Think Quick & Think Quality
One of the things that contractors and builders must not lose sight of in the bid for speed is quality. Lewis Taylor, Timber Frame Consultant at BM TRADA, discusses the importance of build quality, detailing and sole plates.
With the increasing popularity of timber frame, thanks to its speed of build and sustainability credentials, we’re seeing a rise in calls about build quality. With this in mind, we have been working closely with architects, housebuilders and housing associations, providing training courses, reviewing detail drawings and carrying out regular on-site inspections to minimise defects and ensure quality.
Many developers with a tradition of building in masonry construction are now turning to timber frame to achieve better energy performance and enjoy the benefits of speed of construction.
We often work with main contractors to help them ensure a good standard of construction is achieved and to help them minimise call backs and costs associated with possible defects. This saves them money in the long run, and delivers buildings that offer better performance and happier occupiers who enjoy living in them. A large portion of our work is for housing associations, hotel chains, care home providers and other building owners who have a longterm vested interest in the performance and durability of their buildings. As they are normally responsible for ongoing maintenance and repair works, it is important they are satisfied that the design and construction meets their requirements and that build quality has been maintained.
There are a number of key areas – such as structural design, differential movement and thermal performance – which require careful design and construction to ensure quality and compliance with regulations.
The detailing of timber frame buildings evolves as new products come to market, regulations change, or building forms change to suit architectural style, so keeping up to speed with detailing is of great importance. Also, what an architect draws is not always what a builder constructs. For example, a change of floor joist type from a typical
bottom bearing joist to a top hung metal web joist can have implications for plasterboard edge support and fire resistance, vapour control lapping and sealing, routing of services, structural design and manufacture of the timber frame walls, air barrier detailing around floor zones and locating cavity barriers – not to mention the order of work for tasks on site.
While adding in extra checks along the way may seem prohibitive in terms of speed of build – especially when time is key – a focus on build quality can in fact further speed up the process. With an expert pair of eyes overlooking the build, corrective action can be taken immediately if there are any concerns, which saves the time of needing to return to an area and fix it at a later date. This in turn leads to savings, as it not only limits the additional costs involved in going back to the project to correct
any issues, but it’s also possible to identify efficient construction methods and order of work to reduce costs along the way. Furthermore, by focusing on the construction detailing, performance and compliance can also be improved. Inspections from the outset allow us to suggest best practice details for performance, so that the finished
building can be designed to exceed current regulations, while materials and building systems can be assessed against standards at every stage of the process.
One of the most important tasks in constructing timber frame buildings is getting off the ground on a level foundation/sole plate. Dimensional tolerances that ground workers are used to in other forms of construction are often too great for timber frame, so it is important for all parties involved to understand the tolerances required.
Sole plate fixings serve two purposes:
• To locate the plates accurately
during construction so they can be
used as an accurate jig for setting
out the superstructure
• To transfer vertical and horizontal
loads to the foundations once the
building is completed.
Timber frame manufacturers and their erection crews are all too aware of the problems of slab tolerance and are likely to have, within their contract, clauses ensuring responsibility rests with the main contractor. Only when a slab is within recommended tolerances or when a structural engineer gives additional guidance, should construction start on the timber frame. Any areas that are out of level must be identified and corrected, normally using shims. They should be installed below the DPC underneath every stud in the wall panels and be the same crosssectional area as the studs. In effect, they are mini studs transferring the load to the foundations.
For effective point load transfer, the cross-sectional area of the shims should match that of the point load, but some timber frame erectors will only install sufficient shims to level the sole plate, not to support the building. It is sensible to clarify responsibility for undertaking this work to prevent later problems with movement of the frame and sole plate distortion. Adequate support must be installed, not ignored. Sole plates must also be correctly located to the timber frame manufacturer’s drawing. If a
foundation is not within recommended tolerances, an overhanging sole plate reduces the area available for transferring the load of the building. If set back, there remains a small ledge that can collect water, mortar droppings and other debris and may lead to a blocked cavity, as well as opening up a route for moisture to penetrate structural timber elements if it is not addressed.
With speed of build back in the frame, timber is of course the most obvious and best solution for builders, but to make the build as efficient as it can be, the devil is most definitely in the detail. A focus on quality throughout the build process not only leads to more efficient construction and a better end product, but also happier clients.