the appearance of the the works and the quality of the materials roof. and workmanship required for a job. It is 7.2.2 Stripping, sizing and stacking also likely to become part of any contract 7.2.2.1 Once the roof covering is 7.1.9.2 Where alterations to the roof documentation. Estimates will be recorded as far as is possible, the stripping structure of a listed building are prepared on the basis of the specification of the roof can begin, starting at the ridge. necessary, such as the replacement of and thus it needs to be clear and The slates should be carefully removed, rafters, purlins, or valley boards, Listed comprehensive, ensuring that anything cleaned, inspected for damage and Building Consent may be required. which is likely to have a cost implication delamination, and then carefully stacked. Advice from the local planning authority on the work is included. A thorough Damaged slates should be put on one side should be sought before any timbers are specification will enable a fairer and more for redressing. Stone slates should be removed. accurate cost estimate to be prepared by stacked on end, in piles sorted by length the roofers pricing the works. (measured from the fixing hole to the 7.1.9.3 New timber should be dried to a tail). They should NOT be stacked flat. moisture content of between 15 and 20 7.2.1.2 It is outside the scope of this Once sorting to length is completed, per cent and should match the existing in leaflet to provide a model specification, sorting to thickness can follow if this is the size and species. Softwoods should be which needs to reflect local roofing local practice (Fig 8). It is not possible to pressure-impregnated with preservative traditions and practice. Some local stack, inspect, and sort stone slate at roof before being brought to the site and any authorities provide guidance on suitable level unless a large working platform is cut ends or bored holes should be treated specification clauses. As a minimum the provided. Such practice is unsafe on all with insecticide/fungicide before being specification should cover the following but the largest roof platforms, and does used. not encourage the rigorous standards of • scaffolding inspection and sorting required. Stone 7.1.9.4 When timber treatment is • security (stone slates are valuable items) slates are valuable and will need to be considered necessary and appropriate, work • stripping, stacking, sorting and stored in a secure place. should be confined to areas obviously redressing (see 7.1.4 and 7.2.2) affected by active insect infestation or fungal • timber and carpentry (see 7.1.9) 7.2.3 Reroofing decay. Remedial work should be carried out • roofing felt/torching (see 7.1.7) 7.2.3.1 Cover The sorting of the slates with reference to current legislation • insulation and ventilation (if applicable will identify the amount of existing slate including the Control of Substances – see 7.1.7.5–6 and 7.2.5) which is reusable. Sometimes, there will Hazardous to Health Regulations 1998,The • slates, battens and fixings (see 7.1.4 and be sufficient to re-cover the roof with Health and Safety at Work etc Act 1974, and 7.1.6) some patching-in of replacements. Lower the Control of Pesticides Regulations 1986.If • slating (see 7.2.3) rates of recovery will necessitate the bats are present, advice should be sought on • valleys (if applicable – see 7.2.4.6) consolidation of the surviving slates on a the suitability of treatments so as not to • ridges, abutments, hips and verges (see single roof slope, with new slate required cause them harm (see 6.9 Bats). 7.2.4.6–9) to re-cover the remaining slopes. Either

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way, it will now be possible to calculate reproduce slavishly an existing pattern of surfaces, and to avoid variations in the total width of cover available for each margins. This is not the correct traditional thickness from one slate to the next. If length of slate, and the quantity of new method, is unnecessarily expensive and bedding and/or pointing is to be carried slate that will be required. The number of risks the use of inadequate headlaps. The out, this should be applied as work courses for each slate length and the preferable method is to specify the largest progresses, using the minimum mortar batten or lath gauging can also be and smallest sizes of slate and a required. calculated. This will be dependent on the reasonably even gradation of sizes between size of the available slates and should be them. The specific gradation of the 7.2.3.4 Backers Wider slates should adjusted as their length diminishes up the margins will then be the natural outcome generally be reserved for use with backers, roof to ensure that the minimum head lap of the mix of sizes and the correct which are narrow slates placed roughly is achieved for each course (Figs 9 and headlap. In practice new slates in the centrally over wide slates to make 10). The risk of unbalancing the loading largest sizes specified may simply not be provision for the larger number of slates in on the roof should be considered where available due to the limitations of the the upper courses (Fig 9). stone slate is used on one pitch and lighter rock. slates or tiles on the opposite pitch. 7.2.3.5 Wide slates should also be 7.2.3.3 Laying Stone slates should be reserved to close the half bond at verges, 7.2.3.2 Slating gauge Reslating should laid one course or a few courses at a time, hips and abutments. always be carried out by measuring how depending on local practice, working from many courses of each slate length can be thicker to thinner slates across the roof if 7.2.3.6 Underslating Historically, stone achieved with the recovered and new this is the local tradition. The slates should roofs were laid without underfelt or slates together. It is inadvisable to try to be laid to avoid rocking on uneven membranes, although they would

