Ice Dams: What Actually Causes Them (It Is Not Your Gutters)

Every winter, homeowners in cold regions watch ice build along the eaves, see water appear on a ceiling, and conclude the gutters have failed. Contractors get called to replace gutters that were not the problem. It is worth understanding what is actually happening, because the wrong fix is expensive and does not work.

The mechanism

Heat escaping from the living space into the attic warms the roof deck. Snow sitting on that warmed section melts from underneath. The meltwater runs down the roof until it reaches the eave — which overhangs unheated space and is therefore at outside temperature — where it refreezes. Repeat that cycle over several days and you build a ridge of ice at the eave. Subsequent meltwater pools behind that ridge, and standing water finds its way under the shingles and into the building.

Why gutters get blamed

The ice is most visible in and around the gutter, and a gutter full of ice looks like the obvious culprit. But the gutter did not cause the melt-refreeze cycle; it is simply sitting at the coldest point on the roof where the ice was always going to form. Remove the gutters entirely and you will still get an ice dam at the eave.

What gutters and guards do and do not do

A blocked gutter genuinely makes things worse: debris holds water that freezes sooner and provides a starting point for the ice ridge. Keeping gutters clear and draining is worth doing. But no gutter, and no gutter guard, addresses heat loss through the roof. Any product marketed as preventing ice dams is, at best, reducing one contributing factor.

Heat cables

Heat cables melt a channel through the ice so meltwater has somewhere to go. They treat the symptom, they run up electricity costs every winter, and they do nothing about the heat loss causing the problem. They have a place as a stopgap on a roof that cannot easily be fixed otherwise, but they are not a solution.

What actually fixes it

Three things, in order of importance. Air sealing: stop warm air leaking from the living space into the attic through light fittings, hatches, plumbing penetrations and top plates. Insulation: bring the attic floor up to a proper depth so less heat reaches the deck. Ventilation: keep the roof deck cold with adequate soffit-to-ridge airflow, which means making sure insulation is not blocking the soffit vents.

Get those right and the roof deck stays cold, the snow stops melting from underneath, and the dam does not form. That is a roofing and insulation job, not a gutter job — and we would rather tell you that than sell you a gutter system that will not fix it.

Where we do come in

Ice damage to the gutters themselves is real and common: the weight of ice pulls hangers out of the fascia, deforms the trough and tears runs loose. The contractors we work with repair and replace that damage, and make sure the system is clear and draining before winter so it is not contributing. A good one will also tell you plainly when the underlying problem is above the gutter line.

Spotting an ice dam before it leaks

Icicles along the eaves are the visible sign, but the problem is the ridge of ice behind them, on the roof edge itself. If you see a thick band of ice along the bottom of the roof and bare or melting snow higher up, there is heat escaping from the house and water pooling behind the ice. Water stains on ceilings near exterior walls, or damp insulation in the attic, often follow.

Large icicles hanging from gutters are also a safety hazard. They can weigh far more than they look and fall without warning, so keep people away from the area below them.

What not to do

Do not chip at ice on the roof with a hammer, axe or shovel. It is easy to damage shingles, gutters and flashing, and the roof edge is a dangerous place to work in winter. Avoid salt or chemical de-icers poured directly on the roof, which can damage shingles, metal and plants below. A roof rake used from the ground to remove snow from the bottom few feet of the roof after a storm is safer and reduces the meltwater feeding the dam.

Gutters for heavy snow areas

Even when the attic is fixed, gutters in snow country take a beating. The weight of wet snow and ice resting in a full trough can reach several hundred pounds on a long run. Gutters for heavy snow areas are hung differently from those in mild climates: hangers every 24 inches or closer, a heavier gauge of metal, and fasteners driven into the rafter tails where possible rather than only the fascia board. Many contractors in cold regions also hang the gutter a little lower relative to the roof edge, so sliding snow passes over the front instead of tearing the gutter off.

Snow guards on the roof, small brackets or bars that hold the snowpack in place, protect gutters under metal roofs where snow can slide off in a sheet. On shingle roofs they are less common but still used above entrances and decks.

Do gutter heaters prevent ice dams?

Gutter heaters, meaning heat cable laid in the gutter and on the roof edge, do not prevent ice dams in the strict sense. The dam still tries to form; the cable keeps a melt channel open through it so water can escape instead of backing up under the shingles. That makes them useful on roofs where the insulation cannot be reached, such as low-slope additions, cathedral ceilings and porch roofs. The heated gutters page explains how the cable is laid and powered.

Underground downspouts in winter

Buried downspout drains are a common hidden cause of winter trouble. If the pipe has a low spot, water settles in it and freezes, and the next thaw has nowhere to go: it backs up the downspout and fills the gutter, where it freezes again. Pop-up emitters at the end of the pipe can freeze shut. In hard-freeze regions, many homeowners disconnect the buried line in late autumn and fit a temporary above-ground extension for the winter, then reconnect it in spring. The underground drainage guide covers slope and depth.

After the thaw: what to check

Once the ice has gone, walk the house and look up. Gutters that sag between hangers, hangers pulled partly out of the fascia, bent front lips and split end caps are all signs of ice load. Inside, check the ceiling and walls below the eaves for stains, and the attic for damp insulation or frost marks on the underside of the roof deck. Photograph anything you find. If you plan an insurance claim, the photos and the dates matter.

Read next

• Gutters and Gutter Guards for Metal Roofs
• Gutter Water Damage: Foundations, Basements and Splash Guards
• Clogged Downspout? How to Find and Clear the Blockage

More gutter guides

• Why Your Gutters Overflow Even When They Are Clean
• How Often Should Gutters Actually Be Cleaned?
• Seamless vs Sectional Gutters: What It Really Costs You
• Gutter Guards: An Honest Comparison of What Works
• Gutter Pulling Away From the House? Read This First

Ready to get it looked at? See gutter repair in your city or check typical costs in the pricing guide.