Timber Frame

Timber frame is an incredibly flexible system, and almost any design can be built using timber studs and panels. It is very quick to build on site and is ideal for self building because it provides a weathered shell from a specialist supplier, allowing easier project management thereafter. Timber is also a renewable resource; a plus for the eco-conscious self builder and with the appropriate insulation between the studs and in the panels, the energy performance of your finished home will be very high. A growing number of frame suppliers are supplying engineered structural beams, which enable a greater span to be achieved, offering better flexibility for ground-floor room layouts. They are also easier for service installations, therefore ideal for the self builder. Cavity walls were introduced about 100 years ago to control the ingress of damp through solid external walls. You should keep the two cavity skins separate, with a clear gap between - this is very important to stop moisture penetration through the outer skin. The external cladding on the outside, which can be any material including masonry, timber or render, is only for weathering and decorative purposes, rather than taking any of the loading from the roof; it is the timber structure which takes the weight of the roof trusses and the roof covering.

Oak Frame

This is another variation on timber frame, using 'oak post and beam' construction, with centuries-old jointing techniques. The oak can be left on display which gives final house an authentic period look. Oak requires very little maintenance, is very strong and can last for hundreds of years. As one of the most natural of resources, its carbon footprint is very low when all elements are being considered.

The in-fill panels between the posts must meet stringent energy regulations and cope with the shrinkage of the oak. Green oak construction uses unseasoned oak with a moisture content of 20-25%. As it dries out, it shrinks across the grain. This causes gaps around the in-fill panel which, with many systems, will at first need to be filled regularly. Waterproof expanding foam and a sliding joint should be incorporated to keep the effects of movement to a minimum. Build costs will be £1,200 - £1,500 per sqm.

Steel Frame

Another option is steel frame construction, where the timber studs are replaced with metal. As with timber frame, self builders have a complete choice of external facing material, provided that it is properly connected to the steel studs which take the entire load from the roof. Steel frame is often considered where large clear spans are required. Steel frame construction is not particularly common in the UK. A number of manufacturers have attempted to supply steel systems but, due to the exacting standards, they are not always perceived to be as flexible as other forms of construction. The steel framework is erected on site, very much as large industrial units are built. You are required to wrap the whole exterior of the frame with a rigid insulation layer, to prevent heat loss through cold bridging of each stud to the cavity. Additional insulation will then be placed between the studs.

High energy efficiency is possible with steel frame, and good acoustic standards can be achieved. Steel frame is still not common in the UK; its use has increased significantly in the USA over the last five years, and is expected to increase over here too. Few companies will deal with one-off houses, therefore prices are difficult to gauge, however you should expect costs to be in the order £1,000 - £1,200 per m².

Insulated Concrete Formwork (ICF)

This is an ingenious system where the structure of the house is built out of hollow polystyrene blocks known as insulated concrete formwork (ICF), which are then filled with concrete. Externally there a number of choices including a brick skin, an external render directly onto the polystyrene, or a reconstituted stone slip system glued to the face of the polystyrene. This system is ideal for those who want to have a concrete block and beam system as their intermediate floor, lined with a battening system or appropriate clips and a plasterboard ceiling. This type of solid floor is particularly suited to underfloor heating systems and also offers very good acoustic insulation, however service routes within the intermediate floor will need careful planning. The assembly of the hollow polystyrene blocks is reasonably quick; once assembled, the internal cavity of the blocks is filled with ready-mix concrete which provides strength. The result is a concrete external wall, wrapped on both sides with a good layer of polystyrene insulation. Care must be taken not to leave any voids in the concrete, particularly around structural openings; to this end the ready-mix concrete is vibrated within the cavity using a compressed air poker. It is equally important not to get too ambitious with the pour - too much concrete will cause the blocks to burst! There are some important techniques involved in this method, including the metal ties which hold the polystyrene structure together before the concrete is poured in, and some external bracing which can be fundamental to the erection. Lintels over openings are formed in the same way, with steel rods over openings to an engineer's design. On the inside, there is no need for additional insulation, so plasterboard can be adhered directly to the polystyrene.

In many respects insulated formwork is ideal for the self builder because the structure and wall layout is relatively quick, easy and - being polystyrene - not difficult to lift. It is not yet considered a mainstream method (it is probably used by only 1-2% of self builders), but it has certainly become more popular, with more suppliers coming to the market in recent years. A costing of £1,000 - £1,200 per m² would be a fair estimate, but this could be reduced by carrying out a lot of the build yourself - the system is fairly straightforward so this should be within most self builders' capabilities. ICF also offers a relatively short build time and currently meets all building regulations.

Brick & Block

The brick outer skin is indeed the façade of a very 'traditional construction', method, commonly referred to as 'brick and block'. The external wall has a blockwork inner skin which is connected to the external brickwork using wall ties. Lightweight blocks are an efficient way of building, being easy to work with so that a relatively large volume of work can be undertaken in a short period of time. Blocks are manufactured to take different loadings it's important to understand that higher strength blocks are used in certain scenarios. The blocks are light because of the air trapped within them, which keeps the warmth inside the building making them good insulators. A normal size of single block won't meet energy regulations on its own though, so additional insulation is usually inserted into the cavity or packed on the inside of the blockwork by using a composite thermal board or vertical studs with insulation between. Packing out on the inside in this way is also very helpful for electrical and plumbing service installations. The blockwork inner skin is the load-bearing element with this system, and although the brickwork is generally built at the same time, to a large degree the brickwork is weatherproofing again; it is tied to the blockwork usually with stainless steel ties to add structural rigidity to the masonry panels. Bad weather in winter can affect your programme as construction does have to stop in adverse conditions such as heavy rain and frost.

