How an Ice Dam Forms
Four steps, and only the last one happens on the roof surface.
Warm air escapes from the living space into the attic. It heats the underside of the roof deck. Snow sitting on the upper roof melts from below, even while the air temperature stays below freezing. The meltwater runs down the slope until it reaches the overhang, which is not heated from under because there is no living space under it, and refreezes there.
Repeat this over several days and a ridge of ice builds along the eaves. Behind the ridge, meltwater pools.
Shingles shed water running downhill. They are not designed to hold back standing water or water being pushed uphill by an ice ridge. Water finds the fastener penetrations and the course overlaps, and gets under.
Long Island's repeated freeze-thaw swings make this more common here than in places that simply stay cold.
What a Leak Barrier Does
A leak barrier is a self-adhering waterproof membrane applied directly to the roof deck before the shingles go on.
The critical property is that it bonds to the deck and seals around fasteners. A nail driven through it does not create a path for water, because the membrane closes around the shank. That is what makes it effective against standing water, which ordinary felt underlayment is not.
At minimum it belongs along the eaves, extending far enough up the slope to cover the area above the exterior wall line — which is to say past the point where the heated space ends and the cold overhang begins.
Valleys are the other high-priority location, since that is where water volume concentrates. Around chimneys and skylights, and along rakes on exposed elevations, are common additions.
Why the Barrier Is Not the Fix
A leak barrier protects the deck from the results of an ice dam. It does not stop the dam forming.
The cause is heat loss into the attic, and the fix is on the inside. Air-seal the penetrations between the living space and the attic — recessed lights, plumbing stacks, top plates, the attic hatch. Bring insulation to a consistent depth across the attic floor. Then ensure intake and exhaust ventilation are balanced so the attic runs close to outside temperature.
The details are in why attic ventilation makes Long Island roofs fail early.
Roof raking after a storm reduces the material available to melt, so it helps in the moment. It is a treatment rather than a solution, and it carries real risk of shingle damage and of falls.
Getting It Right at Replacement
Leak barrier and ventilation are both far easier to install correctly during a replacement than to retrofit afterwards. Both are also qualifying accessory categories under GAF's enhanced warranty tiers.
Ask specifically how far up the slope the leak barrier will run, since minimum coverage and adequate coverage are not always the same on a low-pitch roof or a house with deep overhangs. Ask whether valleys are included. And ask what ventilation work is in scope.
Older housing stock with plank decking needs particular attention here, which is covered in roof replacement in Valley Stream. Exposure varies much across the island — see South Shore versus North Shore roofing.
How long the finished roof should last is set out in how long a roof lasts on Long Island.
Ice Dams on Long Island: What to Do Next
Frequently Asked Questions
What is a leak barrier?
A self-adhering waterproof membrane installed along vulnerable areas of the roof deck, most importantly the eaves and valleys. It bonds to the deck and seals around nail penetrations, so water that gets past the shingles still cannot reach the sheathing.
Where should leak barrier be installed?
At minimum along the eaves, extending far enough up the roof to cover the area above the exterior wall line. Valleys, around chimneys and skylights, and along rakes on exposed elevations are also common applications.
Will removing snow from my roof prevent ice dams?
It reduces the material available to melt and refreeze, so it can help in the short term. It is a treatment rather than a fix, and roof raking carries real risk of damaging shingles and of falls. The underlying cause is heat loss into the attic.
Do gutters cause ice dams?
No, though they are often blamed. Gutters can hold ice and make a dam more visible, but the dam forms from meltwater refreezing at the cold overhang. Clean gutters help drainage but do not address the heat loss causing the melt.
Does a leak barrier count toward a GAF warranty tier?
Yes. Leak barrier products are one of the qualifying accessory categories that count toward GAF's enhanced warranty tiers, alongside starter strip, ridge cap, roof deck protection, and attic ventilation.
Protect the Edge Before Winter
Leak barrier at the eaves and balanced ventilation are both installed during a replacement. Retrofitting either one later is far harder. Ask us about ice dams on Long Island and you will get a straight answer, in writing.
Request a Proposal
Ice dams on Long Island are a heat problem before they are a roof problem. Warm air escaping into the attic melts snow above, and the meltwater refreezes over the unheated overhang. The resulting ice ridge pushes standing water back under the shingle courses. A self-adhering leak barrier along the eaves seals around fasteners and gives the edge a waterproof layer, but balanced attic ventilation and sealed air leaks are what stop the dam forming in the first place. That is the short version of ice dams on Long Island.
You can armour the eaves, or you can stop heating them. The roofs that never have the problem do both.
Island Roofing Inc. is a full-service residential and commercial roofing, siding, and gutter contractor at 1085 Rockaway Ave, Valley Stream, NY. We are GAF-certified contractors and HAAG-certified inspectors, with four full-time residential crews working across Long Island and commercial projects completed in more than 34 states. Three decades of roofing on Long Island.
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