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Designing with timber

timber house in the Field of Dreams, Findhorn Foundation

timber house in the Field of Dreams, Findhorn Foundation

designing with timber

Are the British inherently nervous of timber buildings? Does this go back to the Great Fire of London? Is this still valid considering how many excellent timber buildings there are in mainland Europe?

Timber can be an extremely adaptable material in terms of house design although the UK has lagged behind most of northern Europe and North America in this respect. If you want to be really creative with timber design then it is best to either

  • design it yourself and hunt around for an engineer who will do the necessary calculations, or
  • find an architect who has good experience in working with timber

advantages and drawbacks

Timber can provide some of the greenest solutions to house building for several reasons

  • timber is inherently low in embodied energy. This is especially true if the timber can be sourced locally.
  • it lends itself well to high insulation values because of the possibility of fully filling the walls floors etc. with insulation
  • it locks up carbon for the lifetime of the building
  • it has very few associated pollutants (providing certain wood preservatives are avoided)
  • it can be very enduring. Some of the oldest buildings in the world are timber
  • at the end of its life it can usually be reused or burnt
  • it plays well with the idea of Lifetime Homes because of the ease of making modifications to the structure

There are some other, not specifically green aspects which favour timber

  • there can be a good deal of freedom in design without resorting to steel structural members
  • it is possible to get a frame up with a roof on very rapidly, so providing dry work space beneath
  • it is possible in some cases to incorporate a timber scaffold system as part of the structure and remove it later
  • many of the skills needed for working with timber are easy to acquire

Then there are some potential drawbacks to using timber as a main form of construction:

air tightness

Probably the biggest challenge is to make a timber building airtight. Although this can be relatively easy with SIPs it can be very difficult with traditional post and beam. With the increasing legislation around air tightness, particularly when reaching levels such as demanded by the Passivhaus standard, some experts have serious doubts about whether timber frame housing (of the post and beamtype) can reliably reach the standards required.

see more on air tightness

it is difficult to design timber frame with a continuous air tight barrier. With masonry construction, plaster can form a continuous air tight layer (although inter-floor areas need careful treatment) and this layer can go round corners and be made to join up without a break. With timber the problem arises of how to seal the joints where studs, joists and beams meet up. Particularly at a 3 way junction such as where an internal wall meets an external wall at floor or ceiling level there is a strong chance that air will infiltrate through gaps at the joints. The cold air may then simply enter the house or it may move through the wall and exit through a similar gap, causing a cooling fin.

Although each such gap may only be a mm. or so (often caused by studs not being completely straight or smooth) they soon add up when there are lots of them. For instance half a dozen leaky vertical timber studs which had a 1 mm. gap at each side would be equivalent to a cat flap being left open all the time. Considering that a house might have hundreds of stud, beam and joist junctions, the possibility of not achieving good air tightness is considerable!

One way to tackle this is to use compressive strips wherever a sheet liner meets the edge timber, top bottom and sides. Another is to use a flexible air tight membrane such as a vapour barrier around the whole of the internal surface of the building. In this case, taping the joints of the membrane will probably not be reliable in the long term and they will need overlapping and trapping by nailing wooden lats over the joints. It also takes a good deal of care to remember to insert the membrane before the internal studs go up.

Timber tends to move over its lifetime and this may introduce leaks if joints do not have flexible seals.


sound proofing

Good sound proofing between rooms can be a challenge. See part E of the Building regulations. The use of timber based internal lining boards and floating floors can be an advantage here.

vermin

rodent damage

This 15mm push-fit plastic cold water plumbing pipe shows the teeth marks of a rat which chewed through it causing a leak in the wall. A way to avoid this is to sheathe the pipe in plasterer’s lath at the point where it comes through the wall.

Timber buildings are inherently more likely to allow vermin to enter. Masonry buildings are designed to prevent this. With timber there is the possibility of long ‘rat runs’ which can mean that one small point of entry can allow vermin access to large parts of the building because of continuous cavities. This should be designed against using steel mesh such as plasterer’s lath.

poor timber culture

Because of the UK’s very limited historical use of timber housing (compared with say Scandinavia or North America), it can be difficult to get information on timber design. Most engineers can do large timber structures, some can do stuff like marine structures (piers etc) but few specialise in housing. North American web sites are heaving with information but the problem is that most of it is not directly relevant to the UK. Many traditional builders have virtually no experience of high quality timber design.


kit houses

The easiest route to take is to buy one of the numerous timber frame kits on the market and assemble it yourself or have a contractor do the work but you are then rather limited by the standard designs on offer. This may be especially true with respect to the external cladding. See more on Timber Frame & SIPs. Usually the kit manufacturers assume that you will want to clad the external walls with bricks or stone etc. to give it some sort of ‘traditional’ appearance. In fact this is counterproductive in ecological terms for several reasons

  • more materials are used than necessary so embodied energy is wasted. This is especially true for the extra foundations required
  • design flexibility is more limited
  • natural lighting and solar gain is reduced because of excessive wall thickness
  • future alterations to the house become more difficult

Possibly the best green approach is to use a SIPs kit house and then clad it with timber and/or a thin cladding such as cement/fibre sheets. See rain screens. In this case you would need to find a manufacturer who was willing to modify the design to your requirements and a contractor who would lay a base and services and then do the external cladding. The manufacturer would probably have their own contractor to do the actual erection of the panels. This could achieve several important advantages

  • the house could be almost all timber of low embodied energy (although delivery distance from the manufacturer is an issue)
  • almost full wall thickness insulation is possible using SIPs so Passivhaus standard of about 300mm can be achieved
  • SIPs panels are relatively easy to seal together for air tightness
  • errection time to ‘wind and weathertight’ stage can be achieved in a few days
  • there can be good design flexibility with SIPs

New on the market is Passivhaus Homes who do a kit house to Passivhaus Standard.

Segal

Another option is the Walter Segal method for self build which offers a very simple method of modular construction based on standard material sizes. The actual constructional skills are very easy to acquire as there are no wet trades. There is some Sketchup 3D modelling software for designing small (up to 30m² floor area) garden sheds, tiny houses etc. using the Segal self build method.

TRADA

TRADA  is a manufacturers’ and suppliers’ association which has, in the past been quite good on timber research and putting out technical information. They also run seminars on a variety of related subjects. They tend to serve the larger building companies but do have the odd course which may be of use to the self builder. However they seem to be becoming somewhat moribund in their thinking, especially now that standards of insulation and air tightness are being tightened up.  They have been a bit slow in catching up with the technical side of domestic design as the improvements in Part L Building Regulations are tightened up and Passivhaus design standards seem to have largely eluded them.

building regulations

Structure

The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

some inspiration

See the inspiring Scottish web site, Designing with Timber and for something a little more esoteric take a look at Oliver Lowenstein’s site called Annular

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