Ice Dams in the Northwest Suburbs: Causes and Fixes

What is an ice dam, and why does it form?

An ice dam is a ridge of ice that builds along the eave of a roof. Heat from the house warms the roof deck, melts the snow above it, and the water runs down to the cold overhang and refreezes. The growing ridge blocks later meltwater, which backs up under the shingles and leaks in.

Picture the roof in two zones. Over the heated part of the house the deck is warm, and snow on it melts from beneath. Over the overhang, which has no heat below it, the deck is as cold as the air. Meltwater travels from the warm zone to the cold one and freezes there, and each cycle adds to the ridge.

The Illinois State Climatologist gives O'Hare an average of 36.7 inches of snow a year for 1991 to 2020, and NWS Chicago places the first snow around 31 October and the last around 14 April. Within that stretch, a cold snap followed by a thaw is the setup, and a week of sub-freezing nights after a snowfall is the engine.

Ice dams are a roof problem in the sense that the water shows up there. They are an attic problem in the sense that the heat comes from below. Treating only the roof end of the chain gives you a repeat each winter.

Why are low-pitch ranches and split-levels so prone to them?

A shallow roof has a long run to the eave, so meltwater travels slowly and has time to refreeze. Postwar ranches also tend to have modest overhangs, thin or uneven attic insulation and few vents, and decades of renovation have added holes in the ceiling for lights and fans. The result is steady heat loss under a slow-draining roof.

The postwar streets of Hoffman Estates, Streamwood and Hanover Park are full of single-storey houses and split-levels. Streamwood incorporated in 1957 with veterans' ranch homes, Hanover Park in 1958, and Hoffman Estates saw its first homeowners in late 1955. The roofs on such houses are low, and many were built when attics were treated as storage and not as part of the building's thermal envelope.

None of this makes a ranch a bad house. It means the attic is the thing to understand. A roofer or insulation specialist who walks yours can tell you which of the usual leaks is present, and the attic ventilation and insulation page describes what the assessment covers.

How do I read what my ice dam is telling me?

The pattern of ice and snow points to the cause. Bare patches in the snow on the roof show where heat is leaking up. Ice only above a bathroom or kitchen suggests a fan or duct problem. Long icicles on one side suggest uneven insulation. Photograph the roof from the ground each time it happens and note the dates.

A roofer who can see a photograph taken the morning after a snowfall learns more than from a visit in midsummer. Snow melting off in patches on a cold day is the roof's own thermal image.

The table matches common signs to likely causes and the first sensible response.

What can I do safely this week?

Rake the lowest few feet of snow from the ground with a long-handled roof rake, move belongings from beneath a stained ceiling, photograph everything, and call. Do not chop ice, climb onto the roof or work from a ladder in winter. The goal this week is to limit damage until the cause can be fixed.

Raking reduces the amount of snow available to melt, which can slow a dam that is growing. Work from the ground, keep the rake off the shingles as much as possible, and do not rake more than the edge zone. Falling snow and ice are heavy, so stand clear of the drop line.

Inside, a bucket and a towel under a drip buy time, and moving furniture away from the wet ceiling spares it. If a ceiling bulges, keep people out from under it. If water is running into a light fixture, switch off the circuit at the panel.

• Rake the lowest few feet of snow from the ground
• Do not chop, chip or pry at the ice
• Do not climb onto the roof or use a ladder on ice
• Photograph the ice, the icicles and any stain with dates
• Move belongings and keep people out from under a sagging ceiling
• Call and say whether water is entering the house

What stops ice dams for good, and in what order?

Seal the air leaks from the living space into the attic, bring the insulation to an even depth, and then ventilate from low intake to high exhaust. The order matters because venting a leaky attic only pulls warm air through faster. Eave protection under the shingles is the final layer of insurance.

Air sealing comes first because leaks carry far more heat than insulation conducts. The usual sources are recessed lights, the attic hatch, plumbing and wiring holes, the tops of interior walls and the chimney chase. Insulation then needs to run evenly across the attic floor, including the edges, without blocking the soffit vents.

Ventilation is the finishing step: intake at the soffits, exhaust at or near the ridge, and baffles to keep insulation from choking the airflow. This guide gives no R-value or code figure, because the sources behind such numbers differ and the village building department is the authority on what applies to your house.

Many of these steps can be coordinated with a reroof, and the roof replacement page describes how eave protection and ventilation fit into a new roof.

What does an eave membrane do, and does it replace the attic work?

A self-adhering waterproof membrane under the first shingle courses at the eave catches water that backs up behind an ice dam and keeps it from reaching the roof boards. It does not stop the dam forming, so it protects against the leak but not the ice. It is a backstop installed when the roof is replaced.

Think of the membrane as a raincoat under the shingles. If meltwater backs up, it has somewhere to sit without wetting the deck. It does nothing about the heat that creates the dam, and ice can still pull gutters off and wreck the eave.

On a house with a history of leaks at the eaves, ask the roofer whether the quote includes the membrane, how far up the roof it runs and whether it also protects the valleys. Do not rely on a code figure from a website; ask the building department what applies.

Do heated cables and roof raking solve the problem?

No. They treat the effect. Heated cables can melt channels through ice so water drains, and raking reduces the snow supply, but neither stops the heat loss that starts the cycle. They can help for a season while the attic is fixed. Used alone, they are a recurring cost and a recurring risk.

Heated cables should be installed by someone who knows the roof and the electrical requirements, and they work best in a pattern that gives water a path to the gutter. They add an electrical load and can wear shingles where they are fastened.

Raking is cheap and harmless if done from the ground with care. It becomes risky when people reach over their heads on slippery ground or when a rake scrapes granules off aging shingles. The ice dam service page explains how a roofer removes ice without damaging the roof.

What about gutters, skylights and wooded lots?

Clogged gutters hold meltwater and freeze solid, which makes the ice heavier and pulls them off the fascia. Skylights and their shafts are thermal weak points. Leaves and needles on wooded lots such as those in Medinah, Wayne and Hoffman Estates make clogging more likely, so clean gutters before winter.

Gutters do not cause ice dams, but they can make the damage worse. See gutter installation and repair for hanger spacing and cleaning. A skylight that sits where snow slides against its uphill edge can show ice earlier than the rest of the roof; the skylight page covers that.