head

fixing hole

laths or battens

tail reduced batten gauge at first head lap shorter course

batten gauge (variable)

margin

side lap

effective slate length

backer

Figure 9 Measuring slates.The length is measured from the fixing hole to the tail.The head lap is the amount by which a stone slate overlaps the fixing hole of the slate in the next course but one below.

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generally have been torched to make them from ground level. The temptation to alter the margin is bigger than courses lower draughtproof. Where torching is an option the pitch of a roof should always be down (Fig 15). The head lap should not or draughtproofing is not required, there resisted: traditional pitches may have exceed one third of the slate length. may be no need to include a felt or evolved in response to regional and very membrane. If a modern specification is to localised conditions. They may also differ 7.2.3.10 Side lap The minimum side lap be adopted, the underslating system might for slates of differing roughness or sizes, is commonly not specified. Frequently, include a vapour barrier, insulation, and even from a single quarry (Fig 19). local rules of thumb are applied, such as roofing felt (see 7.2.5 Insulation and ‘not less than 75mm’ or ‘not less than the ventilation). Ideally, the laths or battens 7.2.3.9 Head lap There are no set head lap’. Genuine local methods known should be fixed in place as the felt is laid guidelines for determining the minimum to be successful should be respected. (Fig 7). head lap of stone slates; it seems to vary from area to area and relates to the pitch 7.2.3.11 Gallets or shales In some 7.2.3.7 Weather tightness Water of the roof. Head lap (the amount the regions where stone slates are thin, they penetration is resisted by ensuring an third course overlaps the first, the fourth are supported with small pieces of stone adequate pitch, head and side lap and, overlaps the second, etc) can be specified slate or metamorphic slate, known as most significantly, making certain that the as a value or as a fraction of the slate gallets or shales, which are bedded in a largest slates (which provide the largest length. In either case it should not deviate small amount of lime mortar at the head side lap) are placed at the bottom of the from established local practice (unless this of the course below.The technique roof. It is important for the weather is known to be inadequate). Lap prevents the slates breaking if, for tightness of the roof to ensure that the measurements are taken from the peghole example, they are loaded by a roof ladder: stone slates are properly sorted, selected, rather than the top of the slate. As the it is mainly required at the eaves where and laid, and sit together as well as stone slates are laid in diminishing the tilt produces a gap between successive possible. courses, it is important to ensure that the courses. Gallets must be used carefully to minimum lap is maintained as the slate avoid forming a path which would draw 7.2.3.8 Pitch It is for good technical length changes up to the roof (Fig 9). moisture into the roof. In some limestone reasons that stone slates are seldom laid at Failure to do so will lead to insufficient slating traditions similar small pieces of so low a pitch that they cannot be seen headlap indicated by pigs, courses where stone or metamorphic slate known as

Figure 10 Section through a typical sandstone slate roof showing structure, diminishing courses, ridge and eaves details.The inset shows a typical Cotswold cussome detail.

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abutment details evolved when lead was unavailable (or too expensive to be used). These details also influence the pitch of the roof. In regions where slates are small and thin, the roof pitch will be generally steep (about 50 degrees), enabling curves (such as the swept valleys of the Cotswolds) to be formed at the intersection of the roof planes. Where stone slates are very large and thick, such as the sandstone slates of the Pennines, the pitches are lower and the roof forms are simpler, with the valleys formed with chevron slates (Figs 11 and 12).

