Ice dams are created by a ridge of ice forming at the edge of a roof. The ridge begins as melting snow that runs down the roof until it gets over a cold spot (i.e, over the soffits) and freezes. This ridge of ice, or ice dam, prevents melting snow from running off the roof, so it pools up behind the dam, eventually working its under the roofing shingles and even down into the house where it can cause significant damage to insulation, ceilings, walls, and even furniture.
Ice dams are caused by an interaction of heat loss from within a house, snow on a roof, and temperatures above 32° F on the top portion of a roof. There are a number of things you can do to prevent ice dams and subsequent damage. Here are some ideas.
Ensure Adequate Insulation in Your Attic
Warm air from your home migrating into the attic can raise the temperature of the underside of your roof enough to start melting snow—particularly when combined with a warm sunny day. Find out the recommended "R-value" (degree of resistence to heat flow) of attic insulation in your climate zone and make sure yours is at least that high.
Insulate and block off any openings into your attic. Just some of these areas include attic access hatches, potlights and ceiling fixtures, plumbing stacks, and HVAC ducts (exhaust fans, for example). Putting insulation too close to a pot light or ceiling fixture can be a fire hazard, though. To get around this, get some wood and build a perimeter of a few inches in every direction from the light fixture.
Make Sure Your Attic is Properly Ventilated
Proper ventilation helps keep your attic temperature similar to that of the outside air and cools the underside of your roof, preventing snow from melting. Common guidelines suggest a minimum 1 square foot of free ventilation area for each 300 square feet of attic ceiling area when there is a vapor barrier at the ceiling.
Make sure the area over your soffits isn't blocked by insulation and the soffits are open to the outside air. A roof overhang of 12 inches or more should provide ample space for inlet (soffit) vents.
Adequate upper ventilation to ensure proper and continuous airflow is also required. Either continuous ridge vents or a number of individual roof vents will normally provide this upper ventilation.
Protect Your Roof
Installation of drip edges or ice shields at your eaves will direct water right off your roof and into the gutters.
Installing a water proofing membrane along the lower edge of your roof (extending back from the edge at least 24 inches) helps keep water out of the roof. Water proof membranes, or ice shields, are often a building code requirement, and they do prevent roof damage.