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Timber frame

overview

‘Timber frame’ can have several meanings:

  • Platform frame where walls are assembled on site into storey height panels and errected and then floored over. This then forms the basis to build the next floor on top.
  • Balloon frame is similar but the walls are double storey. This is now mainly historic because the long timber members for the studs tend not to be available. (It was a North American practice mainly anyway)
  • Structural Insulated Panels (SIPs). Large factory made panels including walls floors and roofs are preinsulated and fixed together on site. A lot of the kit houses in the UK are based on SIPs. More about SIPs
  • Post and Beam, where timber posts and beams are fixed together on site and then the walls are filled in using studwork and sheets or infill. The floors and roof are fairly traditional, using joists (or trusses in the roof). Post and beam can cover anything from the traditional oak timber frame type of building used over many hundreds of years right through to methods like the Walter Segal approach which utilizes standard softwood sections and standard panel sizes. Roundwood framing is also a possibility (as exemplified by the Grand Designs episode showing Ben Law building his cottage).

Oak frame building is a bit of an anomaly.   see more on oak frame

It is hardly a green option to use the traditional oak frame building method in the UK because there is not actually that much oak available. It takes a lot of oak trees to build a house and there has been limited planting of oak over the last century. Furthermore, green oak, which is normally used for such work is notorious for moving (rather than simply shrinking) as it dries in situ. Frames can easily move 50mm out of true (especially when winding and twisting occur) over the first few years and this makes it almost impossible to draught proof joints between the frame and the cladding or internal lining.

Air tightness is becoming one of the major issues for energy conservation. Sealing timber frame houses made of stable kilned softwood is difficult enough. Green oak is almost impossible due to the large number of splits which develop. An interesting example of problems experienced in achieving air tightness is in an AECB article on low energy houses at the Greenoak developments.

A third problem is the cold bridging introduced by the large sections involved. So oak frame is really a nostalgic distraction rather than a green building option.


The first three of these methods offer the advantage of fast erection on site, possibly only a few days. This is useful for getting the building watertight quickly. The post and beam method is slower but more flexible, allowing minor changes to be incorporated as the work proceeds. More of the cost is in site labour and lends itself to self build in the sense of using your own labour.

Of course, with prefabricated kit construction the pricing is more predictable for two reasons. Firstly you can get a fixed price beforehand for both the panels and for their erection. With post and beam you will be relying on estimates calculated by your architect, possibly a quantity surveyor, contractors and sub contractors, etc. Materials might vary in price as you proceed with the work.

design guides needed

One of the problems of designing post and beam houses is that almost all the text books are out of date when it comes to high insulation levels. Usually they show up to 100mm of insulation and the studs tend to act as thermal bridges. Until recently it was assumed that the proportion of timber in a wall was less than 10% when it can often be up around 25% when you include all the noggins, spacers, sole plates etc. When you require something more like 300mm of insulation without thermal bridging (which may well be the case for Passivhaus standard) you have to go for engineered I beams with their thin webs.

Considerable skill needs to go into the detailing of eaves, around openings and at the junction of the ground floor with the wall in order to maintain good insulation. Also there needs to be an approach to air tightness which utilizes membranes or sheet materials which are in simple planar configurations with uncomplicated joints.

In the BRE Green Guide to Specification timber frame rates very highly in most of its forms of construction.

See timber’s green credentials and where to source it

The Forestry Commission do an on-line book called Designing with Timber

There is also a quarterly magazine called Timber & Sustainable Building

There is quite an interesting blog on a self build oak frame project here

walls

With the drive towards super-insulated construction the challenge becomes how to incorporate a large thickness of wall insulation without making the wall too thick (probably around 300mm in the case of a Passivhaus). With timber frame housing this can be achieved quite easily by making almost the entire thickness out of insulation. Ground floors and roofs present less of a problem

The other challenge is to ensure a very high degree of air tightness and this can be done by providing an internal cavity for services so that the wall is never punctured by service runs. The plan detail below is one of the most successful for creating wall panels to fit into a timber frame. It is quite similar to a SIPs panel but has the advantage of having thicker insulation than the kit house suppliers usually provide. The internal lining of OSB 3 has a far lower vapour permeability than the sarking board so a polythene vapour barrier is not necessary. See “Breathing construction

I beam timber wall

The internal lining might be plaster board or a clay based lining board or possibly timber (depending on spread of flame requirements for the building regulations) fixed onto timber spacer strips to allow for service runs. The space can vary from 25mm if it’s for electrics only up to about 40mm if it needs to allow for plumbing.

The OSB boarding (usually 11mm) is a greener alternative to plywood and can be fixed directly to the I beams so that the joints are supported and there is no chance of air leakage. The insulation can be placed in situ before the OSB is fixed. This is easy with mineral fibre insulation but cellulose needs applying slightly damp and it needs to be done by an experienced contractor. The sarking boards need to be vapour permeable and are fixed directly to the I beams, similar to the OSB. Foil faced sarking cuts down on heat loss by radiation. Horizontal softwood battens (probably at 600mm centres) then provide a fixing for the final wall finish in the form of a rain screen.

