The icicles hanging from your eaves and gutters last year may be a faint memory now. But winter is coming, and along with it one of your home's worst enemies—ice dams. Ice dams are continuous chunks of ice that form along the margins of your roof. While frozen, they're no more trouble than the icicles that hang down. But during the warmer parts of a winter day, water melting off the roof pools behind the ice (Fig. A), then seeps back up under the shingles. Sometimes water can work its way 5 or even 10 ft. back up under the shingles. Eventually it drips through the roof into the soffits (the outside overhangs), walls, and worst of all, onto your ceilings. You'll first see rust spots on drywall fasteners, then perhaps peeling paint, sagging drywall and stains around windows and doors (Fig. A). Insurance companies pay millions of dollars to thousands of homeowners annually to repair the damage. But it's never enough to cover the time and aggravation of getting everything fixed.

Now is the best time to stop ice dams, before winter comes and before they build up. In this article, we'll tell you the best ways to prevent ice dams. And we'll also outline the few options you have once you've got them.

Ice dams (and icicles) form when snow melts, runs down your roof and refreezes near the edge. This only occurs when part of your roof warms to above 32 degrees F, warm enough to melt the snow, while the roof edge remains below freezing. This scenario is often the result of a warm attic. In most homes, heat escapes through ceilings into the attic and warms the wood and shingles directly above it (Fig. A). Although the outdoor temperature is below freezing, the snow melts over the warmed section of roof. When the meltwater runs down the roof, it hits the cold edge not warmed by the attic. There it freezes, creating a rim of ice. This rim can grow, trap more water behind it, and bingo—you have a full-fledged ice dam.

The key to preventing ice dams is simply to keep your attic and roof cold. After a snowfall, a cold roof will have a thick blanket of snow. A warmer roof, however, will soon have clear spots where the snow has melted off, and may well have icicles hanging from the eaves.

To keep your roof cold, follow these three steps:

1. Close up attic bypasses
In the average home, about one-third of the heat loss is through the ceiling into the attic. And most of that loss comes from air leaks caused by unblocked walls, gaps in drywall, and cracks around light fixtures, plumbing pipes, chimneys, access hatches and other ceiling penetrations (Fig. A). Air leaks can be tough to stop. You have to climb into your attic, pull or rake back insulation, and plug the leaks using foam, caulk and other methods. Low roof angles make some air leaks difficult to reach. This work is definitely a cool weather project; your attic will be unbearably hot otherwise. Always wear a dust mask, a long-sleeved shirt and long pants to help prevent skin irritations caused by insulation.

Bonus: By stopping air leakage to mitigate ice dams, you'll save energy and reduce both your heating and your air conditioning bills.

2. Measure your attic insulation level
While you're in the attic, check the depth of your attic insulation. Building codes require about 12 to 14 in. of fiberglass or cellulose (Fig. B). Add more if you have less than 8 in. and have had ice dam problems in the past. Blown-in cellulose and fiberglass are usually better than hand-placed batts, because they fill more tightly around rafters, joists and other obstructions, leaving fewer gaps. It's usually worth hiring a professional for this job; you probably won't save much by doing it yourself. However, if you can't find a good price, you can rent a blowing machine from a rental yard or home center. Often, the use of the machine is free with the purchase of insulation.

3. Add roof and soffit vents
Attic ventilation draws in cold outdoor air and flushes out warmer attic air, cooling the attic and the roof in the process (Fig. B). The minimum ventilation area (size of the openings) should be about 1 sq. ft. of vent per 300 sq. ft. of ceiling area (attic floor area), when half the vent area is low on the roof and half is high. Actually figuring all this out is a bit complex; you'd have to examine your existing vents to find the area of each, which is stamped on them. As a rule of thumb, put an 8 x 16-in. vent in the underside of the overhang (soffit) in every other rafter space (Fig. B). (If you're planning to rebuild the soffit, install a continuous 2-1/2-in.-wide “strip” vent, because it will look better.) And install a continuous ridge vent along the peak (Fig. B). If the ridge on your roof is much shorter than the roof edge—on pyramid-shaped roofs, for example—add the common square-shaped roof vents near the peak (Fig. B). Add enough so their ventilating area is about equal to the area of soffit vents. This might deliver a whole lot more ventilation than the minimum requirement, but don't worry. You're unlikely to have too much ventilation.

In addition:

• Some roof types are difficult to vent, especially roofs with angled ceilings and no attic, roofs with skylights, flat roof dormers and low-slope roofs. You may have to rely on the secondary strategies we list in the next section.
• Insulation, especially the blown-in type, can block the airflow (Fig. A). Take a long 1x2 up into the attic with you and poke it through the spaces between rafters over the exterior walls to make sure they're open. Baffles (Fig. B) usually prevent this problem. If you don't have them, add them before installing additional insulation. A shot of air from a compressor hose from the outside will open plugged soffit vents.
• Assessing the venting is tricky on homes that have had aluminum retrofitted over old soffits. If you have persistent ice damming in one area, you might have to remove several aluminum sections to check the venting.

Reminder
Climbing onto your roof can be dangerous. Follow safety procedures.

 Keep the roof cold to minimize ice dams. Upgrade attic insulation to about R-40, plug up air leaks to the attic and improve attic ventilation.