7.2.4.4 The decision to restore original details, such as swept or chevron valleys, which have been replaced by leadwork in the recent past, should be based upon sound evidence for their former existence rather than upon speculation. Changing Figure 11 The limestone slates of the Cotswolds, which are lighter and smaller than most sandstone slates, require a steep pitch and can accommodate curves in swept valleys. the details of the roof may require Listed Building Consent, and advice should be shadows or shales are used to improve the ridge, hip, and valley details should be sought from the local planning authority. weather tightness where the slate's retained wherever possible. New shoulder is not quite good enough. techniques should not be applied without 7.2.4.5 A roof must be capable of Usually they are bedded in lime mortar a careful assessment of their technical withstanding wind forces and preventing and act by blocking the wind from implications, and the adaptation of the penetration of driving rain, so the blowing through the slating and carrying traditional methods to modern practice fixings must be strong and secure enough in water. The way they are laid also turns should be treated with caution. Correct to hold the slates in place without any water which reaches them down and detailing not only ensures that the slipping. Different parts of the country out of the slating. This is acceptable character of the roof is retained but also have achieved these objectives by evolving traditional practice although it is better to that it fulfils its function. various methods, some of which may be avoid the need for them by careful quite localised and unlike those applied to selection of slates of suitable shape. Today, 7.2.4.3 To ensure the roof was similar materials in other regions. They it is also acceptable to use lead soakers for watertight, the valley, hip, dormer, and may also conflict with the the same purpose. recommendations of the British Standard (BS 5534 2003 does not deal with stone 7.2.3.12 Eaves slates The under eaves slate roofing specifically but is the and eaves slates should be laid to project accepted standard usually referred to). over the wall and discharge water away However, many of these traditional details from the building, or into the gutter where have a proven track record and should be one is to be fitted. Local details should be conserved where it is practical to do so. applied (Fig 10). Gutters were not always Care must be taken when replicating used for stone slate roofing. traditional details to implement them correctly. Modification of traditional 7.2.4 Detailing details can be problematic: confusion and 7.2.4.1 The detailing of a roof – its pitch, poor amalgamations of traditional and and the treatment of the eaves, valleys, modern methods have resulted in failures ridges, and dormers – was evolved by local (Fig 17). craftspeople in response to the local stone and regional climate, and may vary 7.2.4.6 Valleys and hips should be formed according to the building type and period. as work progresses, using the appropriate A roof may exhibit a number of stages of a weathering technique. Extra fixings may building’s development and incorporate a be required for hips, which should be number of different details which may be formed with wide slates. Some valleys are equally important. made without fixings. Solid bedding of Figure 12 The heavy, flat sandstone flags of the valleys should be avoided. Some 7.2.4.2 The existing features of an old South Pennines are suitable for lower pitches, and traditional valley detailing requires the roof, such as the mix of slate sizes within the detailing of the roof is simpler to setting of the valley board at an accommodate their large dimensions.The chevron the limits described in 7.2.3.2 (which will valley is an example of the craftsperson’s skill in appropriate level and angle by the slater, determine the number of courses), verge, forming a weather-tight roof. in anticipation of the need to align the

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eaves levels of adjacent roof slopes with the valley slate (Fig 18).

7.2.4.7 Abutments should be formed with lead soakers and cover flashings or fillets to the specified detail. Fillets for abutments to uncoursed stonework and render should be formed with a suitable mortar which may require a stainless steel reinforcement mesh. Back abutments, such as at chimneys, should always include an adequate lead gutter and upstand, and be carried up under the slating. At the intersection of dormer valleys and the upper slope, it is traditional in some regions to use an inverted ridge piece to weather the joint. If this detail is adopted it may need to be backed up with a discreet lead soaker. An alternative detail carries the dormer ridge up into the slating of the main slope.

7.2.4.8 Verges should be bedded and the bedding struck off as work progresses, rather than pointed up after completion. Where verges are formed without bedding or an undercloak, they may be pointed in and a fillet formed under the slates after slating is complete.

7.2.4.9 Ridges should be back bedded in mortar and solid bedded at the joints. To avoid the need for unsightly pointing along the ridge it is preferable if ridge stones have a vertical edge at the bottom of the wing. Mortar should not be visible and the wings should not be pointed except where there are large undulations in the roof surface. Gallets can be introduced under the ridge joints and over the vertical joints in the slating where they coincide (Fig 10).

