Snow Load on Calgary Roofs

Snow load is the weight of snow on a roof, and it depends on the kind of snow as much as the depth. The situations worth paying attention to are the uneven ones, where snow piles up much deeper in one place than another. Watch for signs such as sticking doors, new ceiling cracks and a visible sag.

Key points

• Fresh dry snow is light. Snow that has settled, been rained on and refrozen weighs much more.
• A chinook can raise the weight sharply while the depth barely changes.
• Drifts form in valleys, against dormers and chimneys, on lower roofs and over additions.
• Sticking doors, new ceiling cracks, a sag, new creaking or a bowed rafter are signs to act on.
• Use a roof rake from the ground. Do not climb onto a snow-covered roof.

Weight depends on what kind of snow it is

Depth is a poor guide on its own. Fresh dry snow is light. The same depth after it has settled, been rained on during a chinook, and refrozen can weigh several times more.

That is the pattern that catches people here. A chinook partly melts the pack, water soaks into what is left, the temperature drops, and the whole thing turns into dense ice. The depth on the roof barely changed. The weight went up sharply.

Drifting is the real risk

Wind does not spread snow evenly. It scours it off open slopes and dumps it where the air slows down, which means the deep piles end up in predictable places:

• Valleys where two roof planes meet
• Against dormers, chimneys and anything else standing above the roof line
• On a lower roof section sitting below a taller wall, where snow also slides off the upper roof onto it
• Behind parapets and along the lee side of a ridge
• Over additions, porch roofs and attached garages, which are often the shallower structures

A low-slope porch or garage roof under a two-storey wall can collect several times the snow that the main roof carries. That is where to look first.

Signs worth acting on

• Doors or windows that suddenly stick when they were fine in the fall
• New cracks in drywall at the ceiling, particularly where a ceiling meets an interior wall
• A visible sag or dip in a ceiling line or a roof line
• Creaking or popping that is new, rather than the usual noises a house makes
• A rafter or truss that looks bowed when you check the attic

Any of those together with a heavily loaded roof is worth a call the same day. Standing water and a slow drip in the middle of a flat or low-slope section is also a sign that the load is holding water where it should be draining.

Clearing snow without wrecking the roof

A roof rake with a long handle, used from the ground, is the safe tool. Pull the snow down the slope rather than trying to scrape the surface clean, and leave a thin layer behind. Bare shingles are not the goal. Taking the first metre or two off the eave often helps to relieve a drift and to stop feeding an ice dam. Read more about ice dams and winter leaks.

What not to do: do not get on a snow-covered roof, do not use a metal shovel on shingles, and do not attack ice with an axe or a hammer. Shingles crack when they are cold and the damage does not show until the melt.

Deep drifts on a second-storey roof, or anything you would need a ladder in winter conditions to reach, is work to hand off rather than attempt.

If your roof loads up the same way every winter

If a section of your roof loads up the same way every winter, that is a design and drainage pattern, not bad luck, and it is worth having looked at. We offer free roofing inspections across Calgary and area.

If snow or ice has damaged the roof, see roof repair. For a number on your own roof, use the instant estimate.