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Thawing, refreezing builds ices dams, can breach roof

This is the winter of our discontent.

And here's the biggest winter bugaboo: ice dams, and the leaking of water into the house that often accompanies them. Some people call them ice damns, because they are as frustrating as an exploding computer. And once you get an ice dam and resulting leaks, there is little you can do until snow, ice, and water go away.

How they get startedIce dams occur usually on roofs of houses that have little insulation on the attic floor and little ventilation to cool the attic, making the roof relatively warm. Heavy snow piles up on the roof and just sits there, until during a slight warming period the snow melts from the bottom up. Then during a cold snap the water freezes at the edge of the roof; it is this freezing water that is the ice dam.

Then, because the roof is warm, water behind the dam backs up, getting under the shingles and leaking into the attic and into walls, and even filling windows with water. Sometimes the water goes from the attic into a wall, bypassing the second floor and coming out on a first floor ceiling. Sometimes chandelier shades fill with water, and ceiling fans fail because they become soaked.

As long as the ice dams remain, and water gets backed up by those dams, leaks will occur.

Ice dams can occur on shallow slopes, steep slopes, and flat roofs. Gutters do not necessarily cause ice dams, but when they freeze up they can cause water to overflow the gutter and run down the fascia (the board behind the gutter), under the soffit (the under part of the roof overhang), and down the walls, both inside and out, causing massive amounts of ice to cover the wall. Sometimes it can bulge out wood shingles.

Huge icicles can form on houses with warm roofs and frozen gutters. They are dangerous only if they hang above where people and animals walk. But if they are big enough, they can pull gutters right off the fascia board.

What you can doThe frustrating part about ice dams and the leaks is that there is little you can do until the snow and ice and water go away. Of course, you can try bailing out windows and turning off chandeliers and other fixtures full of water. When they dry out they may very well work OK, but don't try turning them on when they are wet.

The next priority is to get rid of the snow, which is dangerous and uncomfortable work in cold weather. You can buy a snow rake (some hardware stores may still have some in stock) and pull off the snow as high up the roof as you can get. The higher the removal of snow the better, because ice dams can form at the line where you removed the snow. If you can't find a snow rake you can use a garden rake upside down, or wire a wide board to the rake to pull off the snow.

Don't try to chip off ice; that is a sure-fire way of wrecking roof shingles and gutters. Also, if you don't like heights or cannot stand the cold, stay off the roof and off of ladders. A handyman might be able to help, for a fee.

Preventing ice damsYes, ice dams can be prevented.

Once the snow is gone, you can go to work to make the house ice dam-proof. The way to do this is to make the roof cold, so that snow on the roof will melt from the top down, which makes it difficult to form ice dams.

Make sure the attic floor is heavily insulated; up to 12 inches of fiberglass is recommended. Six inches is good, but 12 is better. And there should be a vapor barrier under this insulation to prevent house air from entering the attic, warming the roof. Keep insulation away from the soffit.

And, make sure any attic stairway opening is well sealed and insulated. An uninsulated stairway cover is a notorious loser of heat to the attic.

Finally, an equally important thing to do is to ventilate the attic, to keep it and the roof cold. Many attics have vents at each gable end, which may be adequate to vent out warm air and moisture but most are not.

Proper ventilation for an attic are ridge vents, which are installed the entire length of all roof ridges, and, equally important, soffit vents. And do not consider those round and rectangular vents for the soffits; they are often spaced too far apart to be of use. The best soffit vent is a continuous 2-inch-wide screened strip, going the full length of all soffits. Don't let anyone talk you out of such soffit vents, because they provide necessary ventilation at the bottom of the roof, at the eaves, where ice dams always start.

Some houses do not have soffits; they have what is called closed eaves, which is a design flaw. You can vent closed eaves with an eave vent, which is a special kind of drip edge that screens off a slot in the fascia board. The fascia board must be cut to provide this air slot. To give you an idea where this eave vent is located, it is just above the gutter and below the drip edge.

If all else fails, heating cables can be installed in a zig-zag pattern along the edge of the roof, and in gutters and downspouts. The cables will melt a groove in an ice dam to allow water to go over the roof edges instead of being backed up. They are expensive to operate and can be a fire hazard if left operating on a dry roof. They must operate on a snowy, icy, or wet roof.

Modern extra protectionModern roofs have extra protection to guard against leaks from ice dams. Roofers install a 3-foot-wide rubberized membrane along the edges of a roof. This membrane, of which there are several brands (W.R. Grace Ice & Water Shield is a popular one), will not prevent ice dams, but will keep dammed water that goes up-roof from getting under the roof shingles.

This membrane, under the roof shingles, is a modern variation of installing large flashing of galvanized steel, aluminum, or copper at the bottom of a roof. They are common in Vermont and New Hampshire.

Not only is it important to keep attics and roofs cold, but to avoid putting anything in the attic that could make the roof warm.

One example of what not to put in an attic is a hot-air furnace, fired by gas.

Some builders of large houses have put in two furnaces, one in the basement for the first floor and one in the attic for the second floor. This is a bad move. You can insulate a hot-air furnace but you cannot insulate against an open flame. It is this flame that makes the roof warm, and ice dams result.

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