Winter
Ice Dams Are an Attic Problem
Why your house gets ice dams and your neighbor’s doesn’t: it is what is happening above your ceiling, not on your roof.
· Reviewed by Kevin Strauss
The Answer
Ice Dams In Southern Wisconsin: Why Your House Gets Them And Your Neighbor’s Doesn’t
Ice dams in southern Wisconsin form when heat leaking into the attic melts snow on the roof; the meltwater refreezes at the cold eave and backs up under the shingles. Your neighbor’s roof stays white because their attic is cold — sealed, insulated, ventilated at soffit and ridge. The roof is rarely the fault.
Everything below follows from that: why this part of the state gets them badly, what fixes one this winter, what prevents the next, and what the two houses on the same street are doing differently.
The Mechanism
An Ice Dam Is An Attic Problem
Heat leaking into your attic warms the roof deck and melts the snow lying on it. The meltwater runs down to the eaves, which are cold because there is no heated house underneath them, and refreezes. That ridge of ice then traps the next lot of meltwater, which backs up under the shingles and into your ceiling. The icicles on the gutters are the visible symptom; the dam behind them is the problem.
Notice what is not in that sequence: the roof doing anything wrong. Ice dams are an insulation and ventilation failure that presents as a roofing problem, which is why the ice dam page and the ventilation page are two halves of one answer.
Four Reasons
Why Southern Wisconsin Gets Them Badly
The freeze-thaw pattern
Our winters hover around freezing rather than staying hard-frozen, which is precisely the condition that melts snow by day and refreezes it by night.
The housing stock
Watertown’s median house is from 1969, built before attic air sealing was understood. Recessed lights and bath fans vent straight into the roof space.
Retrofitted insulation
Blown insulation added over unsealed penetrations slows heat without stopping it, and often blocks the soffit intake that would have kept the deck cold.
Roof geometry
North-facing valleys and roofs over porches or additions stay cold and shaded, so ice that forms there never leaves.
The Fix
What Actually Fixes An Ice Dam
This winter: steam removal. Low-pressure steam melts the dam without harming the shingles. Not hatchets, not ice picks and emphatically not a pressure washer, all of which we get called out to repair afterward.
Permanently: air-seal the attic floor first, then insulate, then make sure there is genuine cold-air intake at the soffits feeding exhaust at the ridge. Air sealing matters more than insulation depth, because insulation slows heat and air sealing stops it.
At the next replacement: ice barrier membrane at the eaves, so a future dam backs water onto a waterproof layer instead of into your plaster. The state code requires it only on roofs of 4:12 or less — SPS 321.28(4) — and we fit it on every pitch, because a Watertown eave ices up regardless. What else goes into a replacement.
Prevention
How To Prevent Ice Dams: The Order
Find the heat
Recessed lights, the attic hatch, bath fans venting into the attic, gaps around the chimney chase and plumbing stacks, missing insulation over the top plate. Usually several at once.
Seal it
Foam and caulk at every penetration; a gasketed hatch; bath fans ducted to a roof or wall cap with a damper. This step does more than any amount of insulation.
Insulate
Then, and only then, add depth — with baffles at the eaves so the new insulation cannot block the soffit intake it is supposed to sit behind.
Ventilate
Clear soffit intake, continuous ridge exhaust, sized to the ceiling area as SPS 322.39 requires and the ventilation page explains. Look at the roof after a snowfall: evenly white means it is working.
Straight Answers
Ice Dam Questions
Interior water damage from an ice dam is often covered as a sudden loss. Removing the ice, and fixing the attic that caused it, usually is not, because those are maintenance rather than damage. This varies more between policies than most coverage, so read yours, and photograph the dam and the interior stain before anything is removed. The claims section explains how to document a loss so it is paid.
Pulling the first few feet of snow off the eaves from the ground does genuinely help, because it removes the snow that would melt and refreeze at the coldest part of the roof, and it is safe. Do not get on a ladder to do it, and do not use the rake to hit ice that has already formed — that is how shingles get broken. A rake manages the symptom for one winter; the attic work above ends it.
They manage the symptom on a few awkward geometries — a north-facing valley over an unheated porch is the classic case. They are not a fix, they cost money to run all winter, and they should be the last option rather than the first. If a contractor proposes cables before asking about your attic, they have skipped the diagnosis. The ice dam page sets out the order the fixes go in.
Icicles are the symptom that shows from the street: meltwater running off a warm roof deck and freezing at the cold gutter. Big icicles mean a lot of melt, which means a lot of heat in the attic. The gutter itself is not the cause and cleaning it will not stop them — the gutter page explains what gutters actually protect — though a gutter full of ice is heavy enough to tear its hangers out of the fascia.
Please do not. Every winter someone takes a hammer to a dam, breaks the shingles underneath, and turns a seasonal nuisance into a spring replacement — and no policy covers damage the homeowner did with a hatchet. Falls off icy ladders are the bigger risk. If water is coming in tonight, move what you can, contain what you cannot, photograph everything, and call in the morning for steam. The FAQ covers what else gets blamed on the roof unfairly.
Related
Fixing Ice Dams, In Order
Ice dam removal
Steam removal now, then the attic work that stops it happening next January.
Attic ventilation
The permanent fix: intake and exhaust sized to the ceiling area, soffits actually open.
Roof replacement
Ice barrier at every eave whatever the pitch, as part of every roof we put on.
Seamless gutters
What a gutter does and does not do about ice, and why a full one tears the fascia.
Winter Work
Ice Backing Up Under Your Shingles?
We will steam it off, then tell you what in the attic caused it.