Why are my cedar shakes splitting on the south side in Calgary?

South-facing cedar shakes in Calgary take a beating that shakes on any other exposure simply do not. The combination of intense UV radiation, Calgary's dry high-altitude air, and the city's roughly 128 freeze-thaw cycles a year creates a cycle that splits wood faster than almost anywhere else in Canada.

Why the South Side Is the Problem

The south slope gets the most direct sun year-round. In summer that means repeated heating to very high surface temperatures, which drives moisture out of the wood rapidly. Calgary's elevation and dry air accelerate that drying. Then a chinook rolls in during January, the snow melts off the roof in an afternoon, the wood absorbs moisture, and overnight temperatures drop and it refreezes. Wood fibres expand and contract with each cycle, and over years the grain separates along natural lines, producing the splits and checks you are seeing.

UV degradation compounds this. The south slope loses its protective surface oils and grey-weathers faster than north or east exposures. Once the surface is weathered and the wood is brittle, even moderate hail, which Calgary sees every summer, can crack a shake that would have flexed a decade earlier.

A few other contributors are worth checking. If the attic below the south slope runs hot in summer because ventilation is inadequate, the underside of the shakes dries from below while the sun dries them from above, which accelerates splitting. Calgary's building code requires attic vent area of at least 1/300 of the insulated ceiling area, with at least 25% of that at the ridge and 25% at the eaves; a blocked or undersized ridge vent on the south side makes the problem noticeably worse. Also check whether the shakes were installed with the correct exposure and spacing to allow for seasonal movement; tight installation leaves no room for the wood to breathe.

Age matters most. Cedar shakes installed before roughly 2000 are now 25 years or older. A well-maintained cedar roof lasts 25 to 35 years in Calgary conditions, but the south slope typically shows failure five to ten years ahead of the north slope on the same house.

What to do now. Have a roofer look at the south slope from the ground and, if safe, from a ladder at the eave. Splits that run through the full thickness of the shake let water in; those shakes need replacing. If fewer than roughly a quarter of the south-slope shakes are split, selective repair is reasonable at a cost of about $300 to $1,000 for a small patch, up to $1,000 to $3,000 if the damage is more widespread. If the splits are widespread across the whole slope, a full replacement conversation makes sense. Cedar shake replacement in Calgary runs about $10 to $18 per square foot installed; many owners on a full replacement are switching to architectural asphalt, synthetic shake or metal, which handle Calgary's UV and freeze-thaw cycle better and do not require the same maintenance.

A like-for-like cedar replacement needs no City of Calgary building permit. Switching to a different type of roofing material does require one.

Before any roofer starts work, ask for a written contract with the scope, materials, start date and payment schedule, and check their Alberta prepaid contracting licence through Service Alberta if any deposit is involved. A WCB-Alberta clearance letter is worth requesting as well.

When you want a written quote on that south slope, a couple of local roofers can look at it through Calgary Roof Repairs.