7.2.4.10 If torching is specified, it should be carried out after slating is completed to avoid damage.

7.2.4.11 The use of large stone slates as Figure 13 Eaves slating is a traditional roofing practice in some parts of the country. In some areas stone eaves courses below thatch, tiles, and slate has been gradually replaced by other roofing materials such as metamorphic slate, and the stone slates at the eaves are a remnant of an earlier roofing material.The example shown here is in the village of metamorphic slates may be an original Montacute, where the local Ham Hill stone slates have become increasingly scarce. detail or may be a solution to the shortage of stone slates during the last reroofing. In roof space. This is especially important if especially the need for ventilation. either case, this is now regarded as a roofing felt is used. Failure to provide an Insulation can be positioned above or traditional detail in many parts of the alternative ventilation route can result in below the ceiling, or between, below, or on country and should be reinstated as found condensation and extensive wetting of the top of the rafters (Fig 7iv and v). The first (Fig 13). fabric, or at least a raised moisture two options are relatively straightforward content, with an increased risk of mould and ventilation can be provided fairly 7.2.5 Insulation and ventilation formation and deterioration. simply. However, when insulation follows 7.2.5.1 Wherever an old roof is altered in the rafter line, the situation is more a way that reduces airflow through or 7.2.5.2 If insulation is to be introduced complex and the success of any method beneath it, careful consideration should be into a roof space, a specialist should relies on good workmanship. Where given to the effect on the ventilation of the carefully consider the implications, insulation is laid on top of the rafters, and

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upper courses omitted for clarity

slates swung aside to fix replacement

headlap over peg headlap over nail

replacement slate

Figure 14 Fixings for replacement slates as part of a localised repair must be aligned with the adjacent slates. If the original slates are pegged but the replacement slate is nailed, care must be taken not to compromise the minimum head lap by placing the nail too far below the pegs.

there is a change in the roof line, Listed underside of the roof is accessible, missing their undersides, should be avoided as Building Consent may be required. In torching should also be replaced to help they prevent the salvage of stone slates or most cases when insulation is introduced hold the peg in place. If the underside is reuse and make it difficult to locate into the roof space, it will be necessary to not accessible and the laths or battens are defects. These treatments also serve to seal introduce a vapour barrier. It must be in reasonable condition, it is preferable to roof voids, preventing ventilation and complete, continuous, and sealed at the nail fix the replacement slates. leading to fungal attack and infestation of joints. This may be difficult or impossible Replacement slates should be the same roof timbers. to achieve in practice and this fact should length as the originals and aligned with be taken into account when the ventilation the adjacent slates so as to retain the head system is being designed. BS 5250 lap and ensure that the repair is watertight 7.4 Choosing a slater provides design guidance. (Fig 14). If the new fixing hole would be 7.4.1 Stone slating is a specialist activity lower than originally it should be drilled and not all roofing contractors are either 7.2.5.3 Where proprietary ventilation away from the perpendicular joint of the familiar with the particular methods products are introduced, it is important to overlying slates so as to increase the involved or skilled in the local roofing check with the manufacturer that they are effective head lap. traditions. An assessment and registration suitable for the specific situation. scheme for stone slaters is under Particular attention should be paid to 7.3.2 Where supplies of replacement slates development by the National Federation ensure that adequate laps are achieved. of the same size and thickness are of Roofing Contractors. It is not always Individually designed solutions should be unavailable, it may be impossible to carry safe practice to import a slate from carefully considered to ensure that out patch repairs. In this case, consolidating outside the region, and equally it may not adequate ventilation is provided. If the salvageable stone slates on one part of be wise to import a contractor who is not torching or bedding is employed on a the building may be a solution. experienced in the local stone slate roof, care must be taken not to block the roofing techniques. In many areas of the vents or the flow of air under the eaves. 7.3.3 Where the number of slates to be country, the scarcity of stone slates and replaced is extensive, consideration should the decline in their use has resulted in a be given to renewing whole areas or to loss of traditional skills, and care should 7.3 Minor and localised repairs complete stripping and reslating. Large be exercised when selecting suitable 7.3.1 It is preferable to repair small areas numbers, typically five per cent, of contractors. They should be familiar with of deteriorated stone slate roofing rather displaced or damaged slates probably stone slate roofing in the region and be than to strip the whole roof. It may be indicate that the roof is nearing the end of prepared to supply photographs of work possible to reinstate missing or loose slates its life. If large areas are to be patched, the they have carried out. Better still, they and refix them by swinging the adjacent coursing must match the remainder of the should provide the opportunity to visit stone slates aside if they are pegged: nailed roof, and the head and side laps must be previous work, in order to demonstrate slates will break. Before a stone slate is the same or larger. their competence. Inspection of a roofer’s reused, it should be examined to ensure work should include a check that the that it is suitable for refixing. If the stone 7.3.4 Bitumen-coated fabric applied over stone slates are laid evenly and sit well slates are pegged and torched and the roof coverings, or spray-on coatings to across the roof, and that the sizes of the

