Getting timber frames right

By Dineo Phoshoko

Timber as a building material has a variety of applications, one of them being timber frames. There are limitless options with what can be achieved with timber frames from a design and construction point of view.

When designing buildings, architects often push the boundaries of timber frames – creating ground-breaking designs. “The timber frame is a really versatile means of construction and there are no restrictions on design particular to timber frames, especially on a residential scale,” explains Jacques Cronje, a specialist in timber architecture from Timber Designs.

Echoing Cronje’s view is Peter Bissettt, a timber frame constructor himself, and owner of SA Timber Homes. In Bissett’s view, timber is an extremely strong and lightweight building material – giving designers a myriad of options in bringing their designs to fruition. “Whatever designs can be achieved with the more conventional brick and concrete materials, can usually be achieved or even enhanced with timber,” he says.

Another factor adding to the versatility of timber is the option to use other materials together with timber and combining them to suit the desired end result. Bissett is of the opinion that it is unnecessary to exclude one building material for another. “For example, timber, stone masonry and thatch complement each other very well but timber can also be utilised to enhance a modern look when mixed with aluminium and glass,” says Bissett.

Sound insulation of a timber frame might be a concern, however Bissett asserts that this should not be the case. “A properly built timber home will inhibit the transfer of sound from one room to another.”

Timber frame applications and suitable timber

Cronje explains that timber frames are ideally suited to the scale of residential construction. “Industrial buildings, however, typically have much larger spans, and while these can be achieved using laminated beams, large spans are more cost-effective when built out of steel.” Although large span applications where aesthetics are important have been achieved using timber – for example indoor swimming pools and sports halls – this wouldn’t make sense for industrial applications. This is mainly due to cost.

Bissett further adds that 80 % of the developed world live in timber frame structures. “A large percentage of commercial and industrial structures in other parts of the world are timber structures or a mix of timber and other materials. There are a few timber warehouses in built up areas in South Africa although these are in the minority.” Apart from homes, timber frames are also suitable for warehousing and other large structures although not always as cost-effective as steel. Bissett explains that timber has a known rate of combustion making it safer for fire crews in the event of a fire as it does not collapse as soon as it gets hot. “Timber is also resistant to shock loads, and if designed correctly, can withstand the day-to-day bumps and scrapes in a warehouse.”

SA Pine has been found to be the most suitable type of timber for the construction of timber frames. The timber has unique properties that make it suitable for timber frame construction. “Pine is lightweight, has a very high strength-to-weight ratio, is easy to work with, and if CCA (Copper-Chrome-Arsenate) treated, has a lifespan exceeding most hardwoods,” says Bissett. He adds that external cladding in South Africa, Australia and New Zealand is usually CCA treated Pine. America is different because Cedar is more suitable for the American environment due to its resistance to termite and fungal attack.

Correct standards and common mistakes

All building and construction has to be done according to the correct standards. For timber frame structures in South Africa, the correct standard is SANS 10082. “Every builder should have a copy of SANS 10082, but very few are even aware of its existence except those that belong to the ITC-SA,” Bissett explains. He also distinguishes between the code of practice of timber for the design of timber structures and code of practice for timber structures in general. “Timber frame homes should be designed in accordance with SANS 10163 – the code of practice for the design of timber structures and built in accordance with SANS 10082 – the code of practice for timber structures.”

Building timber frames according to standards removes the margin of error which means quality timber frame structures. According to Cronje, one of the most common mistakes made during the construction of timber frames is insufficient tie-downs. “One needs to keep in mind that timber frame construction is a ‘lightweight’ form of construction, and as such needs to be adequately tied down to the foundation structure to be able to resist high wind loads,” Cronje explains.

Using timber cladding as an example, Cronje elaborates further on what could possibly go wrong if, and when, standards are not properly followed. “From a design point of view there are also factors to consider when using external timber cladding. As it is not classified as a non-flammable material safety distances from SANS 10 400-T apply, so care must be taken to consider appropriate safety distances, as per the SANS tables, from property boundaries when using timber cladding,” says Cronje.

