Wood's a versatile, affordable, and environmentally friendly construction material. But it's also combustible, and somewhat unstable (as we all know, wood swells and shrinks when its moisture content changes).

In the United States, those drawbacks have traditionally limited wood-framed construction to structures no higher than three or four stories. New engineered materials have helped designers and builders achieve longer floor spans or carry heavier loads, but they haven't let buildings grow taller. (For a handy guide to US code provisions for wood construction, you can download the free PDF, "2015 Code Conforming Wood Design," from the American Wood Council.)

But that could be changing. A new material called "cross-laminated timber" is putting wood into contention with concrete and steel for the mid-rise construction market. In the United States, regulations still don't allow it — but in Canada, governments are starting to permit wood construction of mid-rise buildings, and companies are taking on the challenge.

In Canada, the trend started with code revisions in the provinces that allowed light wood-frame construction as high as six stories. British Columbia was the first province to make the change, starting in 2009. Quebec followed suit in 2013, and Ontario joined the club in 2015. Canada's national building code still restricts stick-framed wood construction to four stories, but that looks set to change.

Canadian architects and builders are rapidly building their experience in six-story work. In a 2014 presentation, British Columbia architect Patrick Cotter showcased several exemplary six-story projects, and talked about the technical challenges in building taller structures with wood (see: "Six in Wood, Presented by Patrick Cotter"). (Scroll down in the presentation to see how Cotter addresses issues like wood shrinkage and fire safety). Below, Cotter discusses some of the technical approaches he uses in designing six-story structures with wood materials.

And six stories (or "storeys," as the Canadian spell the word) is no longer the limit in Canada. Quebec recently moved to allow wood construction of buildings as tall as 12 stories — not with traditional stick-frame methods, but with an engineered material called "Cross-Laminated Timber," or "CLT".  Canadian Business makes note of the news here (see: "Quebec allows lumber to be used in construction of buildings up to 12 stories," by Ross Marowits).

Canadian timber companies are pushing CLT technology as a multi-dimensional problem-solver that they hope will help boost the Canadian forestry sector, while addressing problems of housing affordability and also helping to restrain carbon emissions associated with concrete and steel production. With support from the Canadian Wood Council, industry think tank FPInnovations has created a voluminous technical guide to mid-rise wood construction, "Technical Guide for the Design and Construction of Tall Wood Buildings in Canada," which you can buy for about $300 dollars at FPInnovation's website.

Canadian developers have been quick to embrace the mid-rise opportunity. In September, the Globe and Mail profiled a proposed 13-story condo tower proposed by Nordic Structures for Quebec (see: "Quebec firm takes on wood high-rise challenge," by Josh O'Kane). One objection to tall wood construction is fire safety, the paper reported; Nordic Structures is tackling that issue head on.

"In a fire-resistance test, Nordic’s team built a mockup of a wall and floor inside an NRC [National Research Council] furnace, with the weight of 11 storeys applied on top to give a sense of how the lowest floors would behave when exposed to fire," the paper reported. "The Origine mockup’s walls and floors resisted fire for 31/2 hours at 1,200 degrees Celsius – far longer than the required two hours."

A second test, last November, was more of a demonstration," the Globe and Mail noted. "Fire safety officials from across Canada studied a blaze in a one-storey room attached to a three-storey mockup of Origine’s wooden vertical stair-and-elevator shafts. In spite of a full-scale blaze in the room, 'there was no detectable increase in temperature or smoke in the vertical shaft,' says Cameron McCartney, a construction fire-safety research officer with the NRC."