Ice Dams on Chicago Roofs: What Causes Them

Ice dam prevention in Chicago starts with understanding what an ice dam is: a ridge of ice that builds along the eave of a pitched roof and holds back meltwater. The ice at the edge is the symptom. The cause is usually somewhere else, in the way heat gets into the roof.

What causes ice dams on a Chicago roof?

The commonly accepted explanation has three parts. Snow sits on the roof. Heat from the building below warms part of the roof surface and melts the snow from underneath. The meltwater runs down to the eave, which is colder because there is no heated space under it, and it freezes. Over days, that frozen edge builds into a ridge. The next meltwater cannot get past it, so it pools behind it and can work its way up and under the shingles.

Chicago has the cold side of the cycle. The 1991 to 2020 normals count about 121 days a year with a low at or below 32 F, and about 43 days when even the high stays at or below 32 F (NWS Chicago). Winters here swing above and below freezing often, which is exactly what repeated melt and refreeze needs.

Where does the heat come from?

In our experience on Chicago two-flats, bungalows and older frame houses, the heat usually leaks into the roof space through gaps and thin spots:

• Gaps around chimneys, flues, light fixtures and attic hatches
• Thin or missing insulation over the top-floor ceiling
• Attic or roof space that does not move air, so heat builds up under the roof
• Ductwork or bath fans that vent into the roof space instead of outside

We are not giving insulation depths or ventilation ratios here, because we do not have a verified source for them. A roofer, an insulation contractor or an energy auditor can measure your building. Warming a roof from below is a building-envelope problem, and gutters and heat cables alone do not solve it.

What are the warning signs?

• Thick ice along the eave or in the gutters while the rest of the roof is clear
• Large icicles, especially in one area
• Brown or yellow stains on the ceiling or the wall near an exterior wall after a thaw
• Damp or stained insulation or attic framing
• Peeling paint or damp plaster under the roof edge

Water coming inside during a thaw is a reason to call a roofer, not to wait for spring.

What should you do about an ice dam?

For the ice itself, the careful choice is to get a professional to deal with it. Hacking at ice with a hammer or pick can damage shingles and gutters, and climbing a snowy, icy roof is how people get hurt.

For the cause, have the roof space looked at: air leaks, insulation and ventilation. That is what stops the melt and refreeze cycle. Flat roofs get a version of the problem too, with ice forming at frozen drains and scuppers, which is why clearing them matters (see flat roof snow load in Chicago and our flat roof maintenance checklist).

Do you need a permit to fix roof damage from an ice dam?

It depends on the roof. The City lists pitched roofs of at least 2:12 as no-permit for repair, recover and replacement, while any work on low-slope roofs needs a permit (City of Chicago). Express reroofing is available for replacement with no structural work, as long as a licensed Illinois roofing contractor does it (City of Chicago). Rules change, so confirm with the Department of Buildings.

Roofing work for hire in Illinois requires a licensed roofing contractor (Illinois law). If water has damaged the roof deck or the shingles at the eave, replacing a shingle roof on a Chicago bungalow explains what that job involves.

Our roofing crew can find where the water is getting in and where the heat is getting out. For everything else on Chicago roofs, start with the roofing guide.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.