Flat Plastering in Traditional Buildings
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ININFOFORMMATATIOION FOOR HIH STTORRICC BUIU LDDINING OWOWNEERSS Buildings Traditional in Plastering Flat Introduction Plaster has a long history of use as a fi nishing material for internal walls in Scotland. Traditional plasterwork is usually either earth or lime based. Th e two main methods of using lime plaster as an internal fi nish are either to apply it directly onto masonry, commonly referred to as plaster on the hard, or onto a backing of timber laths. Fig. 1 Quick lime being slaked using traditional methods. Th is INFORM provides information on the material make-up and preparation methods of lime plaster, as well as how to apply it. It also considers how to repair plaster, including re- plastering and patch repairs works. Materials for traditional plasterwork Lime Th e main component of the majority of plaster in traditional buildings is lime. Traditionally, most lime used for internal plastering would have been Fig. 2 Matured lime putty for use in traditional non-hydraulic or fat lime. Th is was plastering can be bought from suppliers. formed by burning limestone in a kiln to drive off its carbon dioxide and water slaked and matured putty (Fig. 2). content to leave a white powder known Matured lime for plastering can also as quicklime. Th is was thenslaked be bought from specialist suppliers (immersed in water) to produce lime of lime and traditional building putty (Fig. 1) and left in a pit or tank materials. It is important to note the to mature for around three months diff erence between hydrated lime, lime before use. which sets in the air, and hydraulic lime which sets in the presence of Today lime for plastering mainly comes moisture and would rarely be used for in the form of bagged hydrated plastering traditional buildings. lime. Best practice would still be to mature this in water for around three Gypsum months prior to use in order to bring Another material found in traditional about the benefi ts of traditionally plastering is gypsum (calcium sulphate, or Plaster of Paris) which became popular plaster to provide information on the from the late 18th century. Like lime, it type and grading of aggregate, binder to is burnt to form a powder which when aggregate ratios, the presence of gypsum immersed in water creates a gypsum putty. gauging and other additions. Sand for However, this creates a putty which is plastering should match that used in harder and sets more rapidly than lime original work. Generally sand with a grain putty. Most available modern plasters are size of 1-6mm would have been used for gypsum based. Lime and gypsum are, the fi rst coat, 1-3mm grain size for the however, materials with diff erent properties second and 0.5-1.5mm for the fi nishing and where repairs are being carried out to coat. Sand used in plaster mixes should traditional fl at lime plasterwork the use be sharp, well graded (having a good of gypsum is likely to be inappropriate. mixture of grain sizes) and should be well Lime plasters were sometimes gauged with washed to remove contaminants. In some gypsum, where a small amount of gypsum cases lime plaster, especially the fi rst coat, was added to the mix, and where this often contained hair as a reinforcement is found to be the case its addition may material. Where this is being added it be appropriate in repair work although should always be broken up and well this should not be carried out simply as distributed throughout a plaster mix. Hair a matter of course. Material used for the reinforcement should never be added to a repair of fl at plaster work should match mix prior to maturing, as this will degrade that found used originally on a “like for over time and will be unable to provide the like” basis. strength required. Aggregates The decay of plasterwork To form lime plaster, lime putty is mixed Th e main cause of decay in plasterwork with sand aggregate, to form either coarse is water ingress which can be caused by stuff or fi ne stuff , coarse stuff being formed defective roofs or plumbing and excess of aggregate with a larger grain size than internal moisture (Fig. 3). Water penetration fi ne stuff . Where possible repairs should be into plasterwork will cause staining and like for like, ideally analysing the original ultimately will lead to the plaster becoming Fig. 3 Water ingress and excess moisture is the most common cause of damage and decay to plasterwork. detached from its background and hard, without laths, only two or even collapsing. Where water ingress is the a single coat may have been used. Th is cause of the deterioration of plasterwork was dictated by the standard of fi nishing the building defect which has led to the required. In repair work the number and problem should be rectifi ed before any thickness of coats used should match that repair work takes place. Th e decay of used originally. When plastering, each plasterwork can also be caused by problems coat of plaster should be allowed to dry with the background to which it is applied until fi rm to the touch but not completely such as timber decay in laths or structural dried prior to the next coat being applied. movement within masonry. Vibration Th is will normally take between 1 to 3 can also cause mechanical damage to weeks depending on temperature and plasterwork. Where these causes are present humidity. the fault leading to problems within the plasterwork should be rectifi ed before the Scratch coat plaster itself is repaired. Th e fi rst coat of plaster is known as the Re-plastering work scratch or base coat (Fig. 4). Th e mix Wherever possible, repair work should for a scratch coat is usually one part of be like for like in material properties and lime putty to between 1 to 3 of coarse, methods as these are likely to provide a well graded sharp sand although this repair which is durable and compatible can vary depending on the properties with surviving original work. Most of the sand and lime being used. Th e traditional plasterwork was applied scratch coat will often have hair added in three coats. In some cases where to the mix to give added strength. masonry was plastered directly on the Masonry or lath backgrounds should be Fig. 4 Scratch coat of plaster applied on the hard directly to a rubble masonry wall. (Photo credit: Eden Hot Lime Mortars) dampened down prior to application. reduce the suction of moisture from A scratch coat will normally be one to the other. Once the fi nishing around 9-10mm thick. Whilst still coat is applied a fl oat is used to scour wet the plaster should be scratched the surface to give a smooth fi nish. using a lath scratcher or other wood Th is fi nal coat of plaster can be worked toothed tool in diagonal lines to form in a number of ways to achieve a a diamond pattern. desired fi nish. A trowel can be used to give a fi ne closed fi nish to the plaster. Floating coat Wooden or sponge fl oats will give a Th e second coat of plaster is referred more open and textured fi nish. to as the fl oating or straightening coat. Th e mix is again likely to be 1 Plas ter on the hard parts of lime putty to 2-3 of coarse, When repairing lime plaster which well graded sharp sand. A thickness is applied directly to masonry all of around 9-10mm is again used. A existing, loose materials such as old long fl oating rule or straight edge is plaster or paint should be cleared used to obtain a fl at level surface on away. Masonry joints should be raked the plaster whilst still wet. When the out only enough to give a key for fl oating coat has begun to harden it is plaster to adhere to the wall. Masonry dampened and rubbed or “scoured” should always be dampened prior to with a wooden fl oat to reduce the application of plaster to avoid too likelihood of shrinkage and produce rapid drying which can lead to cracks an open grained fi nish to help the top in plaster. Where a masonry surface coat adhere. is uneven “dubbing out” may be required. Th is involves indentations in Finishing coat the wall being fi lled with plaster of the Th e fi nal coat is the fi nishing or top same composition as the scratch coat coat. It is generally thinner than the to give a more even surface. scratch and fl oating coats, between 2-5mm and uses a fi ner aggregate. The Where plaster is being applied to mix can vary depending on the fi nish laths (Figs. 5 and 6) it should be required. More lime will give a softer applied diagonally across the laths fi nish but one which can be polished with suffi cient pressure to force to a smoother surface. More sand will it through the gaps allowing the give a harder fi nish. Often a simple one hardened plaster behind to form part lime to one part sand ratio will a “plug” or “rivet” to anchor the be employed. Th e fi nishing coat can plasterwork. Th ere is no reason why be applied in two or three thin layers new plaster cannot be applied to in some instances to help achieve a existing laths as long as these are in very fi ne surface fi nish.