Ice at the eaves, and what causes it

What is actually happening

Snow sits on the roof. The part of the roof over the heated house is slightly warm, because heat is getting up into the attic and through the deck. The snow on that part melts from underneath, and the water runs down the roof.

Then it reaches the overhang. The eave sticks out past the wall of the house, so there is nothing warm beneath it. The water hits that cold section and freezes. It does that again the next day and the day after, and the ridge of ice grows until melt water is standing behind it in a pool.

Shingles are designed to shed water running downhill. They are not designed to hold back standing water, so it works its way up under the courses, through the deck, and into the ceiling below or down the inside of the wall. That is why the stain shows up on an interior wall in January after a dry week.

The key point: the roof did not fail. The house leaked heat.

Fixing it in the right order

There is an order to this and doing it out of order wastes money.

Air sealing comes first. Gaps around recessed lights, bath fan housings, the attic hatch, plumbing stacks, top plates of interior walls and the chase around a chimney let warm moist house air straight into the attic. That air carries far more heat than conduction through insulation does. Sealing is cheap, unglamorous and by far the highest return.

Insulation second. Once the air leaks are closed, the insulation actually does its job, and topping it up to a proper depth across the whole attic floor, including out over the wall plates where it is usually thin, keeps the roof deck cold.

Ventilation third. Balanced intake at the soffit and exhaust at the ridge flushes away what heat does get through. It only works if the soffits are actually open, which in a lot of houses they are not, because somebody insulated over them.

Anyone quoting a ridge vent as a standalone fix for ice dams has skipped the first two steps.

What the ice barrier does

The residential code requires an ice barrier at the eaves in regions with a history of ice forming along them, and the standard form of it is a self adhering membrane running from the lowest edge of the roof to a point a set distance inside the exterior wall line. The exact edition of the code your city has adopted decides the details, so ask the contractor which one they are working to.

Be clear about what that membrane is for. It does not prevent the dam. It makes the roof watertight in the area where standing water is likely, so that when the dam forms, the water sits on a sealed surface instead of finding its way through. It buys you a great deal and it does not address the cause.

If a roof is being replaced on a house that dams, the membrane should go on and the attic work should be booked for the same year. Doing the roof and leaving the attic is how people end up doing this twice.

The insurance side

Storm damage and ice dam damage are treated very differently. Interior water damage from a dam may be covered depending on the policy, but the underlying cause, a house losing heat into its own attic, is maintenance, and repeated claims for the same thing are a conversation nobody enjoys.

This is the opposite situation from hail and wind, where the cause is plainly external and the claim is the normal route.