Brick & block is the most popular choice for self builders in the south of the UK, where weather delays are less likely. Most people perceive a brick & block house to be more 'solid' than timber frame, and are therefore prepared to pay a premium. It is much more difficult to reach the U-values and air-tightness required under current regulations and these requirements become more stringent all the time. The 'normal' insulating block is 100mm thick, but to get improved levels of insulation you may want to consider using a 150mm block, and possibly upgrade the cavity insulation - of course, all of this adds to the cost. The weather is probably the biggest factor in the budget management with masonry construction with both heavy rain and frost being a significant risk to the integrity & appearance of the masonry; time delays will add to costs such as scaffolding & plant hire, protection of recently built walls, storage and so on. You also introduce far more water into the structure which will increase drying out time once the house is weathertight. Generally, you should budget around £900 - £1,100 per sqm.

Thin-joint system

Thin-joint system is a variation on a traditional brick and block construction. The blocks used are aerated blocks offering good thermal qualities and are laid using a cement-based adhesive rather than sand and cement, so the joint between blocks is very thin. This enables very rapid construction, and more blockwork can be undertaken per day. Because there is more blockwork and less jointing material it also dries very quickly. The thin joints also offer far better thermal qualities than a traditional 10mm mortar bed. The internal skin of blockwork comes in a variety of thicknesses and could be improved thermally by adding insulation, either in the cavity or on the inside, with composite thermal plasterboard. With certain thicknesses however, no additional insulation is necessary and these blocks can also be used for solid wall construction using waterproof renders on the outside. This method will cost in the region of £1,000-£1,200 sqm.

Pre-cast concrete panels

Reinforced pre-cast concrete panel construction system provides an exceptionally strong building, would be good for higher rise construction and are also ideal for basement construction as long as a suitable waterproofing, (tanking) system is used. The intermediate floor and the panels are fixed with special metal panel pins and dowels and bolted where necessary. Dense concrete like this is a good conductor of heat so the system on its own has practically no insulation qualities, therefore you have to insulate in the cavity on one side before the blockwork and render is fixed, and on the other side only on the interior, because the render is fixed directly to the concrete panel. When insulating in the cavity you effectively trap the concrete on the inside of the thermal layer and in these situations the concrete itself will become highly efficient for thermal mass storage.

Pre-cast concrete is relatively uncommon for the construction of a one-off house, (although ideally suited for basements) - the system lends itself far better to blocks of flats or a terrace of houses on larger site - heavy plant and plenty of space is needed to put the panels in place and the panels are made in a formwork mould. For an individual house, you should budget £1,200 - £1,500 per m², but be prepared to be quoted higher.

Structural Insulated Panel System (SIPS)

SIPS uses a traditional cavity wall construction but with the inner skin being a structurally insulated panel (SIP), the external facing is largely there for weathering and decorative purposes. This is a similar system to the more common 'open panel' timber frame system, but in this instance, a fully insulated and completed frame, often pre-fitted with windows and doors, will be manufactured and delivered to site. The panels themselves are more expensive to make, but savings are introduced later in the build as you are unlikely to need further insulation in the walls to meet current standards. The panels carry the building's structural load, similar to other timber construction methods. The panels are faced on both sides with orientated strand board (OSB), and the two sides are factory-bonded to a rigid insulation, to make a wafer-like construction. The principle is that very large panels can be constructed, using much less conventional timber, as studs are only required at junctions. The larger panels are craned into position so good access to site is essential, but this does make for a very short on-site construction time. A number of the manufacturers will also fix timber battens on the inside face of the SIPS, to ease the installation of pipe work and cabling; this also provides good fixings for the plasterboard. Internal walls can be open studwork if insulation is not an issue.

Using SIPS is probably one of the quickest methods available for building and is very thermally efficient. However, it is not so easy to adjust on-site if changes do need to be made. Using SIPS will give you an overall costing of £1,000 - £1,200 per sq metre. Less than 5% of self builders currently use this system, although there has been a growing interest in recent years.

Summary

Typical build costs will typically be around £900 - £1,000 per sqm for the more common systems, and each system has its own merits and pitfalls. Rather than selecting a system purely on a cost basis, it is perhaps more relevant to consider the following points when deciding on your method of construction:

• The ease with which you can achieve both current & proposed construction standards, especially on insulation & air tightness.
• The amount of work you realistically undertake yourself, as clearly this will save you money on labour cots, thus making your budget go further.
• The speed you want/need the build to progress, especially if you are in rented accommodation
• Whether you are going to 'project manage' yourself or employ a contractor and/or project manager - again this will not only save you money in your overall budget, but will also depend on your experience and knowledge of the system
• The time of year you are starting your build - the winter is not a good time to achieve high quality masonry work.

Notes on costings:
Providing average costings can be tricky - issues such as ground conditions, access to the site, location and proximity of services are a huge factor in the build budget, but generally will be similar for every system. Therefore whilst endeavouring to offer our guidance on pricing for each system, we have assumed that all of these issues are reasonable to deal with. Budget management is all about design and specification, so they vary from project to project. We have based our figures on a 'medium specification' 2 storey, 4 bedroom, 2 bathroom rectangular house with plenty of intermediate supporting walls. Roughly 2,000 sq ft internal floor area, external cladding medium range brickwork with concrete roof tiles on fink trusses. Nothing untoward in terms of access, drainage, ground conditions or room sizes, including mid-range kitchen & sanitaryware. No allowance for basements or use of the roof space for anything other than storage, ready for carpets to be laid & curtains to be hung.

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