Timber is the go-to sustainable building material for our times. Roof design and detailing are important to reap the benefits of using engineered timber. Rachel Sandbrook from Engenuiti answers some questions about the challenges, benefits, and dos and don’ts of roof design.
What are the challenges of using engineered timber such as CLT and glulam for roofs?
It is important to get the design and detailing of the roof right in relation to protecting the timber elements from moisture ingress. Simple measures (executed well) can be enough to ensure that engineering timber products can perform, for example introducing minimum falls on nominally flat roofs to ensure that rainwater does not collect or pond. This isn’t a matter of prescriptive detailing so much as taking a common-sense, fail-safe, project-specific approach.
…and what are the benefits?
Where to start? Timber is the go-to sustainable building material of our time, given its unrivalled capabilities as a natural carbon store and a genuinely renewable resource. Engineered timber projects make use of a high degree of prefabrication, giving rise to a way of combining a high speed of construction with very high build quality, accuracy and low waste. Given our current housing crisis, I think it would be foolhardy not to be looking to make a much wider usage of such a powerful product.
What are the myths about engineered timber roofs?
CLT and glulam can certainly be perceived as being more expensive than more ‘conventional’ building materials. But I think this preconception is often borne of not looking holistically at the possible cost benefits. Certainly, we have experience of CLT options being shown to be more cost effective overall because, whilst they didn’t provide direct savings in the frame, they enabled a substantial reduction in foundation sizes and a dramatically improved frame erection time.
I think there is also sometimes a misplaced fear of putting engineered timber in roofs because it is seen as less moisture-proof. But timber roof structures have been in use (and in many cases have lasted) for hundreds of years. Again, success lies in getting the detailing right.
How can engineered timber be used as a hybrid material in roofs?
Some of our most exciting projects have married CLT with either glulam or steel framing elements to create really exciting long span roof structures. We have built undulating glulam gridshells; muscular steel lattice structures supporting CLT infill panels; and also, some very simple, lean and elegantly detailed CLT and steel frame planar roofs.
How is the issue of moisture tackled in the design of an engineered timber roof?
The key things to consider in designing and detailing an engineered timber roof are: 1. How does the rainwater get off the roof easily and without getting stuck somewhere 2. If there is a failure of the waterproofing system, does our design approach ensure that the impact of this is manageably small? 3. Does our roof have a way of telling us when something is going wrong? If we have a design solution that addresses these questions positively then we can have every confidence in the longevity and durability of our roof.
How do engineered timber roofs perform in comparison to a roof made up for softwood joists or trusses?
Engineered timber roofs offer superior spanning capabilities and an opportunity to create original and exciting geometries. CLT also provides the benefit that it makes an excellent diaphragm for the transmission of lateral loads.
Where a traditional cut roof can offer an advantage, it is in the relative ease with which damaged parts of the roof can be removed and replaced in the event of moisture ingress. It is important to consider this when designing engineered timber roofs and on recent CLT projects, we have made a point of designing any particularly vulnerable areas of the roof so that the panels in this area can be removed and replaced in a straightforward way.
How have you used engineered timber in an innovative or unique way for a roof?
At the Moor Centre in Sheffield, we constructed a free-spanning barrelled glulam gridshell roof. We devised a gridshell node design that uses a fabricated hexagonal steel ring to which the incoming glulam elements are connected by way of glued-in dowelled rods. In the absence of detailed design code or ETA-based guidance, we developed a testing procedure for these glued connections and undertook this testing at specialist facilities in Austria in order to prove the capability of the connections. In going the extra mile in the design process, we have created a node that has the elegance and simplicity to allow the timber to do the talking.