Ice Dams on Your Roof: How to Prevent Them

Icicles hanging from the edge of a residential roof
Ice dams cause some of nature’s most beautiful sights — icicles — but they also wreak havoc on your home. (DepositPhotos)

If you’re fortunate enough to live in the Sun Belt or some other balmy region, chances are you’ve never heard of — or at least experienced — ice dams.

However, in New England and other areas, every winter, ice dams are as common as frostbite and sub-zero temperatures.

Ice dams on house in winter
Ice dams lead to icicles. (DepositPhotos)

Ice Dams: Beautiful, But Problematic

Ice dams are often responsible for one of the most beautiful and iconoclastic of all winter images: glistening crystal-clear icicles hanging from gutters and eaves.

It all starts with a roof blanketed in snow. The snow layer sitting directly on the roof begins to melt, and water runs down the roof under the blanket of snow.

When the water hits the overhanging eave of the house, it begins to freeze. Some water drains into the gutter, where it freezes as well.

As the snow continues to melt and water freezes at the eave, ice eventually builds up along the roof, forming a thick ridge or dam.

Then, as water runs down the roof, it’s blocked by the ice dam, and forced up the roof.

It seems to defy all laws of physics, but water will actually flow up the roof, working its way under the shingles.

House surrounded by ice in the winter
Ice eventually melts, and soon, that can cause damage. (DepositPhotos)

Why Ice Dams Should Concern You

Roofs are designed to shed water that runs down the shingles, not up. However, when ice dams form, over time, water will work its way beneath the shingles and the underlayment, and into the seams between the plywood roof sheathing.

From there, water drips into the attic, where it soaks through the insulation and drywall ceiling, before dripping into the room below. If not detected immediately, the damage can be difficult and expensive to repair, especially if it ruins the floor, walls or furnishings.

The science behind this phenomenon is much more complicated, but if you go back to the beginning, you’ll find a clue to solving this problem.

The entire process starts when snow sitting on the roof begins to melt. Ideally, you want the interior attic temperature to be as close as possible to the outside air temperature. That’s why there’s usually only insulation on the attic floor, not between the roof rafters.

However, if the attic floor isn’t properly insulated or if heated air is leaking through the ceiling via cracks, crevices, ductwork and light fixtures, then the attic will become warm.

And it doesn’t take much heat to raise the attic temperature enough to melt the snow on the underside of the roof sheathing.

And since the overhanging eave extends past the house, it remains cold, which is why the water freezes when it hits the eave.

Man wearing a mask and gloves while installing mineral wool insulation in the attic
Adding insulation to your attic is part of the plan to prevent ice dams. (DepositPhotos)

Ice Dam Prevention

So, now you’re probably wondering what you can do to prevent ice dams.

First, use caulk, insulation and weatherstripping to seal all upstairs ceilings to stop heat from flowing into the attic.

Next, measure the depth of the insulation on the attic floor, and add more, if necessary. Call a building inspector or licensed contractor to find out how much insulation is required in your area.

Confirm that the attic ventilation system is operating properly. Most homes have soffit vents and ridge or gable-end vents, which allow air to flow into the attic along the eaves and out the attic at the ridgeline or gable walls.

Be certain the attic-floor insulation isn’t blocking the soffit vents. And never staple plastic or anything else over attic vents. Remember, you want the attic temperature to be as close as possible to the outside air temperature.

Finally, once the roof is covered with snow, use a snow rake to pull as much snow as possible from the lower portion of the roof. That’ll at least remove the source of water, which will greatly reduce the chance of an ice dam forming.

If your home already has ice dams, there’s not too much you can do about it. Avoid installing electric cables or any other device to melt the ice.

Those products don’t solve the problem that’s causing the dams to form in the first place.

Further Reading