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reduce production costs significantly. Reusing the salvageable material from the same roof will reduce the material costs of the roof. However, new slates should be used to make up shortfalls, to help encourage the market for new stone slates and secure the future for stone slate supplies.

8.2 Grants 8.2.1 Grants for repairs to stone slate Figure 15 ‘Pigs’ (where a long margin is visible above a short one) indicate a failure to maintain adequate head lap and are evidence of poor workmanship, as this roof in West Yorkshire shows. roofs of Grade I and II* listed buildings and scheduled monuments are available slates diminish regularly from the eaves to subcontract work are at risk of having to from English Heritage. Many local the ridge: successive margins should be use unsuitable or inexperienced roofers. authorities also operate grant schemes either the same size or smaller than those It is equally crucial not to be swayed by which include Grade II listed buildings below – there should not be equal price; to be wary of significantly lower and occasionally unlisted buildings in numbers of courses of each margin size tenders; to check that nothing has been conservation areas. The Heritage Lottery (Fig 9). Long courses above short missed; to ensure that all the quotations Fund and English Heritage offer grants courses, known as pigs, indicate that the are based on the same specification; and for ecclesiastical buildings through the gauge and head lap have not been to feel confident that the job will be Joint Grant Scheme for Churches and properly adjusted at changes of slate undertaken to a high standard. other Places of Worship which is length (Fig 15). It is sometimes possible administered by English Heritage. The to check the head lap at the verges, Heritage Lottery Fund also provides provided that the slates are not 8 Costs grants to registered charities, voluntary shouldered. A competent stone slating and public organisations, and to some contractor should be able to provide a 8.1 Life-cycle costs private building owners. detailed method statement that demonstrates understanding and 8.1.1 Stone slates form a high quality, 8.2.2 In some areas, the competence in the historical and technical very durable and beautiful roof covering. Environmentally Sensitive Areas methods of the area. This is a useful They satisfy the aims of sustainability Scheme offers grants to farmers and means of assessing tender submissions. better than any other roof although, in the land managers for the repair of short term, they are also the most traditional buildings, including stone 7.4.2 The local authority’s conservation expensive to purchase and install. slate roofing. officer may be able to provide a list of However, stone slates can prove an specialist roofing contractors in the area. economic choice when account is taken of 8.2.3 For a comprehensive guide to It is essential to remember that it is the the long life cycle of the roof, and the Government, private sector individuals involved in a job, not even longer life cycle of the slates (a high organisations and charitable trusts necessarily the firm, who are important, so proportion of which will be capable of which offer grant aid towards the repair a check should be made to ensure that it reuse when next reslating). Where stone of historic buildings, see is the team whose work has been roofs are properly maintained, total Funds for historic buildings in England and examined (and has proved its reroofing will rarely be required. Wales: this guide is available on the competence) which undertakes the new Architectural Heritage Fund (AHF) contract. Roofing contractors who do not 8.1.2 Stone slates are essentially hand- website, free and regularly updated, at have their own retained slaters but crafted materials and it is difficult to www.ffhb.org.uk

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Appendix 1 Stone slate roofs: checklist for recording construction details Ridges Fixing: bedded or nailed, etc

