A Beginner’s Guide to Timber Frame
Timber frame is the most widely used construction method in the developed world
and tends to dominate in colder climates. In Scotland more than 75 per cent of
selfbuilders use this method, and around one in four new homes built in England
When people talk in general about timber frame they are usually referring to building using prefabricated panels. From the outside it is impossible to tell a timber-framed house from a masonry one, because the external cladding is unaffected by the choice of internal walls. In a timber-framed house the outer decorative ‘skin’ is finished in the same way as any traditional block house, using bricks, stone, timber cladding or rendered blockwork.
Unlike a block wall, which is built one block at a time, timber panels are lightweight and easy to manufacture, and the system lends itself to being prefabricated off-site. Factory-manufactured timber frames guarantee high levels of accuracy and quality, significantly simplifying on-site construction, which can often take just a few days. Timber frames require accurate, level slabs with only a few millimetres of tolerance, compared to brick and block walls, which can accommodate greater discrepancies by varying the depth of the mortar beds.
During construction the softwood frame is usually made up storey by storey, known as platform framing, and exterior wall panels are nailed together. The plywood bracing layer is covered with a moisture resistant breather membrane, nailed or stapled to the external sheathing, designed to prevent rainwater penetration and to allow moist air to permeate through to the outside. This and the polythene vapour barrier mean that the timber frame inner skin is quickly protected from the weather. All other components are usually added on site once the weatherproof shell has been erected: the void is filled with high performance, non-combustible insulation, and the frame is covered internally by plasterboard.
In most cases, the internal dividing walls of a timber frame house are stud partitions, insulated and finished in plasterboard. As the entire inside of the house is formed by a wooden frame the installation of internal services and insulation is straightforward, with fewer ‘wet’ trades involved.
Roofs are constructed in timber and are supported on the structural timber frame, internal load-bearing timber wall panels and party wall frames. A timber frame design is an engineered solution,which means that a timber engineer will need to prove that the structure is capable of bearing the imposed loads. Extending a timber frame house can be more complex than changing the layout of a block-built house, as any alterations must be carefully calculated to ensure that the overall building remains stable.
Some companies take their off-site fabrication one stage further and install the insulation, glazing and doors at the factory. The walls, floors and sometimes even the roofs of these closed panel systems are then delivered to site ready for erection, resulting in an airtight building envelope. Popular in Scandinavia, this method has been adopted by some suppliers in the UK – ensuring faster build times and less reliance on good weather conditions.
Take this idea one stage further and you have modular building – where an entire house can be finished inside and out, with windows, doors, bathrooms and kitchens all factory built in a number of boxes which are simply assembled on site.
Timber frames can be built on the ground on site – a skilled area of carpentry known as stick building. More commonly in the UK, however, the buyer enters into a contract with a timber frame company which specialises in providing a highly engineered frame in kit form. The UK Timber Frame Association (www.uktfa.com) can provide contact information for more than 300 member companies.
Putting up a timber frame is best handled by the experts – and preferably someone already familiar with the chosen system. Most timber frame suppliers insist that their own people, or a builder recommended by them, erect the frame, and to do otherwise may affect the warranty. Some of the larger manufacturers have teams on their permanent staff, but most employ subcontractors as they are needed.
There are numerous commercial timber frame manufacturers in the UK, and these companies can be loosely divided into those offering modern style panel walls and those who provide post and beam designs – where the weight is borne by heavy timbers and the external walls provide a protective layer of weather-proofing. These traditional post and beam styles are ideal for those who prefer the character of exposed timber in their interiors, although it’s important to ensure that structural posts are positioned for maximum effect and minimum inconvenience.
Designing a timber frame house is very different to designing for masonry construction, and the structural calculations are usually carried out in-house by the manufacturer. Although most companies will have standard designs, which may be adapted to suit, they are increasingly being asked to produce plans for one-off bespoke houses. If you decide not to proceed once the frame has been manufactured then the skeleton of the house would be useless to anyone else – a fact that has both financial and contractual consequences.
