Ice Dams in Erie and the Snow Belt: Causes and Fixes

Short answer

Ice dams form when the roof deck is above 32°F while the outdoor air and the eaves stay below freezing. Snow melts on the warm part of the roof, runs down and refreezes at the cold edge. In most snow-belt houses the heat comes from warm air leaking into the attic, plus ducts and thin insulation at the eaves. Heat cable and shoveling only manage the ice; the lasting fix is to air seal the attic floor, insulate, then ventilate, or to seal and insulate the roof deck.

Key points

Three things have to line up: outdoor air below freezing, a roof deck above freezing, and snow on the roof.
Building Science Corporation calls air leakage into the attic the most common and most damaging cause.
Lake-effect bands can drop 2 to 3 inches of snow an hour or more, and deep snow insulates the roof deck from the cold air above it.
Fix in this order: air seal the attic floor, insulate, then ventilate. Or move the insulation to the roof deck.
Don't chop at the ice, and don't add attic vents before the leaks are sealed.

How ice dams form

Snow on a roof is an insulating blanket. Building Science Corporation and a DOE Building America brief both put a foot of snow at roughly R-10 to R-15. Under that blanket, heat coming up from the attic warms the roof deck. Once the deck goes above 32°F, the bottom of the snow melts.

The meltwater runs downhill under the snow until it reaches the overhang. The overhang has no heated house under it, so it stays as cold as the outdoor air, and the water freezes there. Each melt adds to the ridge of ice. Water that backs up behind it can work under the shingles and into the wall or ceiling below. Long icicles along the gutter are often the only visible sign.

House air leaking into the attic warms the roof deck above 32°F; snow melts from below, runs down and refreezes over the cold eave as an ice dam.

Why Erie and the snow belt get hit hard

The National Weather Service explains lake-effect snow as cold air, often from Canada, crossing the open and relatively warm water of the Great Lakes. The lake adds heat and moisture to the lowest layer of air, which rises and builds narrow bands that can produce 2 to 3 inches of snow an hour or more. A mile or two away it can be sunny.

Erie lives with the results. The weather service's validated figures from the Christmas 2017 storm include 32.6 inches in 24 hours (7 a.m. December 25 to 7 a.m. December 26), 93.8 inches for December 2017 and 166.3 inches for the 2017–18 season. The city's previous season record was 149.1 inches, set in 2000–01.

Deep snow that sits on roofs for weeks, long cold spells and repeated thaw-and-freeze cycles are exactly the conditions ice dams need. The deeper the snow, the warmer the deck under it runs for the same attic heat.

Where the attic heat comes from

Air leaks through the ceiling. Warm air rises and escapes through gaps at the top plates, plumbing and wiring holes, recessed lights, the attic hatch, chimney and flue chases, dropped soffits and knee walls. Building Science Corporation calls air leakage "the most common, most damaging, and often the most obvious problem to solve." See air sealing before insulation.
Ducts and heating equipment in the attic. Leaky, lightly insulated ducts heat the attic directly.
Thin insulation at the eaves. Where the roof comes down over the exterior wall there is little room, so the insulation is often thin or compressed. Building Science Corporation recommends at least the wall's R-value over the top plate.

Why heat cable, raking and membranes treat symptoms

Approach	What it does	Removes the cause?
Heat cable	Melts drainage channels through the ice	No
Roof rake from the ground	Pulls snow off the lower roof	No
Ice-barrier membrane at the eaves	Limits water damage when a dam forms	No
Air sealing the attic floor	Stops warm air reaching the deck	Yes
Insulation, full depth at the eaves	Slows heat conducted to the deck	Yes
Ventilation, after sealing	Carries off the heat that remains	Yes
Closed-cell foam on the roof deck	Moves the thermal boundary to the deck	Yes

Building Science Corporation notes that heat cables are still a common answer, and describes them as throwing energy at the problem. The residential code already requires an ice barrier at the eaves in areas with a history of ice dams, reaching at least 24 inches inside the exterior wall line. That membrane matters, but it doesn't stop the dam from forming; it only limits the damage. Raking and cable are reasonable during a storm. They are not a repair.

The fix, in order

Find the leaks. A blower-door test with an infrared camera on a cold day shows where house air is getting into the attic.
Air seal the attic floor. Pull back the insulation and seal top plates, penetrations, chases and dropped soffits. Closed-cell spray foam works well on large, irregular gaps. Around chimneys and metal flues, ENERGY STAR calls for aluminum flashing and high-temperature caulk, not spray foam, with 1 inch of clearance from metal flues and 2 inches from masonry chimneys.
Insulate. ENERGY STAR's zone 5 recommendation is R-60 for an uninsulated attic, or R-49 added where 3 to 4 inches are already in place. Get full depth over the top plates with baffles or raised-heel framing.
Ventilate. Keep the soffit intakes open with a baffle in every rafter bay. Building Science Corporation and the DOE brief recommend a 2-inch airspace under the sheathing where the ground snow load is over 30 lb/ft², twice the 1 inch most codes ask for. Your building department can tell you the design snow load.
Or seal the roof deck. If ducts or a furnace live in the attic, or the roof is a cathedral ceiling you can't get into, closed-cell foam under the deck moves the boundary to the roof. In the heaviest snow-load areas, Building Science Corporation adds a vented layer above the deck. See sealed attics with spray foam.

What not to do

Don't chop or hammer at the ice. It tears up shingles, and Building Science Corporation points out that falling ice kills people every year.
Don't add vents before sealing. Building Science Corporation calls trying to ventilate away gross air leakage bad practice. Seal first.
Don't foam around chimneys or flues. Use metal and high-temperature caulk at the required clearances.
Don't bury the soffit vents. Insulation without baffles blocks the intake and leaves the eaves thin.
Don't count on a full-roof membrane to fix it. It keeps water out longer; the dam still forms.

Want a number for your own attic? Price My Project or call (814) 208-3478.

FAQ

Do icicles mean I have an ice dam?

Not always, but big icicles along the eaves mean snow is melting higher on the roof. The DOE brief calls them the telltale sign of an ice dam hidden under the snow.

Will more attic insulation alone fix ice dams?

Often not. Air moves through and around fiberglass and cellulose, so unsealed leaks keep heating the deck. Seal first, then insulate.

Who should I call first, a roofer or an insulation contractor?

If water is coming in now, a roofer for the leak and the damage. For the cause, call someone who does air sealing and insulation and will test the attic, ideally with a blower door.