For Bissett, the biggest mistake inexperienced builders make is underestimating the amount of work and required skill set needed to erect a home. As a result, these builders often run out of money before the job is completed. “Mistake number two made by most inexperienced builders is not reading up on the codes of practice for timber structures, which often has disastrous financial, safety and longevity issues for the prospective homeowners,” Bissett adds.

Another common mistake highlighted is builders not taking adequate precautions against water standing adjacent to timber or pooling somewhere in contact with timber. “Coating of the structural timber beneath decks is often not done because it is not easily seen. This is short-sighted as it gets wet and takes longer to dry out because it is in the shade,” Bissett says.

Consequences of incorrect timber frame construction

According to Cronje, a timber frame is the basis for the rest of the house, and as such, errors in level or straightness will become obvious at a later stage once the cladding materials are added.

Bissett points out additional consequences of incorrectly constructed timber frames. “Support poles, if used, are often encased in concrete resulting in a short lifespan as water becomes trapped around the pole.” This situation is commonly associated with farmers’ fencing posts being encased in concrete. If there is inadequate bracing of the foundation poles, the structure will ‘rock’ in strong winds or even collapse. Another error that could cause the structure to collapse is inadequate bracing of the wall frames. This can also cause cracks in the cladding.

Some builders do not include the noggins, which are the horizontal members between the upright studs. The noggins are used in the wall frames and it is essential that they are not omitted in a timber frame structure. “These are critical for the structural integrity of the home as well as to help inhibit the spread of fire once it has penetrated the one side of the wall cladding,” explains Bissett. He adds that inexperienced builders sometimes leave out required firestops because of their lack of knowledge of the building codes. “Correct precautions need to be taken for fire so that the structure complies with the fire regulations for the correct fire rating.”

Consulting an engineer with experience in timber design and construction is critical, especially when changes are made to the structural design. Failure to do so could result in overloading which can cause sagging floors, sticking windows and doors and even a collapse in the structure.

“Another consequence of not adhering to the codes is freezing in winter and boiling in summer due to inadequate insulation in the floor, walls and roof as well as installing the correct glazing where required,” Bissett adds.

Creating awareness and adding value to the industry

For people considering building timber structures, the best way to go about it is ensuring that they make use of builders, architects and engineers who have experience in timber structures. “The best way of doing that is to make sure they are members of the Institute of Timber Construction – SA (ITC-SA),” Bissett explains.

According to Amanda Obbes, ITC-SA general manager, the Institute saw a marked upswing in queries and complaints from the public relating to poor workmanship from non-members in the trade in the first quarter of 2019. “This is a recurring phenomenon and can, for the most part, be attributed to a stodgier economy; tougher times make for an environment in which consumers look to cut costs and many desperate contractors are willing to cut corners to secure work.” By using ITC-SA builder members, both clients and builders stand to benefit because they will be able to save a significant amount of money, and time.

Together with its membership, the ITC-SA monitors each of its members, not only in relation to the training they must do on an annual basis to attain and maintain their professional designations, but in carrying out random inspections for quality assurance at both truss plants and building sites, which have a bearing on the member’s status with the Institute.

The ITC-SA plays a pivotal role in regulating and controlling safe, consistent building standards in the industry by way of its involvement in various committees and boards that inform National Building Regulations. The Institute plays a crucial role in the development of training material for the manufacture of nail-plated timber roof trusses, for the erection of timber roof trusses, and to facilitate training in these disciplines, in compliance with the Skills Development Act.

“In trying economic times, it is increasingly common for consumers to be tempted to save money on the construction of a roof, timber home or deck, but this comes at a much higher price in the long run, with much more at stake than just financial burden,” says Obbes. “Consumers who severely cut costs on building projects will most often have to spend even more money to fix the problems that arise from using sub-standard materials and workmanship. In many cases, the problem cannot be fixed, but rather, must be undone before correct building or installation can begin. This is not to mention the physical threat to life and valuable possessions posed by non-compliant structures,” she adds.

Timber as a building material is gaining traction across the world and South Africa is not too far behind. Research has shown the many benefits of using timber to build structures. This has paved the way for timber frames as the base for timber houses. Although there are many positive impacts of timber frame structures, it is essential that they are built according to the correct standards, otherwise the timber structure ends up being unsafe and expensive to build and maintain in the long run.

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