Every January we get the same call. Water is coming through a bedroom ceiling, there is a ridge of ice along the gutter, and the homeowner wants to know which roofer sold them a bad roof. Almost always, the roof is fine. The problem is above the ceiling, not on top of the shingles.
How an ice dam actually forms
Heat escapes from the living space into the attic. That warm attic air heats the underside of the roof deck, and the snow sitting on the outside of that deck begins to melt from below. The meltwater runs down the slope until it reaches the eave — the last few feet of roof that overhang the exterior wall and have no heated space beneath them. That section stays at outdoor temperature, so the water refreezes there and builds a ridge of ice.
Once the ridge exists, the next round of meltwater has nowhere to go. It pools behind the ice and sits there. Asphalt shingles are designed to shed water travelling downhill; they are not designed to hold back standing water. It works its way under the shingle courses, through the nail holes, and into the ceiling below.
Why a new roof does not fix it
This is the part that costs homeowners the most money. A roof replacement puts new shingles over the same attic, with the same heat loss and the same cold eave. The following winter, the same ice ridge forms in the same place.
What a properly built roof does give you is protection when water does back up. Ice-and-water shield is a self-adhering membrane that goes down along the eaves and in the valleys before the shingles. It seals around fasteners and holds back standing water. Massachusetts code requires it, and any roof we replace gets generous coverage — but it is insurance against the symptom, not a cure for the cause.
What actually stops ice dams
Three things, in this order.
Air sealing. Warm indoor air leaks into the attic through recessed light housings, the gaps around the chimney chase, bath fan penetrations, the attic hatch, and the top plates of interior walls. Sealing those penetrations is the single highest-value step, and it is usually the cheapest.
Insulation. Once the air leaks are sealed, insulation slows the remaining heat transfer. Older Massachusetts homes frequently have six or eight inches of settled fiberglass where current practice calls for substantially more. Insulation on its own, without air sealing, disappoints people constantly — moving air goes straight through it.
Ventilation. Soffit vents at the eaves and a ridge vent at the peak let outside air wash the underside of the deck and carry away whatever heat still gets through. The common failure we find is soffit vents packed solid with insulation, which turns the whole system off.
What to do while it is happening
If water is already coming in, the goal is to relieve the pressure, not to win the argument with the ice. Clearing the lower few feet of snow off the roof with a roof rake, from the ground, interrupts the supply of meltwater. Do not go up on an icy roof, and do not attack the ice with an axe or a hammer — we have replaced a lot of shingles that were destroyed by someone trying to save them.
Calcium chloride in a fabric sleeve laid across the dam will melt a channel through it and let the trapped water drain. It is slow and it is a stopgap, but it beats a ceiling coming down.
The honest summary
If a contractor quotes you a roof replacement as the solution to ice dams and never mentions your attic, get another opinion. The roof matters — ice-and-water shield, correct eave detail, clean ventilation — but the cause is heat, and heat is dealt with from the inside.
Getting ice dams every winter?
We inspect the attic as well as the roof, in Boston, Cambridge, Somerville, Quincy, Newton and the surrounding towns.