Ice Daming
An ice dam is a problem of house and building maintenance in cold climates. An ice dam can occur when snow accumulates on the slanted roof of a house with inadequate insulation and ventilation in the attic. Warmth coming up through the roof melts the snow. Meltwater flows down the roof, under the blanket of snow, onto the eave and into the gutter, where colder conditions on the overhang cause it to freeze. Eventually, ice accumulates along the eave and in the gutter. Snow that melts later cannot drain properly through the ice on the eave and in the gutter. This can result in:
- Leaking roof (height of leak depends on extent of ice dam).
- Wet, ineffective insulation.
- Stained or cracked plaster or drywall.
- Rotting timber.
- Stained, blistered or peeling paint.
Under extreme conditions, with heavy snow and severe cold, almost any house can have an ice dam, whereas a house that is poorly insulated and ventilated will have ice dams during normal winter weather. Giant icicles hanging from the eave are one indication of a poorly insulated, poorly ventilated attic.
How to prevent ice dams:
- Keep the roof cold by keeping the attic cold.
- Install more insulation in the attic floor.
- Seal bypasses into the attic.
- Draw more outside air into attic through gable and eave vents.
- Make sure vents aren't blocked.
- Keep gutters clean.
- Install slippery metal roofing along the eaves so that snow and ice slide off more easily. Install real metal roofing such as standing-seam metal panels (either integrated with asphalt roofing, or a 100% metal roof). Do not install a roll of aluminum flashing or inexpensive sheet metal, because the nails and nail holes will be exposed.
- Before installing any shingles, install an adhesive, waterproof, rubberized ice-shield membrane on the sheathing to seal around nail holes and protect the sheathing from backed-up water. These membranes are usually made of rubberized asphalt, polyethylene, or bituthane.
- Switch to a warm-attic system. Make your attic part of the conditioned space of your house. This requires that you insulate the attic roof from both sides, insulate the exterior walls of the attic, and install energy-efficient attic windows.