Concrete and steel enabled the advent of the skyscraper, and in just about a century they helped that form reach mountainous heights. But these materials have an environmental impact that can’t be ignored. That fact is driving a new generation of designers to reconsider wood.

Concrete and steel production is responsible for about 8 percent of the world’s emissions of carbon dioxide, the greenhouse gas mainly to blame for climate change. The majority of both materials go to fuel the construction boom in China, which nearly doubled its use of steel in the last ten years.

Asia’s ongoing building boom is mostly in response to the extreme demand for housing created by its growing and rapidly urbanizing population. More than a billion people will move or be born into Asian cities in the next 20 years. Billions more are already homeless or living in slums. While the density of high-rise living cuts down on transportation and energy emissions, that’s at odds with the carbon content of its two favorite materials.

Looking at a California redwood, which can stand nearly 40 stories tall, it’s not hard to imagine a wood structure reaching such heights. And its carbon profile isn’t just less than competing materials; it is potentially carbon negative. As the poet Bill Yakes wrote, “Trees are our lungs turned inside out.” That is, they grow by drinking up carbon dioxide, exhaling oxygen in return. Every cubic meter of wood stores more than three quarters of a ton of carbon.

Canadian firm Michael Green Architecture just broke ground on what will be the tallest wood building in North America. Designers in Europe and Australia have also gone above wood’s traditional three- or four-story limits. But in the U.S.—where code constraints, economics, and a social stigma prevent construction—the idea has been slower to catch on.

Since they helped set off a flurry of interest in the topic of tall wood construction about ten years ago, a pioneering few designers and engineers have seized on the potential of manufacturing breakthroughs to give one of the world’s oldest construction materials new life. They say urbanization, population, and climate change are on course for a head-on collision that architects have a responsibility to help avert, and wood construction is how.

Seeds to buildings

When British architects Waugh Thistleton set out to the build the Stadthaus building, now called the Graphite Apartments, in the east London borough of Hackney, they weren’t stacking two-by-fours.

Apart from a reinforced concrete plinth and fiber-cement facade panels, the entire building is made from cross-laminated timber (CLT). Essentially huge wood sections that behave like shear walls, CLT panels were the first in a series of material advances that opened up design possibilities for tall timber. Manufacturers like KLH Massivholz GmbH in Austria, where 80 percent of CLT is still made, pile up sheets of wood at 90-degree angels and paste or glue them together into something resembling a jumbo piece of plywood.

“Our biggest job talking to code officials and the fire department was making sure they distinguished between stick-frame and CLT,” said principal Andrew Waugh. “You’re dealing with a more solid robust material. With a stick-frame system you’re relying on the guy on site.”

CLT is assembled off-site, which cuts down on construction errors and time. The Graphite Apartments, a nine-story mixed-use building, was built in just under one year—months less than expected.

A layer of drywall over the thick CLT panels helped the structure earn a fire resistance rating between 60 and 90 minutes, passing code. Heavy timber and cross-laminated timber actually have built-in fire protection; dense wood will burn slowly, charring instead of catching fire all at once. Part of bringing a wood building up to code is providing enough wood so that even after fire produces a “char layer,” there is still enough left to support the structure.

On Green’s forthcoming Wood Innovation Design Center in Vancouver, a pre-charred cedar exterior dramatically improved its fire rating.

Acoustics, another traditional failing of wood construction, is also heartier in CLT towers. An air gap, compressed insulation, and a floor slab totaling about 14 inches overall helped the Graphite Apartments meet stringent UK acoustics requirements.

CLT is not produced in the U.S., nor are newer iterations of high-rise-ready timber panels, like laminated strand lumber (LSL) or laminated veneer lumber (LVL). But as more high-rises are built with wood, Waugh hopes his firm will find a U.S. client.

“The more you build with timber, the more you realize how steeped in concrete we really are,” he said. “It’s still a relatively conservative industry, the construction industry, but when contractors build one they want to build more.”

Waugh built his own CLT home with three friends. He said the wood imparts an emotional value. “It’s a beautiful place to live. You know you’re living in a space captured by a natural material.”

Timber towers

Michael Green, Waugh Thistleton, and several European firms—Berg | C.F. Møller Architects and Dinell Johansson have proposed a 34-story “ultra-modern residential high-rise building” for Stockholm—are the face of the timber tower movement, but they recently added a company from the old guard of skyscraper design to their ranks: Skidmore, Owings & Merrill.

When SOM engineers first floated the idea of a 20-story wood tower, one partner’s response wasn’t the skepticism one might expect from a master of steel-and-concrete structural systems. “Do 30,” he reportedly told them.

“It’s a high standard. We wanted to set a high benchmark,” SOM’s Bill Baker told AN. They chose the 1965 DeWitt-Chestnut Apartment Building in Chicago as their standard, the first building in the world to use the “framed tube” structural system devised by SOM engineer Fazlur Khan.

“We wanted to show not just that it was possible,” said SOM’s Benton Johnson, “but make it competitive with concrete.”

