Ice Dams and Winter Roof Leaks
One sunny winter afternoon I came home to find water pouring out of my ceiling into my family room. “What is happening!!!??” After putting buckets under the streams of water, I walked around to the back of my house. The sun shone on my west-facing roof, melting snow on the roof. The water ran down into a valley that ended beside a wall of the house that shaded the end of the valley. There, with no sun shining on the end of the valley, the water froze and formed an ice dam a foot deep. Water backed up behind the ice dam, ran underneath the shingles, fell through the attic, soaked through the insulation and drywall, and poured into the family room. I fixed the problem temporarily by shoveling the snow off the roof (scary and dangerous! – but I wore a safety harness). Then I installed a temperature and humidity controlled heating cable on the roof to prevent the ice dam from forming again.
While removing snow from the roof and adding a heating cable was my solution, there may be better solutions for most ice dam problems. For all of the suggestions below, take adequate precautions to ensure your own safety if you use a ladder or climb on the roof.
Remove snow from the roof
Use a “roof rake” to pull snow off the roof.
Heat Cable
A heat cable is installed in valleys, in a zig-zag pattern on the eaves, or in other places an ice dam is likely to form. The heat cable keeps a channel clear for water to flow all the way to the roof edge, even if an ice dam is present. The heat cable may also prevent an ice dam from forming. A temperature and humidity controller for the heat cable can be installed so that it comes on only when needed. Unless other methods have been tried unsuccessfully, a heat cable should be regarded as a temporary solution. Electricity isn’t always reliable in a cold climate, and a heat cable or its controller can fail when most needed.
Ice Dam Melter
Various products are available that will melt through an ice dam or that will keep a clear channel through an ice dam so water doesn’t collect behind the ice dam. The basic idea is salt in a sock. The salt melts the ice. Other products use a lighter weight gel in a tube rather than heavy salt in a sock. The sock or tube is placed in the valley or at the eave (perpendicular to the eave edge) where ice is likely to form. This is an emergency solution, since the salt or the gel is damaging to plants and even damaging to the roof.
Adequate Attic Ventilation
A well-ventilated attic allows air to enter the attic from under the eaves. The air flows up through the attic and out through roof vents. This keeps the attic cold in the winter so that snow doesn’t melt off the roof, and then form an ice dam on the eave where the eave is colder than the rest of the attic. It also keeps the air in the attic dry in the winter so condensation doesn’t from on the attic sheathing, which might shorten the life of the sheathing. In the summer, the ventilation keeps the attic cooler, which makes your shingles last longer and reduces your cooling bill.
Many modern roofs have a ridge vent that goes along the entire length of the roof, rather than individual vents lined up across the roof near the roof ridge. Older homes may have gable vents rather than roof vents. A home should not have both gable vents and a roof ridge vent, and should not have both a roof ridge vent and individual roof vents. Having both causes the vents to compete with one another. Instead of the air flowing naturally from under the eaves through the attic and out the top, the air may only circulate in the very top part of the attic between the gable vent, the individual vents, and the roof ridge vent.
Good ventilation requires enough vents under the eaves and near the roof ridge. To ensure a good flow of air, one square foot of net free area (the area of the vent after subtracting for screens and louvers) is needed for 150 square feet of attic. This can be reduced to one square foot of net free area for 300 square feet of attic if a vapor retarder is installed under the insulation. Some insulation batts are “faced” with a vapor retarder, which should be installed with the vapor retarder toward the heated interior (the warm side) in cold climates like Wisconsin’s. Or your house may have a plastic sheet under the insulation that works as a vapor retarder.
Sometimes when insulation is installed in the attic, the insulation is installed all the way against the roof sheathing. This blocks the flow of air from under the eaves into the attic. Properly installed insulation will have an inch or more gap between the insulation and the roof sheathing. For loose-fill insulation, this is accomplished by an insulation chute, as shown in the picture for this article.
An improperly installed powered ventilation fan on your roof may actually decrease attic ventilation. The fan can suck air from the roof vents rather than from the under-eave vents, disrupting the natural flow of air that was designed into the attic.
Adequate Attic Insulation
A well-insulated attic keeps the heat inside the house so that the attic stays cold in the winter here in Wisconsin. Snow is less likely to melt off of a cold roof, and then form an ice dam on the eave. But be careful here! Adding more insulation without first adding a vapor retarder may do more harm than good – see my previous post “Is more insulation in your attic always good?
Ice Dam Membrane
An ice dam membrane can be installed underneath the roofing materials (shingles and tar paper). This membrane extends from the edge of the roof back 24 inches over the heated part of the house. If you were inside the house, 24 inches from the wall, looking straight up through an invisible roof, then the edge of the membrane would be right above you, or even further inside the house. This membrane doesn’t prevent an ice dam from forming, but is water tight and prevents water behind the ice dam from getting into the house. Installing an ice dam membrane is typically done only when the shingles on the house are replaced.
Summary
We’ve reviewed various methods for preventing ice dams or for preventing water entry even if an ice dam forms. Having adequate attic insulation and attic ventilation if the first and best line of defense. If you are re-roofing your house and you live in a cold climate, don’t skimp on installing an ice-dam membrane. Temporary solutions include using heat cable or an ice dam melter. Whatever method you use will be well worth the effort – roof water getting inside a house can lead to expensive and time-consuming repairs.
References:
https://www.michigan.gov/miready/be-informed/winter-prep/preventing-roof-ice-dams
https://extension.umn.edu/protecting-home-rain-and-ice/dealing-and-preventing-ice-dams
