Roof defect
What is an ice dam?
A ridge of ice at the edge of the roof that holds back meltwater until it finds its way under the shingles. The roof is the victim; the attic is the cause.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionDefect
Ice dam — a ridge of ice that forms along the eave of a sloped roof when snow on the warmer upper roof melts, runs down and refreezes on the cold overhang, so that later meltwater ponds behind the ridge and backs up under the roof covering; Michigan State University Extension describes the mechanism and its cures, and the 2021 IRC, Section R905.1.2 requires an ice barrier at the eaves where ice dams have a history.
What it is
The chain of events runs from inside out. Heat from the house leaks into the attic — through ceiling gaps, recessed lights, ductwork, a poorly insulated hatch — and warms the roof deck above the heated space. Snow on that part of the roof melts from underneath. The water runs down under the snow to the overhang, which is outside the heated space and stays below freezing, and there it freezes. Each day adds a layer; the ridge grows; meltwater collects behind it in a pool that has nowhere to go but up, under the shingles, which were never designed to hold standing water. MSU Extension sets out the sequence and the three remedies: seal the ceiling against air leaks, insulate the attic floor, and ventilate the attic so the deck stays as cold as the eave.
The code's answer is a last line of defence, not a cure. The IRC, Section R905.1.2 requires an ice barrier — two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet — from the lowest edge of the roof to at least 24 inches inside the exterior wall line, in areas where ice has formed along the eaves and caused water to back up. It keeps the ponded water out of the house; it does not stop the dam.
Why it matters
Ice dams do their damage quietly: stained ceilings at the outside walls, wet insulation that stops insulating, peeling paint on the fascia and soffit, mould in the wall cavity, and a gutter torn off by the weight of ice. The roof itself is rarely the fault — a new roof over a leaky, under-insulated attic will dam just as the old one did — which is why a roofer's estimate to "fix the ice dams" with shingles alone deserves a second opinion. The IRC ventilation rules in Section R806 and a blower-door or infrared survey of the ceiling are where the fix begins.
Emergency measures — raking snow off the lower roof, laying calcium chloride in stocking-shaped socks across the dam to melt channels — relieve a bad winter. Chipping the ice with tools damages the shingles more than the ice does.
What it costs and who does it
The cure is priced as insulation and air-sealing work by a weatherisation or insulation contractor, often with an energy audit first; the roof-side measures — ice barrier under a re-roof, adequate soffit and ridge venting, heat cable as a last resort — are roofing line items. Interior repairs are a painter's or drywall contractor's. The directory's guide to finding a roof leak helps tell an ice-dam leak from a flashing leak, and its roof repair cost guide explains how the roof-side work is estimated.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
What causes ice dams?
Does a new roof prevent ice dams?
How far should ice and water shield extend?
Next step
Find a roofer who does this
An ice-dam problem usually needs an insulation contractor first and a roofer second, and a good roofer will say so. The directory lists roofing contractors by state and city.
Sources
- Ice dams — Michigan State University Extension
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)
- 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction — International Code Council (ICC)