Michigan Building Science Blog by WeatherGard

Ice damming could ruin your roof.


Every winter, many unlucky homeowners find that their roofs have been badly damaged by stagnant snowmelt. You can prevent it, and here’s how.

Every winter, many unlucky homeowners in the colder, snowier parts of the US find that their roofs have been destroyed or badly damaged by stagnant snowmelt. Unfortunately, even newly installed roofs, installed properly and made of the highest quality materials, are entirely vulnerable.

The one thing that can destroy almost any roof is standing water. Water that sits on the shingling, rather than running off of it, immediately gets under the shingles, as they are not—and should never be—fully airtight or watertight. If the only barrier remaining between the roof deck and the shingling is a layer of tar paper, as is typically the case, the stagnant water can soak through within a few minutes. The roof decking begins to absorb the water; if the inside surface of the wood becomes damp, it may trigger voracious mold growth in the attic.

An example of an ice dam, as viewed from the ground
Figure 1 An ice dam, viewed from the ground, looks like an overgrown collection of icicles.

Matters can get much worse if the amount of water that has permeated the tar paper barrier is too great to be completely absorbed by the roof decking itself. Every winter, too many homeowners are horrified to find areas of sheetrock-finished ceiling soaked and sagging, or hardwood flooring swollen with unwelcome moisture. And occasionally, home-ruining mold problems develop after this kind of catastrophic roof failure.

Wait—liquid water sits still on a sloped roof?

Yes. It’s counterintuitive given that a roof slopes toward the ground, but standing water is a yearly presence on roofs all across the Midwest. Snow builds up on the roof surface and is gradually compressed to form a layer of ice. As the end of winter nears, the ice sheet grows thicker.

Water can be trapped behind an ice dam and ruin your roof.
Figure 2 An ice dam on a steep-pitch shingled roof, during late winter traps water at the eaves, allowing it to penetrate into the home.

When the outdoor air temperature hovers just below 32°F/0°C, the heat inside the house can push the temperature of the roof surface to just above 32°F/0°C. The water travels down the roof, beneath the ice, until it either runs off the roof, or settles in a place where the temperature is low enough for it to refreeze. Most often, the outer edge of the roof is near the temperature of the outdoor air, because the outer extremity of the roof absorbs less interior home heat; as a result, the water will refreeze right there. As this happens, over and over again throughout the winter, a mound of ice forms at the roof’s edge (consult Figure 2 above).

That mound of ice—an ice dam—will inevitably catch snow and ice that would otherwise have fallen harmlessley off of the roof, and so it accumulates mass as the season wears on. If it grows large enough, pools of liquid water can become trapped when outdoor temperatures edge north of freezing. That’s standing water, ready to penetrate shingling and tar paper, rot the roof deck, and maybe worse.

Ice dam on roof
Figure 3 The remains of an ice dam, nearly 6 inches thick, sits at the edge of a roof.


Two steps should be taken to protect a home from this problem; neither is 100% effective, but both are very much worth doing. The first preventive measure involves preventing the roof from becoming warmer than the outdoor air (otherwise, the roof melts water which then flows downward and refreezes at the roof perimeter). The second preventive measure is aimed at reducing the chance that standing water will be able to penetrate the roof deck.

I. Insulate your attic, and do it right. The roof temperature is determined by the indoor and outdoor air temperatures. If the indoor temperature is high, the roof temperature may be high enough to melt ice, even while the outdoor temperature remains below freezing. When this happens, water flows from the warmer parts of the roof to the egdes, where it refreezes, creating an ice dam.

Fiberglass attic insulation
Figure 4 An attic insulated with blown-in loose-fill fiberglass.

Attic insulation is not meant to keep the attic warm during the winter—just the opposite, in fact. It is meant to keep heat from escaping the living quarters into the attic, where it can warm the underside of the roof.

The ideal attic air temperature is within 10°F/6°C of the outdoor air temperature, but don’t expect to achieve that with rolled bats of the insulation from the local home improvement store. Blown-in fiberglass or cellulose (“loose-fill”) insulation is far more effective, as is open-cell foam, which is sprayed into place.

II. Ensure that your roof includes an ice barrier. The outer edge of the roof is vulnerable to standing water from ice damming, so some contractors and home improvement companies (like us!) offer the option of having a wide, water-tight barrier at the perimeter of the roof, to stop standing water from soaking into the roof deck. But beware, roofers don’t generally include an ice barrier by default, and many contractors won’t offer to install one unless you ask. (And if they don’t know what you’re talking about when you ask, find someone else.)

This article was written by WeatherGard and was published in 2014-02-06.