Short answer
Because a pitched roof sheds water that keeps moving and cannot hold back water that sits still. An ice dam at the eave stops the melt, backs it up under the shingles, and pushes it into the house through joints that were never designed to be submerged. The permanent fix is usually in the attic — air sealing, insulation and balanced ventilation — not in the roof covering.
On this page
A roof leaking in February on a still, clear day is not a storm problem. It is an ice dam, and the reason it surprises people is that the same roof handled a driving rain in October without a drop coming through.
A pitched roof sheds water; it does not seal against it
Asphalt shingles are a water-shedding system, not a waterproof membrane. Each course overlaps the one below it, and gravity does the rest. Water arriving from above runs across the overlaps and off the eave. Every fastener, every seam and every edge in that assembly assumes the water is moving downhill and moving quickly.
An ice dam breaks that assumption. Snow on the upper part of the roof melts, the meltwater runs down to the cold eave and the overhang, and it refreezes there because that part of the roof is not being warmed from below. The ice builds into a ridge. Behind the ridge, water pools. Standing water finds the lap joints, wicks backward under the shingles by capillary action, and reaches the deck. From there it follows the framing and shows up as a ceiling stain, a wet top plate, or a dark line on the wall below a window.
Why the eave is always where it happens
The upper field of the roof is warmed by heat leaking out of the living space into the attic. The overhang past the exterior wall line is not — it has cold air on both sides. That temperature difference across a single roof plane is the whole mechanism. Two feet of the roof is above freezing on the underside, two feet is not, and the boundary between them is where the ice builds.
The corollary matters: a house with a genuinely cold attic gets far fewer ice dams, because the snow on the upper roof never melts in the first place.
What ice and water shield actually does
Ice and water shield is a self-adhering, self-sealing membrane laid directly on the deck before the underlayment and shingles. It seals around fasteners driven through it, and it does not care which direction the water is coming from. Where it is present, water backing up under the shingles hits a continuous waterproof layer and runs back out at the eave instead of into the house.
The important detail is how far up the roof it goes. The requirement is not stated as a number of rows. It is measured from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line, measured horizontally. On a steep roof with a shallow overhang, one 36-inch course can satisfy that. On a low-slope roof, or a colonial with a deep overhang, or a roof over a wide soffit, one course does not come close, and two or three are needed.
This is a line item you can check on a written quote. It should say where the membrane runs and how many courses, not just that ice and water shield is included. Valleys, around chimneys, at skylights and along low-slope-to-wall transitions are the other places it belongs.
What the code does not require here
One point worth clearing up, because it gets misquoted in sales conversations: the high-wind underlayment provisions that kick in above 110 mph are keyed to Vasd, the allowable stress design wind speed, which works out to roughly 93 to 97 mph in the towns we serve. The 120 and 125 mph figures printed in Appendix AY are Vult, a different number. The high-wind underlayment trigger does not apply here, and a quote that justifies an upcharge by pointing at the 125 mph figure is misreading the table.
The cure is in the attic, not on the roof
Ice dams are a heat problem wearing a roofing costume. Three things fix them, in this order.
1. Air sealing
Warm indoor air escaping into the attic carries far more heat than conduction through insulation does. The usual culprits are recessed light housings, bath fan ducts that dump into the attic instead of outside, the chase around a chimney or flue, plumbing stack penetrations, the attic hatch, and the top plates of interior walls. Sealing those is unglamorous, relatively cheap, and it does more per dollar than anything else on this list.
Blowing more insulation over an unsealed attic floor buries the problem and traps the moisture that comes with it.
2. Insulation
Once the air leaks are closed, insulation depth is what keeps the remaining heat inside. A great many Connecticut houses — the 1940s to 1970s Capes and ranches around Manchester, the 1920s to 1960s colonials in West Hartford — still carry the insulation depth that was normal when they were built, which is well below what current energy code calls for. Insulation also has to reach the eave without blocking the soffit vents, which is what baffles are for.
3. Balanced ventilation
Intake at the soffit, exhaust at the ridge, and roughly equal amounts of each. Ventilation flushes whatever heat and moisture still get past the first two steps and keeps the deck temperature closer to the outside air. Ridge vent with painted-shut or insulation-blocked soffits is the most common failure we find, and it is worse than no ridge vent at all, because the exhaust then pulls its makeup air out of the house.
Cathedral ceilings, finished attics and knee-wall bonus rooms are the hard cases. There is no attic floor to insulate, the roof assembly itself is the thermal boundary, and the fix usually involves the ceiling from below or a full assembly rebuild from above. Anyone who tells you a new layer of shingles solves a cathedral ceiling ice dam is guessing.
Snow load is a different number
The ground snow load in Appendix AY of the 2022 Connecticut State Building Code is a structural figure. It tells the framing how much weight to carry, not how likely you are to get a dam.
| Town | Ground snow load (pg) |
|---|---|
| TownAvon | Ground snow load (pg)35 psf |
| TownFarmington | Ground snow load (pg)35 psf |
| TownSimsbury | Ground snow load (pg)35 psf |
| TownEvery other town we serve | Ground snow load (pg)30 psf |
Avon, Farmington and Simsbury sit at 35 psf. Elevated, wooded terrain that holds snow longer gives an ice dam more time and more material, but the underlying cause is still the heat leaving your house. A shoreline house in Madison with a badly sealed attic will dam; a Simsbury house with a tight, cold attic largely will not.
The case against re-roofing to solve this
If your shingles have ten good years left, replacing the roof to fix ice dams is spending roof replacement money on a problem the roof did not cause. The honest sequence is:
- Repair the leak damage and confirm where the water is entering.
- Air seal and insulate the attic, and open the soffit intake.
- Rake the lower roof after heavy snow as a stopgap through that winter.
- Replace the roof when the roof itself is due, and run the ice barrier properly then.
The exception is a roof already at the end of its service life, or one where the existing ice barrier is a single narrow strip that stops short of the wall line. In that case the re-roof was coming anyway, and doing the attic work in the same season means the new roof starts life on a house that is not melting its own snowpack. Bundling the two is the version of this that actually pays.
Getting a straight read on it
We inspect the attic as part of looking at a winter leak, because the ceiling stain rarely sits under the point of entry. If the answer is insulation and air sealing rather than a new roof, we will say so.
Someone comes out, gets into the attic, measures the roof and gives you a written price for what is actually needed. You can request a free estimate or read more about our roofing work.
