Ice dam

Frequently asked questions

Why does an ice dam form at the eaves and not on the rest of the roof?

Because the eaves are the only part of the roof with nothing warm under them. Heat escaping from the house into the loft warms the underside of the roof deck over the heated rooms, and snow lying on that part melts from below even while the air stays well below freezing. The meltwater runs down the slope until it crosses the wall line and reaches the OVERHANG, which has outside air both above and below it and is therefore at air temperature. There it refreezes, and the ice that accumulates dams the water still arriving from above. Ponded water sits behind the dam, finds the laps in the covering — which shed water flowing downhill and are not designed to hold standing water — and runs back under them into the structure. The damage shows as stains on the ceiling at the outside wall, which is why an ice dam is so often diagnosed as a roof leak and repaired by replacing shingles that were never faulty.

Is the membrane the fix?

It is the insurance, not the cure, and the distinction matters because the membrane is the part people buy. A self-adhering ice barrier laid from the eaves up past the wall line seals around the fixings through it and keeps ponded water out of the structure when a dam forms. What it does not do is stop the dam forming, and a roof that keeps forming them still loses heat, still grows ice on the overhang, and still drops that ice on whatever is below. The actual cure is to stop the heat reaching the deck: air-seal the ceiling so warm air is not leaking into the loft, insulate it to the depth the climate needs, and ventilate the loft so that the deck runs close to outside temperature. The membrane and the heat control are not alternatives — codes require the membrane because heat control is imperfect in practice — but a specification that provides only the membrane has treated the symptom.

What does a British roof do instead?

Nothing, and mostly it does not need to — which is the point worth carrying across. The British detail at an eaves is an underlay carried into the gutter over a support tray, designed to drain any water that gets past the tiles back OUT, and a ventilated or breathable roof build-up behind it. It assumes water flows; it does not assume water stands. That assumption holds where sustained freezing with snow cover is rare, and stops holding in a hard winter, when the same houses do produce ice at the gutter and occasional leaks at the wall head. The practical consequences run both ways. An American or Canadian detail applied in Britain adds a self-adhering membrane that is rarely required and can trap moisture in a construction designed to breathe. A British detail applied in a cold-winter climate omits a code requirement — which is the more serious direction, because a missing eave membrane is invisible until the first thaw.