How much snow is too much on a Markham roof?

Depth Is the Wrong Measurement

Homeowners look out the window and judge the roof by how deep the snow looks. That is the wrong instinct, and it is the reason roof problems tend to appear in February rather than during the storm.

Twenty centimetres of fresh powder and twenty centimetres of snow that fell three weeks ago, thawed slightly, refroze and then had rain fall on it are the same picture from the driveway and a completely different load on the structure. The second one can weigh over four times the first.

Michael John Lau, a REALTOR® with the Kaizen Real Estate Team in Markham, Ontario and a two-time eXp ICON Agent, raises this with owners of older homes in particular, because the failures that happen are almost never on the night of the heaviest snowfall. They happen in the third week of a cold February when nobody has thought about the roof in a month.

How Much Snow Weighs

Rain falling onto an existing snow pack is the worst case, because the snow absorbs and holds the water rather than shedding it. A mid-winter rain event on a loaded roof deserves attention in a way that a snowfall alone often does not.

The Warning Signs

An Ontario municipal roof snow load advisory lists the signals that matter. These are worth knowing because they are noticeable from inside the house.

• New cracks forming in ceilings or walls, particularly during or after heavy snowfall
• Doors or windows becoming difficult to open or close, which can indicate frame distortion
• New or repeated popping and creaking noises when the structure is under load
• Bowing or deflection of the roof sheathing, visible from inside the attic
• New water leaks, drips or mould in ceilings or the attic
• Visible sagging along the ridge line seen from outside

A door that suddenly sticks in February is the one people dismiss. It is also one of the clearest early indicators, because a frame only distorts when something above it has moved.

Where Roofs Actually Fail

Uniform snow spread evenly over a simple sloped roof is what the design load contemplates. It is rarely what causes a failure.

Failures concentrate at drifts, unbalanced loads, valleys, and places where snow slides off an upper roof onto a lower one. A wind that strips one side of a roof and piles the other is more dangerous than an even blanket, because the structure is being loaded in a way the even case does not test.

Ontario's snow load calculation also treats wider roofs less favourably than narrow ones, which is worth knowing for anyone with a wide or low-slope roof, or a flat roof with parapets and rooftop equipment where snow has somewhere to pile against.

A note on numbers: ground snow load values are published per municipality in the Ontario Building Code's supplementary standard, and they vary meaningfully across the province. Anyone who needs Markham's specific value should get it from that standard or from an engineer rather than from a general article, this one included.

Ice Dams

Ice dams are the more common and more expensive problem, and they are caused by the house rather than by the weather.

Heat escaping from the living space into the attic warms the underside of the roof. Snow on the warm section melts. The meltwater runs down to the eaves, which sit over unheated space and stay cold, and refreezes there. That ridge of ice becomes a dam. Water then backs up behind it and is forced under the shingles and into the house. Rain freezing on top of an existing snow pack produces the same result.

Prevention

• Adequate attic insulation, so less heat reaches the roof deck
• Adequate attic ventilation, so what heat does escape is carried away
• Clear gutters and downspouts before winter, so meltwater has somewhere to go
• Professionally installed heating cables where a chronic problem exists
• Strategically applied de-icers, used carefully

The insulation and ventilation are the actual fix. Everything else manages the symptom.

Should You Clear the Roof?

Sometimes, and carefully.

Ontario guidance suggests considering removal at roughly 60 to 90 cm of accumulation, and is specific about method:

• Remove snow gradually and in layers.
• Never strip one section while leaving another fully loaded, which creates exactly the unbalanced condition that causes failures.
• Do not clear down to the roof surface. Leaving a few centimetres protects the shingles.
• Keep drainage paths open.
• When in doubt, hire insured professionals.

A snow rake with an extendable arm, used from the ground, is the reasonable homeowner tool. A ladder on ice in February is not. Clearing to bare shingles also risks damaging them and can affect a roofing warranty.

What Insurance Covers

Generally covered, as sudden events beyond the homeowner's control: windstorm, hail, falling objects, lightning, ice storms, and structural failure from the weight of ice and snow.

Generally excluded: wear and tear, such as shingles curling, cracking or losing granules with age; maintenance neglect, such as clogged gutters or unrepaired flashing; gradual leaks developing over weeks or months; and mould from ongoing moisture.

Ice dams sit in between, and that is the practical point for homeowners. Damage tends to be covered where it follows an unusually severe weather event and the home was properly maintained. It tends to be challenged where an adjuster finds long-standing attic ventilation problems, inadequate insulation, or pre-existing roof damage, and concludes that neglect caused the dam rather than the weather.

Roof age also affects the payout. A newer roof is more likely to be settled at replacement cost. An older one may be settled at actual cash value, which accounts for depreciation and can pay a fraction of the replacement figure. Asphalt shingles generally last fifteen to twenty years in Ontario.

What This Means in a Transaction

Buying

• Ask the age of the roof and get it in writing. It affects both insurability and the eventual claim settlement.
• Have the attic inspected, not just the shingles. Insulation and ventilation are where ice dam problems live.
• Look for staining at the eaves and in the attic near the eaves, which indicates past ice damming.
• Buying in winter means the roof is under snow. That is a reason to ask more questions, not fewer.

Selling

• Keep receipts for roofing, insulation and ventilation work. Documentation is worth money at the negotiating table.
• Clear the gutters before listing.
• Address visible eave staining rather than hoping it goes unnoticed. Buyers notice, and unexplained staining costs more in perceived risk than the repair costs to fix.

Michael John Lau, REALTOR® · Markham, Ontario · (416) 700-0286
Disclaimer. Michael John Lau is a licensed REALTOR® serving buyers and sellers in Markham, Ontario and the Greater Toronto Area. Guidance is current to September 2026 and reflects Ontario municipal and industry sources. This article is general information and is not engineering, roofing or insurance advice. Ground snow load values are published by municipality in the Ontario Building Code supplementary standard, and structural questions should be directed to a qualified engineer.