Ice Dams on Metal Roofs: How They Form and What Metal Changes
An ice dam needs three things: snow on the roof, a warm deck above the heated space, and an eave below freezing. Metal changes two of them — and leaves the third entirely to you.
Ice damming is the most misdiagnosed roofing problem in Ontario. Homeowners see ice at the eave and conclude the roof has failed. The roof is usually fine. What has failed is the temperature profile across the roof surface, and that is produced by heat loss from the house — which means a new roof of any material, installed over an unchanged attic, will produce the same ice next winter with better waterproofing beneath it.
Three Ingredients, All Required
An ice dam needs snow sitting on the roof, a portion of the roof deck warm enough to melt the bottom of that snow, and a portion further down cold enough to refreeze the meltwater. Remove any one and there is no dam. This is why they form on some houses and not on the identical house next door, and why they form on one section of a roof and not another. The pattern of heat loss decides it.
How It Builds, Step by Step
The sequence is the same every time, and understanding it points directly at the fixes.
- ✓Snow accumulates and forms an insulating blanket over the roof, so the deck under it warms more easily than bare roof would.
- ✓Heat escapes from the heated space below — through insulation gaps, and much faster through air leaks in the ceiling — and warms the deck above the living area past zero.
- ✓The bottom layer of the snowpack melts against the warm deck and the water runs down the slope beneath the remaining snow.
- ✓The water reaches the eave and the overhang, which is over unheated space with outdoor air on both sides, and it is below freezing there.
- ✓It freezes. The next melt cycle freezes on top of it. A ridge of ice builds along the eave and grows upward over days and weeks.
- ✓Meltwater now pools behind the dam instead of draining, and it stays liquid because it is on the warm part of the roof.
- ✓Standing water finds any path available — a seam, a fastener, a lap, a valley, a flashing termination — and enters the assembly.
- ✓Water appears on a ceiling or down an interior wall during a mild spell, which is why it reads as a leak that only happens after a thaw.
It Is a Heat-Loss Problem First
Everything after the second step is a consequence. The deck is warm because heat and warm air are moving from the house into the roof assembly. The usual sources are air leakage at pot lights, the attic hatch, top plates and chases; insulation that is thin or missing at the perimeter where framing is shallow; blocked soffit intake; ducts in the attic; and in half-storey houses, uninsulated knee wall floors. Fixing those changes the surface temperature of the roof, which is the actual variable.
What Metal Changes
Metal genuinely helps, and it is worth being precise about how. First, snow sheds — often before it has had time to sit and melt from below — which removes the first ingredient for large parts of the winter. Second, the surface is smooth and continuous from ridge to eave, so even when ice does form it has no granules to grip and no shingle laps to be forced under; water sitting on a metal panel is sitting on a continuous surface rather than on an overlapping one. Third, the panels shed water faster once melting starts. Together these substantially reduce the frequency and severity of damming compared with an asphalt roof over the same attic.
Where Metal Can Still Dam
What metal does not do is change the heat loss below it. And there are specific locations on a metal roof where snow stays put long enough for the process to run.
- ✓Above snow retention rows, which are designed to hold snow on the roof — this is the trade-off that comes with retention.
- ✓In valleys, where snow from two planes converges, shades itself and consolidates.
- ✓Behind dormers, chimneys and other obstructions that block sliding and create drift zones.
- ✓On north-facing planes and roofs shaded by trees or neighbouring buildings, which get little solar gain to trigger shedding.
- ✓On low-slope sections and porch roofs, where the sliding force is small.
- ✓Along the eave above eavestroughs, where a trough full of ice provides a starting ledge for a dam to build on.
The Defences, in Order of Importance
Air seal the ceiling plane, because air leakage carries the most heat and the most moisture. Get insulation depth uniform and carry it as far out over the exterior wall as the framing allows, using baffles to protect the vent path. Open and balance the ventilation so the deck runs close to outdoor temperature. Then treat the roof as the last line of defence: self-adhered ice and water membrane at the eaves, in the valleys and around penetrations, extending well up the slope past the interior wall line; correct flashing at roof-to-wall junctions; and eave details built for the possibility of standing water. The membrane does not prevent a dam. It prevents the dam from becoming an interior leak.
A useful way to think about it: insulation, air sealing and ventilation determine whether ice forms. Underlayment and flashing determine whether that ice causes damage. Both are worth doing, and only one of them can be done from the roof.
What Not to Do
Do not chip, hammer or chisel ice off a roof; on metal it will mark the finish and can distort a panel edge. Do not put salt or ice melt in eavestroughs or on the roof surface — chlorides are corrosive to metal components and fasteners, and the runoff kills landscaping below. Be cautious with roof rakes: a metal edge dragged down a coated panel scratches it, and a person pulling snow toward themselves is standing exactly where a slab will land.
Frequently Asked Questions
Do metal roofs prevent ice dams?
They substantially reduce them, because snow sheds instead of sitting and there are no granules or shingle laps for water to be forced under. They do not eliminate the cause. If the roof deck is being warmed by heat escaping from the house, ice can still form where snow is held — above retention rows, in valleys, on shaded slopes and over unheated overhangs.
Why do I get ice at the eave but nowhere else?
That is the signature pattern. The roof over the heated space is warm enough to melt the base of the snowpack, and the overhang past the exterior wall has outdoor air on both sides and stays below freezing. The water crosses that boundary and freezes. It is a map of where your insulated envelope ends.
Does ice and water shield stop ice dams?
No, it stops the leak. A self-adhered membrane at the eaves and valleys is a waterproofing layer that keeps water that has pooled behind a dam from entering the assembly. It is important and it should be installed, but it is protection against the consequence, not a cure for the cause.
Will more attic ventilation fix my ice dams?
It is part of the answer, but only if the intake is genuinely open and the ceiling is reasonably airtight. Adding exhaust to an attic with blocked soffits pulls more warm house air into the attic and can make the melting worse. The order is air sealing, then insulation, then balanced ventilation.
I have ice dams on a new roof. Was the roof installed wrong?
Not necessarily. A re-roof replaces the waterproofing but does not change the insulation, air sealing or heat loss of the house unless that work was included. If the melting pattern that caused the dams is unchanged, dams will recur. What should be different is that the water no longer reaches the interior, thanks to eave and valley membrane.
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