Ice dams: a heat problem that shows up at the gutter

How an ice dam forms

The University of Minnesota Extension defines an ice dam as a ridge of ice at the edge of a roof that keeps melting snow from draining off. Its ice dam guide lays out the conditions: there must be snow on the roof, and at the same time the higher parts of the roof surface must average above 32 degrees while the lower edge averages below it.

Snow on the warm part of the roof melts. The water runs down to the cold edge and freezes, and the dam grows as long as melting snow keeps feeding it. Water pooling behind the dam stays liquid, finds cracks and openings in the roof covering, and can travel into the attic, down exterior walls or through the ceiling insulation.

Where the heat comes from

Extension's answer is plain: it is primarily heat flowing from the house that warms the upper roof. Heat moves through the ceiling and insulation, and warm air leaks into the attic through openings such as light fixtures; in many homes, the guide says, that air leakage is the major way heat reaches the roof. Kitchen and bath exhausts that end just above the roof, chimneys, leaky ductwork in the attic and knee-wall spaces all add to it. Sun alone rarely does it.

What gutters do, and do not do

Nowhere in its explanation of causes does the Extension guide name gutters. The dam forms at the edge because that is the part of the roof that stays below freezing, and a gutter simply happens to hang there.

That does not make gutters irrelevant. Blocked drainage makes melting snow a hazard, according to FEMA's Snow Load Safety Guide: when a roof drainage system is blocked or poorly maintained, ice dams can form, concentrating load at the eaves and keeping sloped roofs from shedding snow. The National Weather Service office in Grand Rapids tells homeowners to clean leaves and debris from gutters and downspouts so melting roof snow can flow through them, and to keep them clear of snow and icicles all winter. Michigan's insurance regulator gives the same advice in its winter storm tips.

Put together: a clear gutter keeps the drainage path open, but it cannot stop the melting that feeds the dam. Keep hangers sound and outlets open, and do not expect any gutter, seamless or not, to cure attic heat loss.

What to do when a dam has already formed

Extension's short-term steps:

1. Remove snow from the roof, which takes away one of the ingredients. A roof rake or push broom works, though either can damage roofing materials. The Grand Rapids NWS office notes that long-handled roof rakes let you work from the ground.
2. If water is already coming inside, open channels through the dam by hosing it with tap water on a warm day, working upward from the lower edge. The channel only lasts a few days.

The same guide warns that anyone on the roof in winter, or working on it from below, risks injury and damage to the house, and it recommends hiring professionals. FEMA's snow guidance adds that sharp tools such as picks should not be used to remove snow.

The permanent fix is in the attic

Extension's long-term order of work is to air-seal the ceiling first, so no warm, moist air flows from the house into the attic, and then consider adding ceiling or roof insulation. It does not recommend mechanical attic ventilation as an ice dam solution in Minnesota, because it can create moisture problems and negative pressure in the home. It suggests a weatherization contractor who runs a blower door test, and it advises holding off on interior repairs until ceilings and walls are dry and the heat loss is corrected.

The Grand Rapids NWS page adds that most experts agree attic insulation should be at least R-30, with R-38 preferable in northern climates, and that good airflow from the soffits along the underside of the roof and out through roof vents is essential.

What the code puts under the shingles

The International Residential Code tackles the leak side of the problem. Section R905.1.2 requires an ice barrier where there is a history of ice forming along the eaves and backing up water. It must extend from the lowest roof edges to at least 24 inches inside the exterior wall line, and on roofs sloped 8 in 12 or steeper, at least 36 inches up the slope from the eave. Detached accessory structures without conditioned floor area are exempt. Your state's adopted code governs the details.

Insurance

Michigan's Department of Insurance and Financial Services says homeowners policies usually cover damage resulting from ice dams, in the same winter storm tips. Read your own policy, then use the National Roofing Directory's guide to the roof insurance claim process.

For repairs, see NRD's roof repair guide, or find contractors in Wisconsin and Michigan. Summer's threat to the same gutters is covered in the hail guide.

Frequently asked questions

Do gutters cause ice dams?

Not according to the University of Minnesota Extension, which traces them to uneven roof temperatures caused mainly by heat escaping from the house. Blocked gutters and downspouts can still keep meltwater from draining, so NWS and FEMA both advise keeping them clear.

Should I chip the ice off my gutters?

Extension's emergency method is to cut channels through the dam with tap water on a warm day, working up from the lower edge. FEMA's snow guidance advises against sharp tools such as picks, and working on a winter roof carries a real risk of injury.

What does the building code require to stop ice dam leaks?

The International Residential Code requires an ice barrier where eave icing is a known problem, running from the roof edge to at least 24 inches inside the exterior wall line, and at least 36 inches up the slope on roofs of 8 in 12 or steeper.

Does homeowners insurance cover ice dam damage?

Michigan's insurance department says homeowners policies usually cover damage resulting from ice dams. Coverage depends on your policy, so check its terms before you file.