34 Number of stories in the tallest approved all-wood tower, set to undergo construction in Stockholm, Sweden.
On a cloudy day in early October, the architect Andrew Waugh circles the base of a nondescript apartment tower in Shoreditch, a neighborhood in East London. Shoreditch suffered heavily during the blitz of World War II--"urban renewal, compliments of the Luftwaffe," Waugh says--and then spent decades in neglected decay. Recently, though, the neighborhood has come roaring back. Nightclubs and tech start-ups arrived first on the promise of cheap rent, and residents followed. Along with them came architects, urban planners, and engineers, many of whom make a pilgrimage to the same tower that Waugh now circumambulates.
From the outside, there is nothing particularly flashy about the nine-story building, called Stadthaus, that Waugh designed with his partner, Anthony Thistleton. Its gray and white facade blends almost seamlessly into the overcast London skies. It's what's inside that makes Stadthaus stand out. Instead of steel and concrete, the floors, ceilings, elevator shafts, and stairwells are made entirely of wood.
But not just any wood. The tower's strength and mass rely on a highly engineered material called cross-laminated timber (CLT). The enormous panels are up to half a foot thick. They're made by placing layers of parallel beams atop one another perpendicularly, then gluing them together to create material with steel-like strength. "This construction has more in common with precast concrete than traditional timber frame design," Thistleton says. Many engineers like to call it "plywood on steroids."
When it opened in 2009, Stadthaus was by far the world's tallest modern timber building. Since then, CLT towers have sprouted up everywhere. Waugh Thistleton built a seven-story apartment tower near Stadthaus in 2011, and construction is under way on a 90-foot-tall wood building in Prince George, British Columbia. In 2012, Stadthaus lost the height record to a 10-story apartment building in Melbourne called Forte.
There are plans to go even higher. Swedish authorities have approved a 34-story wood tower in Stockholm, while Michael Green, a Vancouver architect, is seeking approval for a 30-story tower in his city. And the Chicago architecture mega-firm Skidmore, Owings & Merrill recently published a feasibility study for a 42-story tower made predominantly of CLT. It's become a competition among architects to see who can build the next tallest wood high-rise, says Frank Lam, a professor of wood building design and construction at the University of British Columbia.
Why the sudden interest in wood? Compared with steel or concrete, CLT, also known as mass timber, is cheaper, easier to assemble, and more fire resistant, thanks to the way wood chars. It's also more sustainable. Wood is renewable like any crop, and it's a carbon sink, sequestering the carbon dioxide it absorbed during growth even after it's been turned into lumber. Waugh Thistleton estimates that the wood in Stadthaus stores 186 tons of carbon while the steel and concrete for a similar, conventionally built tower would have generated 137 tons of carbon dioxide during production. Wood nets a...