Should cedar shakes be treated or oiled in Alberta?

Treating or oiling cedar shakes can extend their life, but in Calgary's climate the results are mixed and the timing matters more than most homeowners expect.

What treatments actually do

Raw cedar contains natural oils that resist moisture and decay, but Calgary's dry air, intense UV, and roughly 128 freeze-thaw cycles a year accelerate the loss of those oils. Once the wood dries out and checks (splits along the grain), water gets in, freezes, and widens the cracks. A penetrating oil or preservative treatment slows that process by keeping the wood more supple and water-repellent.

The most useful products are penetrating oil-based preservatives, not film-forming sealers or paint. Film-forming products trap moisture beneath the surface, which leads to peeling and accelerated rot. Look for a product designed specifically for cedar or wood shingles, with a mildewcide and UV inhibitor included, given how much sun Calgary roofs receive at 1,045 m elevation.

Timing is critical. New shakes should weather for at least six months before any treatment, so the wood opens up enough to absorb the product. After that, a re-application every three to five years is a reasonable target, though a roof in full sun on a south-facing slope may need it more often. The best window is a dry stretch in late spring or early summer, after freeze-thaw season and before hail season peaks in June through August.

Condition determines whether treatment is worthwhile. If the shakes are already heavily split, cupped, mossy or showing rot at the butts, treatment will not reverse the damage. A roofer who works with cedar can tell you whether the roof is a candidate for treatment or whether replacement is the better call. Many Calgary homeowners with older cedar roofs are switching to architectural asphalt, synthetic shake or metal, partly because those materials need no ongoing maintenance and hold up better against hail.

A few practical points worth knowing before you hire someone to treat the roof:

• Moss and algae must be removed and the roof allowed to dry completely before any product is applied; treating over moss seals it in and speeds decay
• Any contractor applying product on the roof is working at height and must comply with Alberta OHS fall protection requirements, which means protection from a fall of 3 m or more; confirm they have a written fall protection plan and WCB-Alberta coverage before they start
• If your cedar roof was installed before 1990, have a roofer check whether any underlayment or flashing materials contain asbestos before disturbing them
• Some insurers charge more for older wood shake roofs or limit coverage on them because cedar is more combustible than asphalt or metal; ask your insurer where your policy stands before investing in treatment

Treatment is a maintenance cost, not a substitute for a roof that has reached the end of its life. A roofer experienced with cedar can inspect from the ridge down and give you an honest read on whether the deck and shakes are worth preserving.