Seasonal

Ice Dams in Rain Gutters, Explained: How Winter Breaks Them

By Pittsburgh Gutter Repair · October 28, 2026 · 8 min read

Early November, and the first snow is a few weeks out. This is the explainer on ice dams in rain gutters: what actually happens to a gutter between December and March in Pittsburgh, why the damage doesn't show until the thaw, and why the oldest houses in the city have it worst. The to-do list is in winterizing gutters and the heat-cable decision in its own post; this is about the physics.

Twenty-eight inches of snow, and dozens of thaws

Pittsburgh gets about 28 inches of snow in a winter, but it rarely stays. The temperature crosses freezing dozens of times between December and March. Each crossing means water that was liquid becomes ice — and expands by about 9% doing it — then becomes liquid again and finds somewhere new to sit. A gutter is a trough designed to hold water, so it holds ice, and every cycle works on every joint. Frozen gutters aren't a single event here; they're a season of freezing, thawing and refreezing.

How ice dams in rain gutters form

An ice dam is not really a gutter problem, though the gutter is where it shows. Heat leaks from the house into the attic — through an uninsulated hatch, around recessed lights, up the walls of a Cape Cod's knee-wall rooms, through a third floor under a slate roof in Squirrel Hill — and warms the roof deck from underneath. Snow on the warm part of the roof melts. The meltwater runs down under the snow to the eave, which hangs past the wall and is cold, and refreezes. That first ridge of ice is the dam. Everything after it backs up behind it, refreezes, and the dam grows up the roof. Water that can't get past it goes under the shingles and into the wall, and that's the ceiling stain in the corner of the front bedroom.

The gutter sits below the dam and fills solid. On a north-facing eave in Mt. Washington or Brighton Heights that never sees the sun in January, that ice in rain gutters can stay put until March.

What a gutter full of ice weighs

Water weighs about 8.3 pounds a gallon, and a 5-inch K-style gutter holds roughly 1.2 gallons per foot. A 40-foot run full of ice is roughly 400 pounds, plus the icicles hanging off the front lip, plus whatever snow has slid off the roof and is sitting on top. Spike hangers at 36 inches were never asked to hold that. Hidden screw hangers at 24 inches are far better suited to it, and at 16 inches on a north eave they give the gutter a real margin. That's the whole reason the hanger spec matters more here than in a warm climate.

Gutter damage from snow sliding off the roof

A slate or metal roof doesn't hold snow the way asphalt does. When the sun hits it or the attic warms it, the whole blanket can let go at once and land on the gutter. That's why gutters on slate roofs in Highland Park and the older streets of Mt. Lebanon are often hung lower and further back than on shingle, with snow guards on the roof above them, and why 6-inch gutter in heavier gauge earns its place there. A 5-inch .027 gutter on spikes under a slate roof is at real risk of ending up on the lawn.

Freeze-thaw and the corner seam

The sealant at a mitered corner is a bead of polyurethane between two pieces of aluminum. In summer it flexes with the heat. In winter, water sits in the corner, freezes, expands into the joint, thaws, and leaves the joint a hair wider than before — dozens of times a season. A corner that was tight in October can weep by March. That's why leaking corners are among the most common spring gutter repairs, and why resealing in the fall, when the joint is warm and dry, works better than waiting until April when it's already open. The downspout elbow does the same thing more violently: water freezes in the bottom elbow, expands, and pops a pushed-together joint apart or splits a crimped one.

Why box gutters suffer most

A hung aluminum gutter holds ice on the outside of the house. A built-in box gutter on a 1900–1930 brick home holds it inside the roof edge, in a wooden trough, on a metal lining that's been through many decades of winters. The trough is deeper than a hung gutter, so it holds more ice; the lining seams are soldered tin or galvanized, which crack rather than flex; and when the outlet freezes, the meltwater has nowhere to go but through the first crack it finds, into the cornice and the top of the wall. That's the peeling cornice paint and the stain at the top of the plaster that Highland Park and Shadyside owners notice in March. A cable in the trough keeping the outlet open through February is cheap insurance; the box gutter page covers the relining that fixes it properly.

What the damage looks like in March

• A run pulled away from the fascia, showing a gap at the top: hangers backed out under ice load. Re-hang.
• The front lip folded down along a stretch: sliding snow or a large ice load. Straighten or replace the section.
• A corner dripping that didn't drip in October: freeze-thaw opened the seam. Reseal.
• A downspout split at the bottom elbow or separated at a joint: ice expansion. Replace the downspout.
• Peeling paint on the fascia below a corner: overflow from a frozen gutter ran behind it all winter. Check for soft wood.
• Water stains on an inside ceiling near an outside wall: ice dam, and the attic needs attention before the gutter does.

If you see any of these, ice dam gutter repair in Pittsburgh is usually a re-hang, a reseal or a downspout, priced on the gutter repair page. Photograph the damage before anything is touched; if a ceiling or wall was damaged too, check your policy or ask your agent what it covers.

Questions to ask after a hard winter

When someone looks at the gutters in spring, these questions get you a useful answer rather than a general one:

• Did any hangers pull out, and is the fascia sound where they were?
• Is the front lip bent anywhere from ice or sliding snow?
• Which corners or end caps opened over the winter?
• Did any downspout split at an elbow or separate at a joint?
• Is there any sign the attic is losing heat at the eave, such as staining on the soffit?

Ask for the answers in a short written list with photos. It tells you what to fix now and what to watch next winter.

The short version

The gutter can't prevent any of this. What it can do is survive it: heavy gauge, hidden hangers close together, 3x4 downspouts with screwed elbows, corners sealed in warm weather, and a clean trough going into December so the ice has nothing to grip. Do those and the March inspection is a formality. Skip them and March is when you're booking repairs, which is also when local gutter crews are busiest.

Common questions

How do you prevent ice dams in gutters?

Start in the attic, not the gutter. Air-sealing the attic floor and adding insulation keeps the roof deck cold, so snow doesn't melt from underneath and refreeze at the eave. After that, keep the gutters clean going into December, make sure downspouts flow, and consider a heat cable only on eaves that still ice up. Gutter heat cables in Pittsburgh covers when a cable is worth it.

Can ice pull gutters off a house?

Yes. A run full of ice can weigh hundreds of pounds, and spike hangers that have already backed out of the fascia can let go under that load, especially when snow slides off the roof on top of it. Hidden screw hangers at 24 inches or closer, fastened into sound wood, are what keep a gutter on the house through a Pittsburgh winter.

Why do ice dams form on some houses and not others?

It comes down to how much heat reaches the roof and how much sun the eave gets. A house with a warm attic, recessed lights in the top-floor ceiling or knee-wall rooms melts snow from underneath; a well-insulated house next door doesn't. North-facing eaves that stay in shade all day freeze that meltwater fastest, so the same street can have ice dams on one side and none on the other.