Buildings are getting the wooden tree-tment

Nice try big bad wolf. The three little pigs have the power of science on their side now.
19 November 2021


Wooden roof timber A-frame


To build tall buildings like the skyscrapers you see in London, the materials we use by far the most are concrete and steel. But more and more, architects are now using this special form of wood called cross laminated timber, or CLT, to make structures that are taller and taller. Iacopo Russo went to visit Dave Lomax from Waugh Thistleton Architects to have a look at what is going to be the tallest office building made of timber in the whole of London...

Iacopo - We're in Rivington street and I've just had the first sight of the black and white building.

Dave Lomax - The basic principle of the building is that we're providing six floors of new office space in the middle of the street, a really tight little site with a railway on one side and small roads and all those sorts of things. And the principle here is that we've chosen to build the building in a way that is better for the environment. So instead of just doing the normal thing, going for a concrete frame and pouring lots and lots of grey stuff into the ground, we've elected to build it out with pieces of wood.

Iacopo - I can see layers in the wooden planks forming the floor. What kind of wood is that?

Dave Lomax - That particular material is one called LVL and that stands for Laminated Veneer Lumber. What that is, is a whole series of hundreds and hundreds of veneers, veneer meaning specifically that it was peeled from the tree rather than cut from the tree. That product is incredibly strong and the species that goes into that is beech, so those are hardwood elements, that means that they're much stronger. With a really strong material like L.V.L. Made from beech, we can have smaller columns, smaller beams.

Iacopo - We just entered the construction site. The first thing I noticed as I entered the building is the smell. It's very nice, woody and also construction smell, I guess.

Dave Lomax - Absolutely. The way I describe it to people is my dad's woodwork shop. Typically this is a building made out of beams and columns. So that's very like a steel structure, slightly different to a usual concrete structure. A concrete structure would normally have columns, just needs a flat slab. Now the centre of the building deals with the horizontal movement, all buildings move horizontally with the load of the wind, if they don't have something to stop them. Here, what we use is CLT. CLT is quite thick, you can see them on the edge there. Average everyday chunks of spruce cut into planks, and then laid in five layers, each layer at 90 degrees to the next. And that allows it to work a bit like the slab you'll see in a concrete building. It spans in two directions.

Iacopo - I suppose fire is one of the biggest concerns that people have with timber. Is this building going to be safe against fires?

Dave Lomax - All building materials suffer when there is a fire, when we have a piece of steel and we expose it to 300 degrees of temperature, it also is very likely, very quickly to start to fail. But we do things to protect it from burning in a way that is dangerous to those fighting the fire or those being in the building at the time. So our approach to wood is pretty much the same. We know exactly how timber performs, how long it takes to degrade. We know that this building can be on fire in certain places for a certain amount of time, which should be long enough for everybody to evacuate the building and, if they decide to do so, for the fire brigade to put it out.

Iacopo - How long is this building gonna last?

Dave Lomax - It's a really sensible question. I'm going to give you the politician's answer first. It comes with a warranty for 60 years. There's absolutely no reason wooden buildings can't last forever, we just need to look after them. So the principle with wood is, it's great if it gets wet and then dries out again. It's only when we start to trap water in certain places that we start to get problems, because then we get a cycle of mould and fungus growing and rot. All of this wood is really, really carefully treated in the way it's manufactured. It arrives to the site at 14% no more moisture content, that's monitored. Then we test it again before we put things like waterproof membranes on the roof, so that we know we're not trapping water inside. But beyond that, belt & braces. You may even see some, when we go downstairs, there are some little tiny holes in the roof.

Iacopo - So we got close to a square column here and we're just going to touch it to see what kind of sound it makes.

Knocking - [knocking on column]

Iacopo - I was expecting it to be a lot more hollow as a sound. Instead it's very solid.

Dave Lomax - This material particularly on the CLT panels, you can see they've got five or sometimes seven, sometimes three thick layers in them. If you see the surface of this material here, I mean, we couldn't count them. There's probably a hundred layers in that column that you can count right now. So what we've got is lots and lots and lots of layers, all packed tightly together through the surface of the column.

Iacopo - What's gluing the wood together and the wood planks?

Dave Lomax - It's not a particularly technically complex glue, but the difference is that all of these parts are pressed together in a really big mechanical press. So that's either a hydraulic press or it's a vacuum press. And that happens in the factory. So it's not just the performance of the glue, which needs to be one that can absorb into the lignin, into the cell structure of the timber. It also needs that kind of high pressure applied to it to give you the really strong adhesion we have. The basic principle is sticky stuff, and pressure.

Iacopo - Do you think wood is a material for the 21st century?

Dave Lomax - I think it absolutely is. It's not the wood itself, the wood is no different, it's always been the same. But now these incredibly clever processes are working with a size and scale of material built to an incredibly tight tolerance in a factory in a way we couldn't have done even 40 years ago. That's why timber is the material of the future because we're able to use it in such innovative and safe and clean and tidy ways.



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