“Burn baby burn” and other myths about wood
You wouldn’t build a skyscraper out of wood, would you?
For many people, any talk of building with wood brings to mind the Great Fire of London, with entire streets going up in flames.
In fact, after four fifths of the city was lost in 1666, it was decreed that houses and buildings could only be built out of brick or stone.
And yet engineers and architects have been quietly looking again at the use of timber as a construction material.
It might surprise you to know that there are already some fairly tall structures where wood is the dominant building material.
In Canada, at the University of British Columbia, there’s an 18-storey, 53-metre tall student accommodation block which is currently the highest inhabitable structure made from timber. It opened this summer and houses 400 students.
And there are already plans afoot to top this. Construction is due to start on a 21-story building in Amsterdam, and the 24-story HoHo building in Vienna is due to open in 2018.
The HoHo building is a mixed-used development containing a hotel, apartments, a restaurant, wellness centre and offices. When you compare this timber building, which will be 76% wood, with a similar steel and concrete structure, its construction will save a massive 2,800 tonnes of CO2.
And that highlights one of the benefits of working with timber – the positive environmental impact. It can also be cheaper to work with, and the more it is used the cheaper it will get. I think we will see it used a lot more in social housing.
Why haven’t we used wood historically?
One of the main objections to wood is not the associated fire risks, but the fact that it’s a natural substance so it doesn’t have the same strength properties in every direction. Timber is also sensitive to moisture content changes, which can lead to warping and distortion – not something we want to see in a building.
We are now getting around the downsides of wood with CLT – cross-laminated timber.
With CLT, each layer in a panel has the wood grain running perpendicular to the one in the adjacent layer. It is a little like plywood, but with thicker laminations.
The cross gluing overcomes the risk of warping and gives the panels very good dimensional stability. This enhances timber’s high strength to weight ratio, so that CLT behaves more like steel than like standard timber.
It can also speed up the build, as panels can be cut to precise dimensions offsite. Clearly this is positive from an economic point of view, and also reduces the impact of a building project on local residents and businesses.
What about the fire risk though?
With the Grenfell Tower fire fresh in our minds, it might seem like timber is an odd choice of material to use in construction.
Yet there are several factors at play. It’s true that timber can pose a fire risk, but it’s a nuanced area.
Timber panels and beams burn slowly and predictably. They are capable of the 30, 60 and 90-minute resistance times that many building regulations insist on. Steel can lose structural integrity quickly in certain situations. So clearly it’s the context of where different materials are used that’s important.
We can construct timber buildings safely, and there are definite benefits to doing so, but we need to proceed with caution.
If you need help with the structural elements of a project, whatever materials you’re using, please do get in touch.