Snow loads on roofs: when to worry

Roofs in Canada are designed to carry snow, and most come through normal winters without trouble. The risk rises with unusual conditions: heavy wet snow, rain on snow, deep drifts against walls and equipment, and ice from blocked drains. Knowing where load builds up, and the warning signs of overload, tells you when to act and when to call in a qualified assessment.

How roofs are designed for snow

The National Building Code of Canada, and the provincial codes based on it, set design snow loads using climate data for each location, adjusted for the roof's shape, exposure, slope and surroundings. The design also accounts for rain falling on snow. Because these values vary widely between places, a roof in one city can be designed for a very different load than the same roof elsewhere.

Three things are worth knowing:

• Older buildings were designed to the code of their time, which may differ from current requirements.
• Changes since construction, such as new rooftop units, solar panels, a taller neighbouring building or an addition, can change how snow accumulates.
• Condition matters. Corrosion, rot, damaged connections or overloaded framing reduce the capacity a roof was built with.

Where snow load concentrates

Average depth across a roof tells you little. Load piles up in specific places:

• Drifts at steps in roof height: where a low roof meets a taller wall, wind deposits deep drifts on the lower roof.
• Against parapets and rooftop equipment: snow builds up on the downwind side.
• Valleys: on sloped roofs, snow slides and collects where two slopes meet.
• Sliding snow: snow sliding off a higher sloped roof can land on a lower one.
• Ice and ponds: blocked drains let meltwater freeze into heavy ice. See roof drains and ponding water.

Wet snow, rain and ice

The weight of snow depends greatly on its density. Fresh, dry snow is light. Settled, wind-packed and wet snow is much heavier, and ice is heavier again. A late-season storm, or rain falling on an existing snowpack, can add significant load without much change in depth. That is why depth alone is a poor guide to weight.

Warning signs of overload

Watch for these, especially after heavy snowfall, rain on snow or a thaw and refreeze:

• Visible sagging of the roof line or ceiling
• New cracks in walls or ceilings, especially near the top of walls
• Doors and windows on upper floors that suddenly stick or won't latch
• Creaking, popping or cracking sounds from the roof structure
• Sprinkler heads pushed down below the ceiling line
• Water leaks appearing during cold weather
• Bent or twisted steel joists or bracing, where visible

Removing snow safely

Snow removal from roofs is hazardous work. Falls from roofs, through skylights and through hidden openings are a serious risk, and removing snow unevenly can make the load worse.

• Use trained workers or a contractor experienced in roof snow removal.
• Plan fall protection for the roof's edges, skylights and openings before anyone goes up. See the fall protection hierarchy.
• Know where skylights, drains, vents and equipment are before snow hides them. Mark them.
• Remove snow in a balanced pattern so you don't create uneven loads, and don't pile it in one area of the roof.
• Don't cut into or scrape the membrane; leave a thin layer of snow on the roof.
• Keep the area below clear of people and vehicles.

For homes, a roof rake used from the ground can clear the lower part of a sloped roof without climbing. Stay clear of power lines.

Planning before winter

• Know your roof: structure type, age, and any changes since it was built
• Clear drains and scuppers in fall
• Identify drift-prone areas from past winters
• Mark skylights and openings so they are visible under snow
• Arrange a snow removal contractor and a trigger for calling them
• Know who to call for a structural assessment if signs appear

How ehZee can help

We can inspect roofs before and after winter, identify drift and ice areas, check drainage and look for signs of past overload or structural distress. Where a structural capacity question arises, we recommend involving a structural engineer. See our roof inspections, or call 1-833-888-0091.

Common questions