Snow Does Not Land Evenly on a Flat Roof

Snow arrives in Metro Vancouver every few years in a quantity that matters, and every time it does, the same conversation happens in strata offices across the region. Somebody looks out at the roof, or at a photograph of it, and decides whether it needs clearing.

That decision is usually made on the wrong number.

The field depth is not the load

Snow does not settle evenly across a flat roof. Wind picks it up and redeposits it, and it drops it wherever something interrupts the flow of air across the surface.

The result is that a roof with a modest, uniform-looking cover across its middle can be carrying several times that depth in specific locations. Structural design accounts for this explicitly, because the drifted load rather than the uniform load is what governs on most low-slope roofs.

The places where it accumulates are predictable:

Against parapets, on the side sheltered from the prevailing wind. Every flat roof in this region has parapets, which is one reason the problem is universal here rather than occasional.

At roof steps, where a lower roof section meets a taller wall. This is the classic drift location and frequently the highest load on a building.

Behind mechanical equipment, screens, and the kinds of rooftop installations discussed in the post on rooftop equipment and membrane penetrations. Every unit added to a roof creates a new drift location that was not in the original design.

In valleys between roof sections, and in any recessed area of a complex roof geometry.

Someone assessing the roof from a doorway sees the field. The load is at the edges and behind things.

Coastal snow is heavy

The weight of snow comes from its water content, not its depth. Dry continental snow at a given depth weighs a fraction of what wet coastal snow weighs at the same depth.

Metro Vancouver gets the wet kind. Snow here falls at temperatures near zero, it contains substantially more water than the same depth would in a colder region, and it compacts under its own weight into something denser still.

Then it rains on it. This is the sequence that produces the highest roof loads in this climate: a snowfall, followed within a day or two by a Pacific system delivering rain onto a snow pack that absorbs and holds it. The depth does not increase. The load does, substantially.

That combination is common enough here that it should be the planning assumption rather than the worst case. The atmospheric river patterns described in the post on atmospheric rivers and building envelopes do not stop because there is snow on the roof.

Drains first

If a roof is going to have work done on it during a snow event, clearing the drains and the area around them is the highest-value action available.

A drain buried under snow, or blocked by ice at its inlet, means meltwater has nowhere to go. As the snow melts, water accumulates on the roof, and water at a given depth is considerably heavier than the snow that produced it. A roof that was managing its snow load can be overloaded by the melt if the water cannot leave.

Ponding also does what ponding always does, which is the deterioration mechanism set out in the guide on ponding water warning signs. The freeze-thaw cycling on a roof with standing water and near-zero temperatures accelerates it further.

Overflow scuppers matter here too. They are the redundancy that keeps a blocked primary drain from becoming a structural problem, and they are frequently the first thing to ice up.

Snow removal damages roofs

This needs saying plainly, because the damage is invisible at the time and shows up months later as a leak that nobody connects to the snow.

Metal shovels cut membrane. Ice chippers puncture it. Heavy equipment tears it at seams and flashings. Anyone clearing a roof is standing on the waterproofing, and the tools that work fastest on snow are the ones that work fastest on membrane.

Snow removal done properly uses plastic tools, leaves a residual layer of snow in place rather than scraping to the surface, avoids concentrating removed snow in piles that create new point loads, and treats every flashing, drain, and penetration as an obstacle to work around rather than through.

Ice is worse than snow in this respect, because removing it mechanically almost always means removing some of what is underneath. Where ice has bonded to a membrane surface, leaving it and managing the drainage around it is usually the better decision.

When removal is the right call, and who decides

Snow removal is not automatically correct. The work carries real risk to the membrane and to the people doing it, on a wet, cold, slippery surface at height, and those risks are not hypothetical.

Where there is a genuine structural concern, the decision belongs to a structural engineer who can assess the specific roof, its design capacity, and the actual accumulated load. That is a different question from whether the roof looks snowy.

The signs that escalate this immediately, at any hour, are structural rather than roofing: doors that stick or frames that visibly distort, new cracking in interior finishes, any visible sag in a ceiling, and unusual sounds from the structure. Any of those during or after a snow event means clearing the affected area and getting an engineer, not scheduling an inspection for the following week.

Ice dams and where they form

Snow that melts and refreezes creates ice, and ice on a roof concentrates in the places that cause the most trouble.

Meltwater running to a cold roof edge, a scupper, or a drain inlet freezes there and builds up. The ice then blocks the drainage path, and subsequent meltwater backs up behind it. On a flat roof with parapets, that produces standing water held by an ice dam at the only outlet.

Water backed up on a roof reaches heights the membrane upturn was never designed for, which is the reason the base flashing height described in the post on roof-to-wall junction failures matters. A roof that performs in every rainfall event can leak during a thaw because the water level is higher than it has ever been.

Heat trace at drains and scuppers is one response and it requires the electrical provision to be there. Keeping the drain area clear is the other, and it is the one available to every building.

Access and safety during a snow event

Anyone going onto a roof during or after snow is working on a slippery surface at height, frequently with obscured hazards.

Skylights, roof openings, and changes in level all disappear under snow. The fall protection anchors described in the post on anchor load testing may themselves be buried and unreachable, and the roof hatch may be frozen or blocked.

That means roof access during snow is a planned operation with a defined access route, known hazard locations, and fall protection arranged in advance. It is not a caretaker with a shovel deciding to have a look.

For most stratas the correct answer during a significant snow event is that nobody goes onto the roof without a specific reason and proper arrangements, and that the reason is established by someone qualified to judge whether the load warrants the risk. The buildings that have an incident during a snow event usually had one person making that decision alone, in a hurry, on the morning it snowed.

What to do before the snow

The useful preparation is unglamorous and takes an afternoon.

Know where the drains are and confirm they are clear before winter, which is standard practice under the approach in the post on fall building maintenance for strata. Know where the roof steps and drift locations are on your specific building. Know who you would call, and have their number somewhere other than in one person’s phone.

Have a decision made in advance about who authorizes roof access during a snow event and under what conditions, because the alternative is making that decision at seven in the morning with a caretaker already on the roof.

The buildings that handle snow badly in this region are rarely the ones with structural problems. They are the ones where nobody had thought about it until it was on the roof.

Sources and further reading

• National Research Council Canada, National Building Code of Canada, snow load and drift provisions, referenced 2026
• Canadian Roofing Contractors Association, CRCA Roofing Specifications Manual, winter roof maintenance guidance, referenced 2026
• WorkSafeBC, Occupational Health and Safety Regulation Part 11, fall protection requirements for roof work, referenced 2026
• Allweathercoating roof service records, Metro Vancouver, 2026