Ice and water barrier: what it does and what it cannot fix
It is a good backup layer. It is not a cure for the heat loss that builds ice dams.
An ice and water barrier is a self-sealing rubberized membrane that goes under the shingles along the eaves and other leak-prone spots. If water gets pushed back up the roof by an ice dam, the membrane stops it reaching the wood deck. It is a backup. It does not stop the ice from forming.
What it actually does
The membrane has a sticky underside. It bonds to the roof deck and seals around the nails that pass through it. That is the whole trick. Ordinary felt or synthetic underlayment is only a water-resistant sheet, and water forced sideways under shingles can find its way past the nail holes.
Ice dams force water backward, up under the shingle edges. A sealed membrane gives that water nowhere to go. It is a backup layer on a part of the roof that takes a beating.
Where it is normally installed
Along the eaves, and often up the slope to a point above the exterior wall line. Many roofers also use it in valleys, around chimneys and skylights, and at low-slope sections. Local building codes in snowy regions often require it at the eaves. Check what your local building department asks for before a re-roof.
You cannot add it on its own to an existing roof. It sits under the shingles, so it goes in when the shingles come off. That is a decision for a re-roof, not a weekend job.
What it cannot fix
A membrane does not change the temperature of the deck. The ice dam still forms, the icicles still hang, and the gutters still fill with ice. Heat leaking from the house is the cause, and the barrier ignores it. Our page on what causes ice dams covers that chain in full.
Treat the barrier as the second line. The first line is air sealing, continuous insulation and balanced ventilation, as laid out in ice dam prevention. A roof with a membrane and a leaky ceiling will still grow ice dams. It will just leak less when it does.
How to check without climbing
You cannot see it from the ground once the roof is finished. Look at your records from the last re-roof, or ask the roofer who did the job. From inside the attic, you may see a dark, rubbery material on the underside of the deck near the eave, but in most cases you will not. Do not go up on the roof to look. Photos in the contract or invoice are the safe way to find out.
Questions to ask a roofer
Ask where the membrane will start and where it will end. Ask whether it covers the valleys and penetrations. Ask which product they plan to use, and whether it meets the local code. A roofer who can answer in plain terms usually knows the job. Ask them also how they plan to deal with attic air leaks and ventilation, since a new roof on a leaky attic repeats the old problem.
Do you need it?
If you live where snow sits on the roof for weeks, yes, when the roof is replaced. If you get a few flurries a year, it is a lower priority, though still useful around chimneys and valleys. For a roof that is not being replaced soon, put the effort into the attic. That is where the real fix is.
Frequently asked questions
Does ice and water barrier stop ice dams?
No. It stops water from reaching the deck if an ice dam forms. The dam still forms unless you fix the heat loss from the house.
Can I add ice and water barrier to an existing roof?
Not by itself. It goes under the shingles, so it is installed when the roof is replaced or the shingles are removed.
Where does ice and water barrier go?
Along the eaves, often up past the exterior wall line, plus valleys and around chimneys and skylights. Local codes may set the minimum.
How do I know if my roof has it?
Check your roofing contract, invoice or photos from the last re-roof, or ask the roofer. You cannot see it from the ground.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NOAA / National Weather Service. Winter — National Weather Service safety hub
- NOAA / National Weather Service. What To Do If You're Caught in a Winter Storm