Timber frame is often perceived as being a more expensive option than traditional construction, but the difference is usually a matter of a few hundred pounds – money which should be recovered thanks to the time savings. Project costs are reduced not because timber frame is cheap, but because factory fabrication allows for greater predictability and a faster pace.
The package deal is one of the highly seductive elements of buying a timber framed home, and manufacturers can offer anything from the design and assembly of the frame right through to a complete ‘turn- key’ build, and provide various levels of service depending on your location. They can supply doors, windows, staircases and other joinery as part of the overall package, but make sure you study the small print carefully so that you’re certain about what is being provided for your money.
One of the main benefits of building with timber frame is reputedly the fast build speed which, it is claimed, can be as much as a 30 per cent reduction on traditional construction times using standard format blocks. A tip is to make sure that your frame is ordered in plenty of time to avoid losing this advantage, as a 12 week wait is not unusual.
Once the foundations are completed the panels can be assembled on site, the roof constructed, sheathed and battened, and finally the windows and external doors fitted into their openings. A kit can take anything from three days to several weeks to erect, depending on the size, the site and weather conditions.
With the roof left watertight by the erection crew the internal trades can start work as soon as the shell is weathertight and, as the walls are dry-lined, there is no drying out time required for wet plaster prior to decorating.
As timber frames are largely hollow it is relatively easy to insulate them to a high standard while maintaining a clear cavity between the frame and the external walling material. A timber frame house, by its very nature, has less mass than a pure masonry home and will therefore heat up and cool down more quickly – making it prone to fluctuations in temperature. Heat retention will therefore be a major consideration. Timber is a naturally insulating material, however, and timber trade technology has developed so that properties can be designed which require no space heating systems at all – completely eliminating heating bills.
An increasing number of selfbuilders want to build very low energy houses, in advance of current building regulations. Masonry block builders are usually limited to building with wider cavities, while timber framers can specify thicker wall sections which accommodate a thicker layer of insulation between the studs.
Whereas 90mm timber frames were once the industry standard, and the 140mm stud option was deemed a luxury, selfbuilders are now choosing the 140mm option, which is capable of accommodating Scandinavian specifications of insulation. Reflective vapour control layers and airtight membranes have also been introduced which further enhance thermal performance and increase the overall U-value.
Thicker walls are never particularly desirable, as they take up valuable living space and cost more to construct, but the external wall thickness of a timber frame clad in bricks can be kept below 300mm while still maintaining a cavity and delivering U-values of 0.20 W/m2K or less.
The heavier and denser an object is the more sound it will absorb, and one advantage of a masonry-built house used to be the ability to construct solid pre-cast concrete upper floors, which reduce the passage of sound between floors. Now, however, it’s possible to have a poured screed (www.screedflo.co.uk) pumped onto timber floors which is also ideal for carrying underfloor heating pipes.
Good detailing in timber frame houses can also reduce the impact of airborne sound transmission, and there are numerous acoustic insulations on the market, as well as heavier wall linings, which will give dramatic results, and mean that new timber frame homes are as quiet to live in as block houses – although the additional cost of materials should be taken into consideration when making your budget calculations.
Timber is a renewable resource which is non-toxic and completely organic. The UK Timber Frame Association believes that no alternative building material offers the combination of sustainability, renewable, natural, recyclable and low embodied energy.
Benefits to the environment include the fact that wood is effectively a carbon neutral material – even allowing for transport – as growing trees absorb carbon dioxide and produce oxygen. The carbon is stored for the life of the tree and the building and, at the end of its life, the wood can be recycled into new products or burned for energy as a substitute fossil fuel.
Converting timber into a usable building material takes far less energy and generates fewer greenhouse gases than any other mainstream alternative, including steel and concrete. Energy efficient, well insulated timber frame homes built using CFC-free materials are an ideal replacement for old housing stock.