The prototype isn’t pure wood. A concrete core and joints mean the system uses about one quarter as much concrete as the actual Dewitt-Chestnut. Structural steel anchors the building at its base, using about 15 percent as much steel as a typical composite system.

SOM’s report examined five schemes with varying amounts of timber, steel, and concrete, trying to replicate the landmark building’s structure. They focused on reducing the weight of the floors, where most of the material weight is contained. Wood high-rises already built in Europe, such as the Graphite Apartments in London, use a lot of load-bearing walls to hold up the structure. But that would limit the building owner’s options for renters, Johnson said, as would the immovable columns placed throughout.

To make the Dewitt-Chestnut system work without drastically shrinking the floorplate or beefing up the structural system, SOM zeroed in on what engineers call the boundary condition—its mathematical pressure point. To illustrate, Johnson built two stacks of tile samples and placed a ruler on top to span the distance between. He balanced a can of La Croix water on the ruler, the building’s floor in this example. The ruler bowed beneath its weight, but its edges also flared up, making a slight u-shape. But with a few more tiles placed on each stack to pin down the ruler, it held its shape.

In his example, the ruler is a solid timber floor, while the tile stacks are reinforced concrete wall joints and beams. Without concrete, SOM’s engineers determined the Dewitt-Chestnut would need custom 13.5-inch CLT panels to support the floorplate’s core-to-window span. That would be too expensive and would use more material in just the floors than the whole of the original building.

“It just started solving all these problems for us,” Johnson said. “You have the concrete to hold it all together—basically all this timber coming together and concrete sealing it at the joints.”

It would take about 12 million cubic yards of timber to build, the report estimated—less than one-hundredth of one percent of the annual North American timber harvest.

Scaling back

Even if engineers can solve these problems, there is still a stigma involved with tall wood structures. Antony Wood, executive director of the Council on Tall Buildings and Urban Habitat, counted timber towers among the “quiet revolutions” happening in tall building design.

“I think the fear of timber is that it’s an organic material,” he said. “It’s not manufactured to provide a structural member like steel or concrete is.”

Wood rots, so it must be kept out of the rain. SOM’s system swaps wood for a steel frame at the building’s base to prevent water damage during flooding.

Most critics worry about fire. Tall timber skeptics seized on a structural fire at the job site of a six-story wood building in Richmond, British Columbia, in 2011. In the city just south of Vancouver, what would have been the first wood-frame six-story building in Canada burned to the ground on May 3. Steel companies were quick to blame the wood frame’s flammability. But Canadian Wood Council President Michael Giroux pushed back, noting the construction team hadn’t yet installed safety features, including fire sprinklers.

“To suggest that the outcome of the May 3 fire at the Remy project in Richmond would have been the same if the building had been fully completed, is not plausible,” he wrote.

Even tall timber’s champions concede the material isn’t suitable for super-tall buildings. But they say building codes, which in many places restrict wood to only low-rise construction, isn’t up to date with structural engineering advancements.

“It’s time to reconvene and reconsider what we’re doing,” Waugh said. “We need to densify our cities to leave ground for agriculture and wildlife. Condensed cities are much more efficient places. But I don’t think these Babel-sized towers are the way.”

And some go as far as to say the threat of climate change means wood high-rises are our only choice.

Wood world

In 2009, the government of British Columbia endorsed a “culture of wood” requiring designers of public buildings to prove they can not use wood before considering other materials. With millions of acres of forests in the U.S. and Canada devastated by mountain pine beetles, it was a prudent move for a province home to one of the world’s busiest forestry sectors.

But if wood construction is going to take off on the scale envisioned by its pioneering architects, Michael Green said, the “wood first” policy will have to become “carbon first.”

“We need to create incentives around climate change instead of seeing it all as a hindrance,” he told AN. “Let all industries benefit—it allows the concrete and steel industries to make their case. By no means is one exclusive of the other. Let’s use all materials where it’s most appropriate.”

While at MGB (mcfarlane green biggar ARCHITECTURE + DESIGN), Green released an open source platform for wood tower construction—a structural system to engineer tall buildings 12, 20, or 30 stories high. Several iterations later, his wood-based structural systems have started a conversation in Vancouver, where he is based.

Green said the warmth of wood interiors and buildings’ retreat from airplane-scale heights could help solve another problem of modern high-rise construction: social sustainability. Whereas many residential skyscrapers are isolating, new typologies developed with wood in mind—not traditional forms grafted onto wood frames—could change the mindset.

As with British Columbia’s “wood first” policy, the UK’s performance-based code has created an opportunity for timber construction, while U.S. code remains constrictive. But it wasn’t novelty that ultimately built Waugh Thistleton’s Graphite Apartments. At a cost of about $2,200 per square foot, the building was 15 percent cheaper than if it had been made from concrete.

By 2050, concrete use is predicted to reach four times its 1990 level. And production of steel and concrete are on track to balloon, eclipsing advances in recycling and materials science that could shrink their carbon footprints.

“We need to really hit reboot on how we build environments,” Green said. “As architects we owe it to ourselves to push these boundaries.”