Durability of Cross Laminated Timber (CLT)

Much has been said about the benefits of CLT over the past 10 years. All of it true! It’s proven to be a wonderful material: commercially beneficial, sustainable and a pleasure to interact with whether as user or designer. But like all materials, CLT needs to be understood and effort needs to be put into both harnessing its opportunities and mitigating against its vulnerabilities.

CLT in the UK is most commonly specified as an untreated softwood. It is therefore required to be kept dry through appropriate detailing, and it is in this area that a bit of work needs to be done to create a better industry understanding. It’s also very easy so there’s no need for concern.

And it really is an issue. A number of flat roofs/terraces in particular have shown signs of decay and have led to some saying CLT shouldn’t be used as the structure for a flat roof. Again, there’s no need to be concerned as simple detailing will see that such things don’t occur. Please also understand that we aren’t here to advocate or pitch CLT. Our practice places a strong emphasis on putting the right material in the right place. It’s just that CLT can be demonstrably the right material in such applications.

A few recommendations;

• Provide falls to the CLT. This will allow water to be managed more easily on site and will also help the building to fail-safe in the event of failure of the membrane. 1:40 designed fall to achieve a post deflection 1:80 fall is a good starting point.
• Provide a hot melt or liquid applied membrane as soon as possible on site. This membrane works well with CLT. It stops water tracking across the surface of the CLT in the event of failure and CLT is typically tolerant of very localised areas of degradation. Keeping CLT as dry as possible during construction is necessary for taking care of the timber, and membranes of any form should not be placed onto excessively wet timber however stressed the project programme is.
• Consider using a second membrane. In conjunction with the hot melt or liquid applied membrane, a second membrane on the top surface of the roof makes good sense. It allows a second means of protection in the event of failure. The top membrane is inspectable and maintainable. The bottom will catch any water that may penetrate and will take it away from the roof using the fall. This two-lines-of-defence approach is standard fare when constructing a basement so why should a roof be any different?
• Consider where water would go in the event of failure. Just chase it through so that any water that has penetrated the top membrane and is moving over the second has somewhere to go. Openings in parapet and routes into drained cavities are good here.

by Nathan Wheatley