Ice Dams Form on Roof

Problem: Ice dams form at roof edges, causing backup of snow water which can damage home.

Background: Ice dams cause mil lions of dollars of damage to homes in northern areas every year. They are especially prevalent when snow and weather conditions react with poor attic ventilation and insulation. Snow melts next to the shingles, runs down under the top layer of snow, and freezes near the edge of the roof. Additional snow water backs up under the shingles, resulting in soaked insulation; stained, cracked, or spalled plaster or wallboard; damp, smelly, and rotting wall cavities; and stained, blistered, or peeling wall paint inside and out side the house.

What to do: The most effective remedy is to improve both attic ventilation and insulation to keep roof temperatures as close to the outside temperature as possible. All other emergency measures are short-term, and have drawbacks. They may include using a roof rake, hosing ice dams with tap water on a warm day, or having the roof steamed. Removing snow from roofs can be dangerous, water runoff from hosing can damage shrubbery, and steam can expand and contract the roof deck.
Room ceilings should be insulated heavily to minimize heat losses and reduce attic temperatures, and the attic area should be ventilated sufficiently so outside air sweeps out any warmed attic air. Make sure insulation doesn’t lose effectiveness because of bridging, wires, or ceiling fixtures. Also check for uninsulated chimneys, gas vents, warm exhaust piping, or other sources of heat. (A rule of thumb is that attics should have 1 square inch of ventilation opening for each square foot of ceiling area.)

Special advice: Heating cables, arranged in a saw tooth pattern near the eaves, are sometimes installed to help prevent ice dams. They are generally ineffective: melting is limited to only a few inches from the cable, melting often causes secondary ice dams higher on the roof, and cables use large amounts of energy. Never chop through the ice dams down to the shingles, or use a blowtorch, because you may cause roof damage.

Helpful hint: Just improving attic insulation will not prevent ice dams; it must be done in conjunction with adequate ventilation using ridge vents, soffit vents, roof louvers, or power vents. Insulation also must not block air passages, especially immediately above outside walls.