Missouri and Illinois sit in the worst possible band for this. We do not get the steady deep freeze that keeps a northern roof uniformly cold and snow-covered all season. We get cycling: thaw, freeze, thaw, freeze, sometimes several times in a week. Every one of those cycles drives water into a gap and then expands it as ice.
How an ice dam actually forms
- Snow sits on the roof.
- Heat leaking from the house into the attic warms the roof deck above the living space, and the snow touching that warmed deck melts.
- The meltwater runs down the slope and reaches the eave and gutter, which hang out over open air and stay at outdoor temperature.
- There it refreezes, building a ridge of ice at the edge.
- Subsequent meltwater pools behind that ridge, standing on the roof surface.
- Standing water works up under the shingle laps by capillary action, reaches the deck, and comes through into your insulation, ceilings and walls.
Note what is not in that list: the quality of your shingles. Asphalt shingles are a water-shedding system, designed for water moving downhill. They are not a waterproof membrane and they were never intended to hold back a pond.
The three causes, in order of importance
- Air leakage from the house into the attic. The biggest one and the most overlooked. Recessed lights, bath fan housings, plumbing and wiring chases, the attic hatch, and gaps at the top plates of interior walls all pump warm, moist household air straight into the attic.
- Inadequate or unbalanced ventilation. Intake at the soffits and exhaust at the ridge should keep the deck close to outdoor temperature. Blocked soffit vents — usually insulation stuffed into the eaves or painted-over perforations — break the whole system, because exhaust vents cannot pull air that intake vents are not admitting. See soffits and fascia.
- Thin or poorly distributed insulation, particularly out at the exterior wall plates where depth is usually least and heat loss is greatest.
Contributing factors: complex rooflines with many valleys, low-slope sections, north-facing slopes that never get sun, and gutters packed with debris that hold meltwater at the eave. Skylights and dormers create their own melt patterns.
What freeze-thaw does beyond the dam
Even without a full ice dam, cycling is hard on a roof. Water enters a hairline crack in a pipe boot, a nail hole, or a lifted shingle edge, then expands roughly nine percent as it freezes, widening the gap. Repeat that fifty times over a winter and small defects become real openings. It also pries at flashing laps, cracks aging sealant, and splits already-weakened decking. This is why a minor issue found in October is a genuinely different repair than the same issue found in March.
The right fixes, in the right order
- Air seal the attic floor. Sealing the leaks that let warm air up is the highest-value work, and it lowers your heating bill at the same time.
- Restore intake ventilation. Clear the eaves, install baffles so insulation cannot choke the airflow, and make sure the soffit vents are genuinely open.
- Balance exhaust at the ridge or with roof vents, matched to the intake.
- Bring insulation to depth, evenly, especially over the wall plates.
- Install ice and water shield at the eaves when the roof is replaced. This self-adhering membrane seals around fasteners and protects the vulnerable lower roof if a dam ever does form. It is protection, not prevention — but it is the single best insurance policy on a Midwest roof.
- Keep gutters clear so meltwater has somewhere to go. See gutters and your roof.
What not to do
- Do not chip, hammer or pick at the ice. You will damage shingles and gutters, and standing under falling ice causes serious injuries every winter.
- Do not use rock salt directly on shingles or in gutters; it corrodes metal and damages plantings below.
- Do not get on an icy roof. Ever. See DIY roofing safety.
- Do not rely on heat cables as a solution. They can manage a chronic problem spot, but they treat the symptom, cost money to run all winter, and fail while the underlying heat loss keeps going.
If water is coming in right now
Move what is underneath, get a container down, and if a ceiling is bulging with trapped water, puncture the low point with a small hole from a safe distance to let it drain in a controlled way rather than letting the drywall come down. Photograph everything. Then call. A roof rake used from the ground can pull snow off the lower three or four feet of roof and slow the process, but it does not undo a dam that has already formed. Our 24/7 emergency service runs through the winter, holidays included — see also what to do during a winter storm leak.
Free storm inspection, photographs included, no obligation. Call 1-800-564-6232 — a person answers, 24 hours a day — or book online.
Frequently asked questions
What causes ice dams on a roof?
Heat escaping from the house into the attic warms the roof deck and melts the snow on it. That meltwater runs down to the cold, unheated overhang, refreezes into a ridge of ice, and backs standing water up under the shingles.
Do ice dams mean my roof is bad?
Usually not. Ice dams are primarily an attic problem caused by air leakage, insufficient insulation and blocked or unbalanced ventilation. A perfectly sound roof over a warm, poorly ventilated attic will still form dams.
How do I get rid of an ice dam safely?
Do not chip at it and do not climb up. Use a roof rake from the ground to remove snow from the lower few feet of roof so no new meltwater feeds the dam, and call a professional if water is entering the house. Chipping ice damages shingles and gutters and is dangerous.
Does insurance cover ice dam damage?
Interior water damage caused by an ice dam is often covered under a homeowners policy, while the underlying insulation or ventilation deficiency is not. Photograph the damage as it happens and report it promptly.
Will ice and water shield stop ice dams?
It does not prevent dams from forming; it prevents them from leaking into the house. The self-adhering membrane seals around nails and protects the eaves, which is why it is standard on our replacements in Missouri and Illinois.