Pitch

Slating Head lap

Number of courses

Margin for each course

Batten/lath gauges

Side lap: minimum and range

Gallets

Single lap

Shadow slates

Dormers Pitched: mono or duo

Hipped

Cheeks: slated or other

Junction weathering

Abutments Weathering: mortar or metal

Gables Plain or coping

Fixing for coping

Flush, overhang or oversail

Hips Mitred

Covered: stone, tile, lead, etc

Detail: roll top, etc

Valleys Open

Mitred

Chevron

Swept

Laced

Other

Eaves Plain or sprocket

Slating detail

Rafter centres Range or average

Fixing system Battens or laths

Slates fixed to rafters

Pegs or nails

Torching Full, partial or none

Slating felt Laps: head (up slope)

Laps: side (across slope)

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Appendix 2 Stone slate roofs: checklist for recording materials Ridge Material

Dimensions

Shape

Copings Material

Dimensions

Shape

Slates Limestone

Sandstone/Gritstone

Other

Composition

Dimensions: length range x random

Thickness: average or range

Gallet or shadow and material

Battens/laths Split

Sawn

Species

Dimensions

Fixings

Slate fixings Pegs: sawn or split

wood: species

other: bone, etc

dimensions

Nails: hand made

machine made

wire

dimensions

material and composition

Mortar Bedding

Pointing

Flashings and soakers Material

Dimensions

Torching Composition

Slating felt Composition

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Glossary Gallet a small piece of stone slate or and placed so that they provide at least metamorphic slate bedded in lime mortar the minimum side lap over the slates in the Backer narrow slates laid centrally over a at the head of a slate to support the slate course below (Fig 9) wide slate to accommodate the above. Synonym: shale diminishing size and therefore increasing Regularly of diminishing slating: the number of slates in subsequent courses as Gauge the spacing of laths or battens up system whereby each successive margin is work progresses up the roof (Fig 9) the roof slope measured from the top the same size or smaller than those below. edges of successive laths or battens. In It does not mean that there is an equal Batten sawn wooden support for stone slating, the gauge is always variable number of courses of each margin size hanging or nailing stone slates. Synonym: (Fig 9). (Fig 9). lath. NB: in this technical advice note, the word lath is reserved for cleft supports Head the top edge of a stone slate as laid Shadow a thin piece of slate used in the (Fig 7). (Fig 9) Horsham district to improve the weather resistance of a roof when, because of a Bedding use of mortar in spots or fillets Head lap the amount by which a stone shortage of stone slate, the head lap is to prevent stone slates from rocking. In slate vertically overlaps the stone slate in reduced to less than the normal some areas, it is used to improve weather the course next but one below (Fig 9) minimum. Originally the shadow was a tightness. thin piece of Horsham stone but it is now Lath cleft wooden support for hanging normally a metamorphic slate. It was Cussome see Eaves stone slates (Fig 7). Synonym: batten.In always used in conjunction with mortar this technical advice note, the word batten is bedding and pointing. Technically this is Defrassing the removal of insect reserved for sawn supports. an undesirable method; however, it has attacked sapwood from timber. The been in use for about 100 years and material subjected to insect attack is Margin strictly the area, but more appears to work in many cases. termed frass. commonly the length, of the exposed part of the slate Shale see Gallet Delamination splitting of stone slates along the bedding planes, most often seen Mossing use of moss or other vegetable Shoulder (verb) to remove the top at the heads of slates. Delamination material to windproof the joints and gaps corners of a stone slate; (noun) the top should not be confused with surface between stone slates corners of stone slates. Excessive flaking. shouldering can result in a leaking roof. Overburden in quarrying: useless Diminishing the system whereby slates material which overlies a bed of useful Side lap the amount by which a stone are sorted by length and laid with the material slate laterally overlaps the stone slate in longest at the eaves diminishing to the the course below (Fig 9) smallest at the ridge. It is essential that the Perp joint Perpendicular joint: the minimum head lap is maintained when vertical joint between adjacent slates in a Tail the bottom edge of a stone slate as there is a change of slate length between course laid (Fig 9) two courses. This also ensures that each successive margin is the same size as or Pig a course with a larger margin than Tiering see torching smaller than those below (Fig 9). the course(s) below resulting from poor setting out and a failure to maintain Tilt the lift provided to the eaves course Double lap stone slates laid so that each adequate head lap (Fig 15) to ensure that successive courses lie course overlaps the course next but one correctly without gaps at the tail. On the below (Fig 9) Pitch the angle of the rafters to the main areas of the roof slope, the tail of horizontal. The pitch of the stone slates each stone slate rests on two thicknesses Dressing the process of shaping the will be significantly less because they are of stone slate. At the eaves, the first full stone slate and producing the edge detail resting on each other, but this is taken course rests on only one thickness - the using either a chisel edged hammer or a into account by the traditional rafter pitch eaves slate. Essentially, the tilt replaces the bladed tool. Regional differences exist for and lap relationship for the slate and the missing thickness. the edge detail which may be square or locality. bevelled. Synonyms: trimming, fettling Torching lime and hair mortar applied (Yorks, Lancs) Pointing use of mortar to fill the vertical to the undersides of stone slates to render joints and to seal the tail gap of stone them windproof. Synonym: tiering Eaves of stone slates: the short course laid slating. Pointing may show (undesirable) at the eaves under the first full course. or be raked or held back. Often associated half torching/single torching: application of The method of placing and supporting the with bedding. lime and hair mortar between the top eaves stone slates varies regionally. edge of the lath or batten and the Synonyms: under eave(s), cussome Random undersides of the slates (Fig 7i) (Cotswolds) of stone slate: variable length and width of roofing: slates laid with reducing length full torching: application of lime and hair Fixings nails or pegs up the roof slope and the widths selected mortar between the top and bottom edges

