Design & DIY

Is cedar rot resistant?

Should we build with cedar? We heard that it’s a myth that cedar is rot resistant. It has something to do with the wood we harvest now being from second-growth trees that don’t have the resins they had a hundred years ago. Is that true?
—Jack and Cammy Holden, Haliburton Lake, Ont.

The “resins” you’re describing are called “extractives.” They’re chemicals, produced naturally by the tree, thought to make the wood resistant to decay. “In general, extractives build up over time,” explains Sara Robinson, a research assistant professor in the School of Forest Resources and Environmental Science at Michigan Technological University. Studies into extractives (plus anecdotal evidence) going back many years claim that second-growth cedar is less rot resistant than old growth cedar. But according to Paul Morris, a research leader in durability and building enclosure with FPInnovations, a forest research centre, “there is more recent evidence suggesting that second-growth wood is just as good as old growth.” In 2001, scientists from Oregon reported that the decay resistance in second-growth western red cedar was similar to the decay resistance in old-growth cedar. And 2011 results from a five-year study comparing four test sites (including one in Petawawa, Ont.) showed that old- and second-growth western red cedar and yellow cedar were the same when it came to decay resistance.

One thing’s for certain: Wood durability varies among individual trees, and the location within the tree itself. Lumber cut from the heartwood—the innermost part of the tree—contains the 
most extractives, so it’s more durable than lumber from sapwood (the outermost wood). “Lower-quality wood is a mix of both heartwood and sapwood, as opposed to only heartwood,” says Robinson. If you know what you’re looking 
for (the heartwood of eastern red cedar, for example, is a bright red-pink colour), you can choose the most durable wood from the lumberyard. Also, “less decay resistant” doesn’t mean “useless.” Cedar, old growth or not, is a long-lasting and durable wood, says Robinson. “I’d build 
a deck out of it.”