How an ice dam forms
Those thick icicles hanging off the gutter in February usually have a ridge of ice behind them, frozen along the bottom edge of the roof. That ridge is an ice dam. It holds back meltwater, and the trapped water is what ends up inside the house.
Most ice dams start with heat in the attic. Warm air from the living space rises into the attic and warms the underside of the roof deck. Snow on the upper part of the roof melts from below, even on a day that never gets above freezing.
That meltwater runs down the roof under the blanket of snow until it reaches the eave. The eave hangs out past the heated walls of the house, so it stays as cold as the air outside. The water refreezes there, and every melt-and-freeze cycle adds another layer to the ridge.
Once the ridge gets tall enough, new meltwater has nowhere to go. It pools behind the ice and works its way up under the shingles. Shingles shed water that runs downhill, but they can't hold back standing water, so it soaks into the roof deck, the soffit, the top of the wall, and sometimes a ceiling.
Why Front Range winters make it worse
Snow on a Front Range roof rarely stays frozen for long. Strong sun by day and hard freezes at night melt and refreeze it again and again, and each round gives the ridge at the eave another chance to grow.
Shade adds to the problem, because on a north-facing roof the edge stays frozen while attic heat keeps melting the snow above it. An eave shaded by a big tree or a second story does the same thing on a roof whose upper part gets afternoon sun. In the foothill towns, snow piles deep on the north side of a roof and sits there long after the sunny side is bare.
The dam also gets heavy, since ice that fills the gutter and hangs off it as icicles puts its whole weight on the hangers. In a bad stretch that load can tear them out of the fascia and bring a section of gutter down.
Where the attic heat comes from
In most homes, the heat leaks up from the rooms below. It slips through gaps around recessed lights, the attic hatch, and the holes where pipes and wires pass through the ceiling. Thin or uneven insulation lets even more of it through.
Ventilation is the other half of the picture. Soffit vents let cold outside air in at the eaves, and vents higher on the roof let it out, which keeps the roof deck closer to the outdoor temperature. When the soffit vents are blocked by insulation, paint, or a solid panel, warm air stays trapped against the roof and melts the snow from underneath.
What helps keep an ice dam from starting
An attic that stays cold matters most, and open soffit vents are part of keeping it that way. Clear, properly pitched gutters matter at the edge, since a trough full of wet leaves and pine needles freezes solid and adds ice and weight at the eave. Guards help for that reason too, though no guard stops ice dams on its own.
- Clean the gutters after the last leaves and needles come down and before the first hard freeze, so there's no wet debris left to freeze in the trough.
- Check the pitch. A level run, or one with a dip somewhere along its length, keeps a puddle that turns to ice overnight. Water still standing in the gutter a day after a rain marks the spots that freeze first.
- Make sure every downspout is open at the bottom. One that's plugged with debris or frozen shut backs water up into the gutter, where it freezes and feeds the ridge.
- Look at your soffit vents from the ground. Vents that are painted shut, covered by solid panels, or missing altogether can't feed the attic any air.
- Have the attic side checked. Sealing the gaps where house air leaks in and filling thin spots in the insulation deal with the heat where it starts.
If there's already ice on the eave
Stay off the roof. Chipping at a dam with a hammer or a shovel tears up shingles and dents the gutter, and an icy roof is a dangerous place to stand. A ladder leaned against a gutter full of ice can bring the whole run down.
From the ground, a roof rake can pull snow off the lower edge of the roof after a storm, which takes away some of the snow that feeds the dam. Keep it well clear of power lines. If water is already showing up on a ceiling or wall, have a roofer look at the leak, then fix the attic and eave conditions behind it before next winter.
Soffit repair from Stoked
Stoked's soffit repair covers the eave side of this: vents that are blocked or painted over, and soffit that has gone soft from past ice dams.
More articles
- Signs of Soffit Damage and What They MeanSagging panels, peeling paint, water stains, and chewed gaps all point to trouble at the eave. The water that rots soffit usually soaks the fascia above it first.
- How Soffit Vents Help Your Attic BreatheYour attic pulls fresh air in at the soffit and lets it out up high. When insulation or paint blocks that intake, heat and moisture stay trapped under the roof.