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of the laths or battens and the undersides References Hughes, T G, Macdonald, S, and Strange, of the slates (Fig 7ii) P, in Wood, C (ed), 2003 Saving British Standards Institution publications: stone slate roofs: the revival and Unweathered of stone roofing: rock which BS 5250: 1989 Code of practice for enhancement of the stone slate roofing is too deep to have been subjected to control of condensation in buildings industry in the South Pennines, a weathering and consequently has to be BS 5534: 2003 Code of practice for model area of Britain, English Heritage split by mechanical action or frosting after slating and tiling Research Transactions, 9, James and extraction James, London Other publications: Morris, W, 1890 On the external covering Weathering the process by which rocks Brereton, C, 1995 The repair of historic of roofs, SPAB (Society for the are broken down and decomposed by the buildings: advice on principles and Protection of Ancient Buildings), action of external agencies such as wind, methods, 2 edn, English Heritage, London rain, temperature changes, plants and London North Wiltshire District Council, 2001 bacteria. In the development of weathered Cotswold District Council, 2000 Stone tiling, Chippenham. Also stone slates, it is often very thin clay or Cotswold stone slate roofs: technical guide, available at mica beds which are weathered out. Cirencester http://www.northwilts.gov.uk/ Department of the Environment, and follow the planning links Department of National Heritage, 1994 Planning Policy Guidance: Planning and Internet only: the historic environment (PPG 15), Architectural Heritage Fund (website London regularly updated) Funds for historic Derbyshire County Council, 1996 buildings in England and Wales Derbyshire stone slate roofs; guidance for www.ffhb.org.uk owners of historic buildings, Matlock Collyweston Stone Slater’s Trust, 2002 ——, 1996 Derbyshire stone slate roofs; See www.csst.org.uk technical advice and model specification, English Heritage Stone Roof Matlock Working Group A format for regional EH, SRWG and IHBC, in prep Stone stone-slate roofing guides, Roof Conservation: a Best Practice www.stoneroof.org.uk/format.htm Guide (Pending print publication, see www.stoneroof.org.uk/best.html)

Figure 16 Langley Chapel, Shropshire.This Harnage stone slate roof includes both sandstone and calcareous sandstone, which overlie each other in the same quarry. The calcareous sandstone is identifiable by the presence of white lichen.

