Ice Dam Prevention in Calgary: Why the Same Roofs Fail Every Winter

The physics, in plain language

The National Research Council of Canada laid this out in Canadian Building Digest 89, Ice on Roofs: ice damming needs snow on a sloping roof, outside air below freezing, and a source of heat (or solar radiation) melting the snow from underneath. CMHC’s ice damming research reduces it to three ingredients:

1. Snow on the roof.
2. Upper roof areas at or above melting temperature.
3. Lower roof areas below freezing.

Remove any one and you get no dam. The heat for ingredient two almost always comes from inside your house: warm air leaking through the ceiling into the attic, plus conducted heat through thin or gapped insulation. It warms the sheathing, the snow in contact with the deck melts, the water runs down to the overhang — which is not warmed by the attic and sits at outdoor temperature — and freezes there.

Natural Resources Canada adds the counter-intuitive proof: a completely uninsulated attic usually has no ice dams, because the roof is so warm snow melts as it lands and never accumulates. A well-sealed, well-insulated attic also has no ice dams, because the roof stays cold. It is the houses in between — partly insulated, leaky at the top plates — that dam up every year.

Why the same houses, every winter

• Storey-and-a-half and split-level roofs. NRCan describes the classic melt line where a knee wall meets the ceiling. Calgary’s older stock is full of them.
• Thin insulation at the eave. Where a baffle is needed to keep the vent path open, there may only be about 100 mm of room left for insulation — a cold-weather hot spot right above the coldest part of the roof.
• Complicated roofs. Valleys, dormers and big overhangs mean more thermal bridging and more places for ice to start.
• Point sources of heat. A leaky attic hatch, a bath fan discharging into the attic, ducting in the attic, or old incandescent pot lights. The City of Calgary specifically recommends replacing incandescent fixtures below the attic with LEDs because they generate enough heat to melt roof snow.
• Blocked eavestrough. Full troughs freeze solid first, giving the dam a foundation.

The damage path

In the order it usually happens:

• Water under the shingles onto the underlayment and eave membrane. If either is old, torn or absent, it reaches the sheathing.
• Soaked and rotting sheathing at the eave, then rot in the fascia, sub-fascia and the truss tails.
• Wet insulation, which loses R-value, which warms the attic further and makes the next dam bigger.
• Stained ceilings and walls, lifting drywall tape, peeling paint on the interior of exterior walls, and mould in the insulation and on the sheathing.
• Failed eavestrough. Ice weight pulls hangers, deforms the trough and separates the seams; downspouts split.
• Falling ice. NRC notes both the structural weight of ice at the eaves and the hazard of falling icicles over a walkway or driveway.

On the insurance side, the Insurance Bureau of Canada notes that policies commonly respond to sudden and accidental water damage, while ongoing or repeated damage is usually treated as a maintenance issue and may not be covered. Translation: the third winter of the same ice dam is your problem, not your insurer’s.

What actually fixes it, permanently

Make the roof deck cold and keep it cold. Same three steps, in this order:

1. Air seal the attic floor. Top plates of interior and exterior walls, plumbing stacks, wiring, bath fan housings, chimney chases with a non-combustible fire stop, dropped ceilings, party walls, and a weatherstripped, insulated attic hatch. Every bath and kitchen fan gets ducted out through solid insulated metal duct with sealed joints, sloped to the outside, discharging clear of the soffit vents.
2. Insulate to R-50–R-60, with the eave detail done right. CMHC’s floor is at least 300 mm of loose fill or batts. The part that stops ice dams is the outer edge: baffles in every truss bay, then the best insulation you can fit above the top plate — spray foam or extruded polystyrene held 25 mm off the sheathing with insulation packed behind it, as NRCan details. That is the core of an attic insulation upgrade.
3. Confirm venting meets code. The National Building Code – 2023 Alberta Edition, in force provincially per the Government of Alberta, requires unobstructed vent area of at least 1/300 of the insulated ceiling area (1/150 for slopes under 1 in 6 or joist-framed roofs), with at least 25% of the openings high, at least 25% low, and 63 mm of clear space above the insulation.

When the roof is due anyway, this work is cheapest done together. At tear-off we can see and correct the eave: proper eave protection membrane — code requires it from the roof edge at least 900 mm up the slope and to a line at least 300 mm inside the inner face of the exterior wall — new drip edge, replacement of any rotten sheathing and fascia, and clean intake at the soffit. If the sheathing is soft, a residential roof replacement with the attic corrected in the same job is far cheaper than doing it twice.

Emergency removal, done safely

If water is coming in right now:

• Protect the inside first — buckets, move contents, drain a bulging ceiling through one small controlled hole.
• Work from the ground with a roof rake on an extension handle, pulling snow off in strips. Stay off icy ladders and clear of power lines.
• Clear the last 1–2 m of roof above the eave, which is the part that matters.
• The City of Calgary’s method for an existing dam is to make holes through the ice to expose the roof and place de-icer in each hole. Do not swing at it.
• Call for steam removal if the dam is thick, over an entryway, or you cannot reach it. Low-pressure steam is the only method that removes ice without damaging shingles.

What NOT to do

• Do not chip, chisel or axe the ice. CMHC puts it bluntly — attacking ice dams with an axe or ice pick is a good way to shorten shingle life, and a good opportunity to fall off a ladder.
• Do not spread rock salt or ice melter on shingles. It corrodes flashings, fasteners and eavestrough, damages plants below, and CMHC notes the corrosion and environmental damage risk even with the stocking-in-the-gutter trick.
• Do not use a pressure washer, torch or heat gun. You will strip granules, melt sealant or start a fire.
• Do not climb an iced roof. Nothing up there is worth a fall.