Non Traditional Housing

Timber-Frame Housing - Part One - Structure

Introduction

Modern timber-frame construction has been developed from North American and Scandinavian methods and bears little resemblance to the traditional, heavy oak-framed buildings of the late Middle Ages. Indeed, most modern timber-frame houses are virtually indistinguishable from their brick and block counterparts. Although it is not difficult to find examples of timber-framing from the first part of this century, it did not become a popular form of construction until the 1960s. By the beginning of the 1980s some 20% of new houses were timber-framed, but a burst of adverse publicity reduced this percentage considerably during the middle of the decade. In the last few years, improved design and more rigorous quality control have helped to reinstate the reputation of timber-housing and it is once again becoming a popular form of construction.

Timber-frame construction offers several potential advantages over more traditional forms of building. These include:

1. Fast construction and therefore quick return on borrowed capital. 2. Less dependence on traditional ‘wet’ skills. 3. Reduced drying-out time due to elimination of wet trades. 4. Reduced dead-load. 5. Improved quality control 6. Less reliance on non-renewable resources

In addition, timber-frame construction can easily achieve very high levels of thermal insulation, resulting in capital savings in heating equipment and lower costs-in-use.

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Principles of construction

Since the 1920s external walls have mostly been in cavity construction, with an inner leaf of blockwork and an outer leaf of brick. The blockwork carries the loads from upper floors and roof, with the external brick leaf providing additional stability and environmental protection. In timber-frame housing, the external (and internal) loadbearing walls are formed in preservative-treated timber and comprise a series of timber studs nailed to top and bottom timber-plates. The outside of the studding is covered with plywood or other suitable sheet material, to stiffen the frame and help prevent deformation caused by wind-loading and the inside is covered with a lining of plasterboard. To keep the frame dry, it can be clad in timber boarding, tiling, or protected by an outer leaf of brickwork. In this country the latter is by far the most common.

After the First World War some houses were available as timber framed buildings. Many of these were in ‘kit’ form and imported from America. The walls are usually 100mm thick, covered internally with fibreboard and externally with asbestos sheeting or similar.

Although the frames can be made up on site, they are usually prefabricated into storey-height panels in lengths of up to 3.60 metres, this being the heaviest panel two men can handle (larger panels can be made for crane erection).

The timber frame system described in this section is known as the Platform Frame (the Balloon frame is more common in North America), and is the most common method presently used in the UK. Specific construction details may vary from manufacturer to manufacturer but the principles remain the same.

Platform frame Balloon frame

Storey height panels. Full height studs framed Upper storey built on in factory or on site. Floor floor platform. is attached to studs.

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Panel design

The studs and plates are made from stress-graded softwood and are usually about 100 x 50mm in cross section; 140 x 50mm studs can be used where extra thermal insulation is required. Where openings need to be incorporated for doors and windows, extra studs are introduced either side of the opening to support the timber lintels, which are of a deeper section to safely carry the loads above.

Timber framing is factory pre-fabricated. This should ensure good quality control. Computer aided design produces drawings and cutting lists for the individual panels. The purpose of the sheeting, nailed to the outer face of the frame is explained below.

2 number 200 x 50mm timbers to form lintel

Cripple studs to support lintel

To prevent the panels from deforming under wind-load (sometimes called racking) the outside face is covered or sheathed in a sheet material, such as bitumen impregnated fibreboard, strand board or plywood. The latter is the more common in the UK, and is available in widths of 1200mm to suit the spacing of the studs. The plywood is usually 8 or 9mm thick and is typically nailed at 150mm centres or so, to the studs and plates. Where vertical joints occur in the sheathing, a small gap of 2-3mm should be left to allow for moisture and thermal expansion.

Sheathing 2.4m high and 1.2m wide.

Racking can occur due to wind loading It is prevented by covering the outer face of the panel with a sheet material.

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Foundations and substructure

It would seem logical to use suspended timber-floor construction for the ground floor, but in practice, most developers use either strip foundations with a ground-bearing slab or piled foundations, possibly with precast ground beams.

