Ice Dams on Roofs and Winter Roof Safety in Calgary

A fresh, lofty snow can add 20 lbs. of weight per cubic foot, but wet snow, or partially thawed and refrozen snow, can add as much 60 lbs. per cubic foot of weight. According to Environment Canada, and based on historical numbers on weather data gathered from 1981 to 2010 at the Calgary International Airport, Calgary sees 54.2 days of snow per year resulting in 129 cm of accumulation on average. The first snowfall of autumn for Calgary usually arrives in September or October, with the last snowfall typically happening in April or May. For about 86 days, or one-quarter of the year, Calgary has at least a centimetre of snow on the ground.


Fortunately, Calgary roofs are designed for our Northern Canadian climate and while this includes some additional snow load, the best practice is to have heavy snows promptly removed. To prevent damage to the roof and reduce the risk of injury to workers, snow removal is best left to the pros.


Your roof was designed to withstand most snow loads. Three primary factors help each rafter stand up to the load:

  • Building codes specify that rafters withstand a snow load expressed in pounds per square foot (psf). The higher the psf snow-load requirement, the deeper the rafter must be (or the more closely spaced to its neighbours). A measure of a rafter’s bending resistance is its moment of inertia (moi), or its inertial resistance to movement in the form of bending.
  • The roof deck collects the snow load and transfers the weight to the rafters. For any rafter, the portion of the roof deck that transfers this load is the tributary area. It extends outward in both directions from the center of the rafter’s thickness midway to the next rafter. Because rafters are typically spaced 16 inches on center, this amounts to 8 inches (in both directions) from the rafter’s centre line. The smaller the area, the lighter the load each rafter carries, and the stronger your roof.
  • For most roofs, the duration of a snow load is brief. Within a few days of falling, most of the snow slides off, melts, or undergoes sublimation, the process by which it is transformed from ice crystals directly into vapour.

While engineered to withstand most snow loads, there are issues that can cause concern. Uneven snow loading from large drifts can cause a roof to fail. Shallow roofs adjacent to or below taller, steeper ones are especially vulnerable to a load of snow sliding down from above. Low-sloping roofs over porches, carports, and hastily built additions (which also often have undersized rafters) can be vulnerable when the snow flies.


Previous home renovators that have removed the collar ties (located about one-third of the way down from the ridge; the supports connect the rafters and counter the spreading effect created by snow loads) to gain additional head room, dampening its moment of inertia, can place your roof and family in danger. This can lead to your rafters cracking and breaking under the load. Previous fire damage, pest infestations, rotting timbers, and even the house settling and twisting can put additional strain on your roof's ability to withstand a heavy snowfall.


When it comes to safety, snow melting and refreezing forms ice that can fall to the ground, injuring people and damaging property. It can also cause ice damming and winter leaks. If you notice a significant accumulation of ice at the eaves of your roof, do not attempt to do it yourself because you could get seriously injured or cause considerable property damage. This ice can release without warning, with great force, and inexperienced workers may damage your roof.


If you have ice damming problems, or are concerned about snow accumulation damaging your roof, contact your professional Calgary roofer, Century Roofing Limited. Whether you’ve noticed problems or simply want to extend your roof’s life, put Century Roofing Limited to work for you and call today at 403-235-5457 to schedule a free estimate for roof repair at your home or business.

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