Therefore, some architects, such as the architectural firm Waugh Thistleton, are pushing for a return to wood as the main building material. Wood from forestry actually absorbs carbon, not emits it: as trees grow, they absorb CO2 from the atmosphere. As a rule, a cubic meter of wood contains about a tonne of CO2 (depending on the type of wood), which is equivalent to 350 liters of gasoline. Not only does wood remove more CO2 from the atmosphere than it does during production, it also replaces carbon-intensive materials such as concrete or steel, doubling its contribution to reducing CO2 levels.
“Because a timber building weighs about 20% of a concrete building, the gravity load is greatly reduced,” notes architect Andrew Waugh. “This means that we need a minimal foundation, we don’t need a huge amount of concrete in the ground. We have a wood core, wood walls and wood floor slabs, so we keep the amount of steel to a minimum.” Steel is commonly used to form internal supports and to reinforce concrete in most large modern buildings. However, there are relatively few steel profiles in this wooden building,” Waugh says.
Between 15% and 28% of new homes built in the UK use timber frame construction each year, which absorbs over a million tons of CO2 per year. The report concluded that increasing the use of wood in construction could triple that figure. “Savings of the same magnitude are possible in the commercial and industrial sectors through the use of new engineered systems such as cross-laminated wood.”
Cross-laminated timber, or CLT, is a building site staple that Andrew Waugh is showing in East London. Because it’s called “engineered wood,” we expect to see something that looks like chipboard or plywood. But CLT looks like ordinary wooden boards 3 m long and 2,5 cm thick. The point is that the boards become stronger by sticking together three in perpendicular layers. This means that CLT boards “do not bend and have integral strength in two directions.”
Other technical woods such as plywood and MDF contain about 10% adhesive, often urea formaldehyde, which can release hazardous chemicals during processing or incineration. CLT, however, has less than 1% adhesive. The boards are glued together under the influence of heat and pressure, so a small amount of glue is enough for gluing using the moisture of the wood.
Although the CLT was invented in Austria, London-based architecture firm Waugh Thistleton was the first to build a multi-storey building that was used by Waugh Thistleton. Murray Grove, an ordinary gray-clad nine-story apartment building, caused “shock and horror in Austria” when it was completed in 2009, Wu says. CLT was previously only used for “beautiful and simple two-story houses”, while concrete and steel were used for taller buildings. But for Murray Grove, the entire structure is CLT, with all walls, floor slabs and elevator shafts.
The project has inspired hundreds of architects to build tall buildings with CLT, from the 55-meter Brock Commons in Vancouver, Canada to the 24-story 84-meter HoHo Tower currently under construction in Vienna.
Recently, there have been calls for planting trees on a massive scale to reduce CO2 and prevent climate change. It takes about 80 years for pine trees in forestry, such as European spruce, to mature. Trees are net carbon sinks during their growing years, but when they reach maturity they release about as much carbon as they take in. For example, since 2001, Canada’s forests have actually been emitting more carbon than they are absorbing, due to the fact that mature trees have ceased to be actively cut down.
The way out is the felling of trees in forestry and their restoration. Forestry operations typically plant two to three trees for every tree cut, which means that the greater the demand for timber, the more young trees will appear.
Buildings that use wood-based materials also tend to be faster and easier to build, reducing labor, transportation fuel and local energy costs. Alison Uring, director of infrastructure company Aecom, cites the example of a 200-unit CLT residential building that took just 16 weeks to build, which would have taken at least 26 weeks if it had been built traditionally with a concrete frame. Similarly, Wu says the newly completed 16-square-meter CLT building he worked on “would require about 000 cement truck deliveries just for the foundation.” It only took them 1 shipments to deliver all the CLT materials.