When the substructure is complete, a preservative treated sole-plate is normally fixed on top of the DPC and this forms the base for the wall panels (some manufacturers dispense with the sole-plate and fix the panels direct to the substructure). The sole-plate provides a level, square and accurate surface on which to fix the wall panels, and ensures that the DPC is not displaced during erection of the shell.

The plate can be fixed in position by either expanding bolts or shot-fired pins, which pass through the DPC and penetrate the blockwork or concrete below. The latter method is preferable for two reasons. Firstly, the nail driven through the DPC creates a tight collar which prevents damp penetration and, secondly, the use of nails does not require trimming of the wall panels where they fit over the heads of the bolts. Although both methods described require puncturing the DPC, there is no evidence to show that this has caused significant problems. Nevertheless, in recent years, a third method of fixing the sole-plate has been used and this involves the use of galvanised metal strips, which secure the sole- plate to the substructure without having to puncture the DPC (shown below).

The panel is nailed to the sole plate Sole plate fixed with galvanised steel brackets

DPM below slab

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Panel junctions

Where panels join side by side they are secured by nailing, and to ensure correct alignment, an additional piece of timber, known as a head binder, can be nailed to the top-plate. Where panels adjoin at the right angles, the panels can be specially designed, or additional studs can be introduced at the corner. Both these examples are shown in the diagrams below.

Head binder keeps the panels in alignment. If it is omitted the panels can twist and crack the internal finish.

Head binders can often be omitted on the top storey, but only if trusses sit directly over studs.

Panels nailed together

Intermediate floors

The construction of the first floor is very similar to that used in more traditional houses. The joists are usually at 400 or 600mm centres and are secured in position by nailing to the head binder. A header joist is nailed round the perimeter of the floor and, besides providing a firm base for the upper floor panels, prevents fire from entering the floor cavities. Once the joists are in position, the chipboard or plywood deck is laid right across the joists, and this forms the platform for the next lift of wall panels

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The construction at floor level can seem a bit complicated. The left-hand graphic above shows the joists running onto the head binder. A header joist running around the perimeter prevents fire from entering the floor void. Where the panels run parallel to the floor joists the construction is as shown in the above right-hand graphic. The inner header joist sits slightly proud of the head binder to provide a fixing for the edge of the plasterboard ceiling.

Most systems in the UK use the platform method of construction. This means that the upper floor panels sit on the chipboard decking. If this chipboard gets wet for any reason and needs renewing, there are obvious problems.

The outer face of the panel is protected with a breather paper. This is applied in the factory and is designed to protect the panel during construction (from rain). It also provides protection after construction is complete in case any rainwater finds its way across the cavity. In the UK most timber framed houses have an outer cladding of brickwork fixed after completion of the frame and roof.

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When the upper floor panels are in position, another head binder is nailed along the top and then the trussed rafters can be fixed in position. Some forms of construction omit the top head binder, but if this is the case the trussed rafters must coincide with the studs in the wall, to prevent excessive load on the top rails or the panels.

Internal party walls and partitions

The timber stud partition is the most common form of internal space separation used in timber-framed houses, and it is common to find it used for both loadbearing and non-loadbearing walls. On the ground floor, some partitions will be loadbearing and will support the first-floor joists. On the upper floor level, it is unlikely that any of the partitions will be carrying load, due to the almost universal use of trussed rafters which can easily span from wall plate to wall plate, without internal support. Non- loadbearing partitions are likely to be fabricated from slightly smaller studs than their loadbearing counterparts (typically 75 x 50mm). Unlike the external walls the partition panels do not require sheathing (unless extra racking resistance is required) and are usually covered both sides in plasterboard.

Party walls are normally formed using two sets of panels. These walls must provide good sound insulation and fire protection. Means of achieving this are in a later section. Party walls and partitions usually contain raking braces to help prevent racking (distortion) of the building.

Questions

1. What are the advantages of timber framed housing? 2. What is the purpose of the external sheathing? 3. How are the panels protected from rain during construction? 4. Describe typical construction at first floor level 5. What do you understand by the term ‘platform frame’?

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