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Further information Building Conservation and Research Team Environmentally Sensitive Areas Scheme: for English Heritage information contact Planning and local technical advice: 23 Savile Row The Department for Environment, contact London W1S 2ET Food and Rural Affairs Local planning authority, 020 7973 3314 Nobel House Conservation Officer Grants: for Grade I and II* listed buildings 17 Smith Square Bats: contacts contact London SW1P 3JR Local authority, English Heritage 020 7238 6907 Nature Conservation Department 23 Savile Row Regional stone slate types, areas of use, sources English Nature London W1S 2ET and manufacturers: contact Northminster House 020 7973 3000 Stone Roofing Association Peterborough Grants administered by the Heritage Lottery Ceunant PE1 1UA Fund: contact Caernarfon 01733 455000 The Heritage Lottery Fund Gwynedd LL55 4SA Exhibition: English Heritage’s exhibition 7 Holbein Place 01286 650402 The Roofs of England: A celebration of London SW1W 8NR www.stoneroof.org.uk England’s stone slate roofing traditions is 020 7591 6000 available for loan free of charge. The For information on funding (various sources): exhibition provides information on contact stone slate roofing including The Architectural Heritage Fund production, traditional techniques, (AHF) conservation, and practical issues. For Clareville House enquiries regarding the exhibition or to 26–27 Oxenden Street comment on this Technical advice note: London SW1Y 4EL contact 020 7925 0199 www.ffhb.org.uk

Figure 17 The misinterpretation of historic details and their modification with modern materials can cause difficulties.The cement pointing to this Purbeck stone slate roof is unsightly and technically problematic.

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Figure 18 The hip of the roof at Stokesay Castle, Shropshire has been formed with lime mortar, which disguises the introduction of a lead soaker beneath to ensure that the roof is weather-tight.

Figure 19 These small Cotswold stone slates have been laid at a steep pitch to form the huge roof of Great Coxwell Barn, Oxfordshire.

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Sourcing stone slates

• New stone slates rather than second-hand should • New stone slates should match the existing ones as be used. closely as possible in terms of geological type, colour, • Reclaimed slates should be used only on the building texture, size, and thickness. It is important to or group of buildings from which they were removed. recognise the slates particular to your area. Sandstone • Substitute materials, such as artificial slates made of and limestone slates should never be substituted for fibre resin, concrete tiles, ‘reconstituted stone’ and so each other, nor should they be used together. on, are inappropriate alternatives to real stone slates • Stone slates which have been sawn to thickness, rather and are not suitable for use on historic buildings. than split, can be technically and aesthetically unacceptable and, if they fail to meet these criteria, should not be used on historic buildings.

English Heritage Regional Offices

LONDON NORTH EAST WEST MIDLANDS 23 Savile Row Bessie Surtees House 112 Colmore Row London W1S 2ET 41–44 Sandhill Birmingham B3 3AG Tel: 020 7973 3000 Newcastle upon Tyne NE1 3JF Tel: 0121 625 6820 Tel: 0191 269 1200 EAST OF ENGLAND YORKSHIRE Brooklands House NORTH WEST 37 Tanner Row 24 Brooklands Avenue Suites 3.3 & 3.4 Canada House York YO1 6WP Cambridge CB1 1DJ 3 Chepstow Street Tel: 01904 601901 Tel: 01223 582700 Manchester M1 5FW Tel: 0161 242 1400 EAST MIDLANDS 44 Derngate SOUTH EAST Northampton NN1 1UH Eastgate Court Tel: 01604 735400 195–205 High Street Guildford GU1 3EH Tel: 01483 252000

English Heritage Customer Services: 0870 333 1181 © English Heritage 2005 (text) E-mail: [email protected] © 2005 Illustrations and photographs copyright of the authors or Website: www.english-heritage.org.uk sources cited in the captions

Authors: Terry Hughes (Slate & Stone Consultants); English Heritage Product Code 51024 Chris Wood, Building Conservation and Research Team, English Heritage; Susan Macdonald (1998 text) Editing: Margaret Wood, English Heritage Publishing Design: Michael McMann Production: Elaine Pooke, English Heritage Creative Services Printer: Hawthornes

Front cover Stone slate roofs are laid to diminishing courses, with the longest slates used at the eaves, diminishing to the ridge.This maximises the use of the available rock from the quarry or local delph.