What Causes Ice Dams (And How to Stop Them for Good)
Step through an interactive cutaway of a New England roof and see the two fixes that end repeat ice dams.
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Winter Roof Guide
Frozen Gutters And Heat Cables: What Works On A New England Roof
A gutter full of ice in February usually started as standing water in November. Here's why gutters freeze, what heat cables can and can't do, and the fall checks that keep water moving when the cold arrives.
Why Gutters Freeze Solid
A gutter freezes when water sits in it. Moving water drains before the cold can get a grip, so a gutter that carries every drop to the downspout rarely fills with ice. The trouble starts with anything that makes water stop and wait.
- Leaves and seed pods. A clog left over from fall holds a pool of water behind it, and that pool turns into a block of ice on the first hard night.
- A sag between hangers. Gutters lose their slope as hangers loosen. Water collects in the low spot, freezes, and the added weight pulls the sag deeper.
- A downspout frozen at the bottom. Extensions, elbows, and pipes that run underground freeze first because they sit in snow. Once the outlet is shut, the whole downspout fills, then the gutter.
- Shade. A north-facing gutter under trees may not see sun for weeks in January. Whatever water reaches it stays frozen.
- Meltwater from a warm roof. If heat escaping through the attic melts snow all day, a steady trickle runs into a gutter that's well below freezing, and the gutter builds ice from the bottom up, night after night.
Ice in the gutter does damage of its own. It pulls on hangers and fascia, overflows as icicles and ice on the siding, and gives an ice dam a foundation to build on. When that ridge of ice climbs from the gutter onto the shingles, water starts backing up under the roof. Our guide to what causes ice dams walks through that sequence with an interactive cutaway.
How To Prevent Frozen Gutters
Most of the work happens in October and November, before the first real snow. None of it needs you on the roof.
- Clean the gutters after the leaves are down. Around Tewksbury and Billerica the oaks hold on into November, so a second pass late in the fall catches what the first one missed. Finish by running a hose into each gutter and watching where the water goes.
- Watch the gutters in a rainstorm. Water should run steadily toward the downspouts. A spot where it pools, or spills over the front edge in the middle of a run, is a sag or a clog, and it's where ice will start.
- Clear every downspout outlet. Make sure the bottom of each downspout drains onto open ground or a splash block, away from the foundation. A flexible extension lying in a snowbank freezes shut, so check them through the winter too.
- Re-hang loose sections. A gutter that has pulled away from the fascia can't hold its slope. Tightening or replacing hangers is a small repair in the fall and a hard one once the gutter is full of ice.
- Deal with the attic. If your roof sheds snow in stripes, or ice builds at the eaves every winter, the gutter is catching a symptom. Air sealing, insulation, and ventilation stop the daytime melt that feeds frozen gutters. Our attic insulation and ventilation guide covers what to look for.
Gutter guards help with the fall half of the problem by keeping leaves out. They don't stop a gutter from freezing, and snow can sit on top of a guard until it melts into the gutter below. Count them as a cleaning tool rather than a winter fix.
Do Heat Cables Work?
Heat cables do one thing well: they keep a drainage path open where they're installed. A cable laid in a zigzag along the roof edge, run through the gutter, and carried down the inside of the downspout melts a channel so water can get off the roof instead of sitting at the eave. On a gutter that freezes every year, that path can be the difference between a dry ceiling and a wet one.
What a cable can't do is stop the melting that makes the ice. The heat leaking out of the attic keeps producing meltwater, and above the top of the zigzag that water can still refreeze and build a dam the cable never touches. That's why we treat cables as a patch for a known trouble spot and fix the attic for the rest of the house.
Where heat cables make sense
- A short stretch of eave under a valley, where water from two roof planes collects in one spot
- A low-slope porch or addition roof that meets a taller wall
- A gutter in permanent winter shade
- A cathedral ceiling or finished attic with no room to add insulation or vent space, where the attic fix is hard to reach
What to know before installing them
- They have to be on before the ice builds. A cable buried under a solid gutter of ice works slowly, if at all. Most people switch them on for the season or put them on a controller that turns them on in freezing, wet weather.
- They use electricity every hour they run. Self-regulating cable adjusts its output to the temperature around it, while constant-wattage cable runs at full output whenever it's on. Ask which kind you're getting.
- The wiring belongs to an electrician. Roof de-icing cable runs outdoors in water and ice. The electrical code calls for ground-fault protection on these circuits, so have a licensed electrician wire it to the manufacturer's instructions.
- Clips shouldn't go through shingles. Use the clips made for the cable, fastened so they don't punch new holes in the roof. Every fall, look the cable over for cracked jacket or loose runs.
If Your Gutter Is Already Full Of Ice
Leave the ice in the gutter alone. Chipping at it dents the gutter, cracks seams, and tears hangers out of the fascia, and a ladder leaned against an icy gutter is a fall waiting to happen. A few things are worth doing from the ground:
- Rake fresh snow off the lower three or four feet of roof with a long-handled roof rake, so there's less meltwater feeding the gutter
- Keep the downspout outlet clear, so whatever does melt has a way out
- Keep people and cars out from under heavy icicles and ice-loaded gutters
- Check ceilings and walls along the outside of the house for stains or drips
If the ice has climbed from the gutter onto the shingles, or water is showing up inside, it's time for help. Our ice dam and roof snow removal crews use low-pressure steam, which melts a drainage channel without breaking shingles or gutters, and we hold a 48-hour response on those calls all winter.
When the thaw comes, walk the gutters again. Ice season is hard on hangers, seams, and the fascia board behind them. If a run has pulled loose or the fascia has gone soft, our seamless gutter crews can repair it, or replace it with gutters formed on-site to fit the house. Most seamless gutter projects in the Tewksbury area run about $3,000 to $5,000, and the estimate is free.
Let's find out why your gutters freeze every winter.
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Frozen gutter questions we get every winter
Do heat cables prevent ice dams?
They keep a drainage channel open along the stretch of roof edge, gutter, and downspout where they're installed, which can protect a known trouble spot. They don't stop the heat loss from the attic that causes the melting, so ice can still form above the cable. Air sealing, insulation, and ventilation prevent ice dams across the whole roof. Cables are a patch for the spots those fixes can't reach.
Why do my gutters freeze every winter?
Because water is sitting in them. The usual causes are leftover leaves, a sag where the gutter has lost its slope, a downspout outlet frozen shut at the bottom, and a steady trickle of meltwater from a roof warmed by attic heat. Fix whatever makes the water stop, and the gutter has far less to freeze.
Should I chip ice out of my gutters?
No. Chipping dents the gutter, cracks the seams, and pulls hangers out of the fascia, and working from a ladder against an icy gutter is dangerous. Rake fresh snow off the lower roof from the ground, keep the downspout outlet clear, and call for steam removal if ice is building onto the shingles or water is getting inside.
Do gutter guards stop gutters from freezing?
Not on their own. Guards keep leaves and twigs out, which removes one of the main causes of standing water, but snow can sit on top of a guard and melt into the gutter below, where it can still freeze. Pair guards with good gutter slope, clear downspouts, and a well-sealed attic.