The Mechanism
Heat escapes into the roof assembly
Mostly by air leakage rather than conduction — gaps around light fittings, the attic hatch, plumbing stacks, wiring penetrations, bath fan ducts, and the top plates of interior walls. Warm air rises, finds those gaps, and carries far more heat with it than passes through the insulation itself. That is why air sealing comes before adding depth.
Snow melts from underneath
The deck above the heated space rises above freezing while the outside air stays below it. The snow melts at the bottom of the pack, against the roof, on days when nothing else outside is melting at all.
Meltwater runs to a cold eave
The overhang has unheated air beneath it, so it sits at outdoor temperature. Water reaching it refreezes. So does water reaching any other cold part of the roof — which is why dams also form above unheated porches and garages.
Ice builds, water backs up
And standing water gets under roofing that was designed to shed water downhill, not to hold it. It shows up inside as a stain at the top of an exterior wall, often weeks after the dam formed.
Notice what is absent from all four steps: the roofing material.
Three Honest Claims
Metal reduces dams
True. It sheds snow, and snow that has left the roof cannot melt on it. Less fuel, smaller dams. A real and worthwhile benefit.
Ice-and-water membrane prevents damage
True, and different. Membrane at the eaves stops the backed-up water reaching your ceiling. The dam still forms; it just stops costing you plaster. This is why it is not optional here.
A metal roof prevents ice dams
False. The mechanism is heat loss and it is untouched by the covering. A badly air-sealed, badly ventilated house under a metal roof still forms dams. We have seen it.
All three of those get said by roofers. Only two of them are true, and the false one is the one that sells roofs.
In This Order, For This Reason
Air-seal the ceiling plane
The highest-value step and the one most often skipped, because it is unglamorous and invisible. Seal around light fittings, the attic hatch, plumbing stacks, wiring holes, chimney chases and the top plates of interior walls. A contractor who does blower-door testing is measuring where the air actually goes rather than guessing.
Insulate properly, especially at the eaves
Adding depth over unsealed penetrations is a partial fix. Once sealed, depth matters — and the eaves are where insulation is habitually thinnest and most compressed, which is precisely where the dam forms.
Ventilate so the deck stays cold
Soffit intake and ridge exhaust, with a clear path between them. Insulation stuffed into the eave blocking the soffit vents is one of the most common faults there is. On a cathedral ceiling with no ventilation path this gets harder and may need a different assembly strategy.
Then roof
With ice-and-water membrane at the eaves and valleys, and metal if it suits the building. Now you are buying reduction and protection on top of a cause that has been dealt with, rather than instead of it.
If you are re-roofing anyway, the ventilation work belongs in that job. The deck is open, the access will never be better, and it will never be cheaper. Bolting it on afterwards means paying for access twice.
Working Out Which Kind You Have
The single best indicator: does snow melt off your roof while it is still lying on the ground? If yes, the roof surface is above freezing while the air is below it — which can only mean heat from the house. That one observation is worth more than any amount of speculation.
Whole-roof dams along the entire eave line usually mean general heat loss: a warm attic, inadequate insulation, poor ventilation, or all three.
Localised dams over one area usually mean one identifiable thing — a bath fan venting into the attic rather than outside, a cluster of recessed lights, a dropped soffit over a bay window, a section where insulation was disturbed during other work, or a cathedral ceiling section with no ventilation path. These are frequently the cheapest of all to fix.
Valley dams can be partly geometric: valleys collect snow and meltwater from two planes, so they build ice more readily even with modest heat loss.
The ice dam check walks through this in five questions.
Getting Through This Winter
- Rake from the ground with a proper roof rake, clearing the first few feet above the eave after a heavy fall. You are removing the fuel closest to the cold edge, not clearing the whole roof.
- Do not get on the roof. People are seriously injured doing this every winter in this region, and a metal roof under snow is as slick as anything you will stand on.
- Do not chip at ice with a hammer, shovel or axe. You will damage the roofing, the flashing or yourself, and it returns within a week.
- Watch for the power service drop before raking. It often runs near an eave and an aluminium rake is an excellent conductor.
- Heat cables keep a channel open so water escapes. A reasonable stopgap while you arrange the real work; not a repair.
- If water is coming in, clearing snow from above the dam reduces the meltwater being produced, which is the useful emergency measure.