This gives a total wall thickness of about 440 mm which is not excessive in terms of footprint. Nor does it cause problems with excessively deep windows. There is very little thermal bridging through the webs of the OSB beams (compared with normal timber stud work which does actually increase the heat loss quite considerably.

The I beams may not be needed if the panel is simply infill and not structural. In that case they can be substituted by OSB spacers which would be cheaper and easy for any joiner to run up.

Note that external insulation can be added to an existing wall with a similar detail using OSB, battens and a rain screen.


Check this out if you are interested in a kit house

Passivhaus Homes

Passivhaus Homes with their “PH15 System, a timber framed solution, designed to simplify all the technical aspects of building a Passivhaus, enabling you to create your energy efficient home, one that will be fit for now and the future”.


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)

Note that compartmentation (for instance a party wall), can be achieved using timber frame construction clad in plasterboard or a similar none combustible material.

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)

Resistance to the passage of sound

The degree of sound insulation required within and between houses and flats is covered in Part E of the Approved Documents.

4 comments to Timber frame

  • bobthebuilder

    Hi Mr.Robert Smith
    No, you don’t need to be part of any organisation but you would do well to have a good grip on how the building regulations are getting much stricter around air tightness as we move towards the 2016 Zero Carbon standards. There is a lot of information on this web site about this

  • mr robert smith

    hi could anybody tell me if i need to be part of a organisation to manufavcture a timber frame , i have my own small joinery business with a cnc and i ave just won planning permission to build 3 houses but would like to do them myself
    thanks

  • bobthebuilder

    Hi Z Brown.

    Thanks for your comments. You are quite correct on the shrinkage figures you quote but the 50mm I referred to is not shrinkage but total movement. That is, the movement which can occur through twisting and bending of the timber as it dries. This can cause movement of the structure as a whole and put the frame out of alignment. 50mm might be on the high side but 25mm would certainly be considered normal over two extremities of a large house. This is the sort of movement which could tear or loosen joints on an air tight membrane.

    As we approach 2016 and zero carbon standards, air tightness becomes critical. With timber frame construction generally it becomes very difficult (but not impossible) to ensure air tight joints between posts, beams etc. and the surrounding surfaces. Taping to oak, which will probably split in the future is not realistic for the long term. Using a continuous vapour control layer/air tight membrane usually entails a lot of complicated joints where walls, roofs etc meet. Relying on membranes which get covered by other materials is a problem because if the fan test fails then how do you locate the problem? Enter ‘Allowable Solutions’ at this point.

    Sorry about the ‘about’ page. We are busy rearranging the site menu system and this page is about to go.

  • Z Brown

    Your comments on air tightness and timber frame, both stick frame and heavy structural timber frame, are very bias and quite misleading. Airtightness has almost nothing to do with the building materials used and everything to do with design detailing. You will get more air leakage from a single incorrectly installed uPVC window frame in a masonry built house than you will from a well detailed and professionally designed and build green oak structure or timber frame build.

    Cold bridging is an issue with timber frame if you have exposed members that bridge from inside to out. This is only a real issue in mock Tudor style timber frame. It is an issue with any building material that bridges though. In fact timber has a lower rate of heat loss than other traditional building materials like brick, stone, concrete or steel. If any of these form a cold bridge the rate of heat loss for an equal surface area is significantly higher. The conductivity of hardwood of a desity of 700kg/m3 (green oak is about 950kg/m3 and dries to around 690kg/m3) is 0.18W/mK. Softwood at 500kg/m3 is 0.13W/mK. Brick at 1700kg/m3 is 0.84W/mK. Concrete at 2200/m3 is 1.59W/mK. Steel reinforced concrete at 2400/m3 is 2.50W/mK. These figures are quoted from the Building Construction Hand book by Chudley and Greeno 9th Edition.

    You have taken your 50mm shrinkage from TRADAs Green Oak in Construction book but you have used it in quite a misleading manner. The calculation used to achieve this MAXIMUM settlement assume a total of 1250mm of cross grain timber over a 2 storey traditional jettied build. This equates to the more accurate estimate of 4% across the face of a horizontal member. The huge majority of builds have closer to 500mm of cross grain timber over 2 stories. This would equate to closer to 20mm of vertical settlement. Vertical members have negligible shrinkage along the grain of 0.01%. Assuming 2 storeys of 2300mm this would give 4.6mm of shrinkage. These issues are not unique to timber frame. All buildings go through an initial settlement followed by seasonal settlement. Although this is more pronounced in timber frame builds the design and detailing of the overall build allows or these initial and seasonal changes.

    If you are going to make bold claims about a number of building methods you really should properly research the subject in an appropriate manner and lease with experts in the field. You appear to want the site to come across as an authoritative and experienced source of information but the About page is void of anything bar an email address and sitemap. Is any of the content of the site more than mere opinion?

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