During the cold winter months when the snow falls and the temperatures remain low, a phenomenon known as ice damming can occur on your roof. Left unchecked, ice damming can do substantial damage to both your roof and the interior of your home, so it pays to know what to look for and what to do about it.
Ice damming begins when snow piles up on the roof and outside temperatures remain low enough that it doesn’t melt off from above. At the same time, heat being lost from inside the house begins to melt the snow from below. As the bottom of the snow layer melts, a thin film of water begins to form between the top of the roofing and the underside of the snow. This water runs down the top of the roof, beneath the snow, until it reaches the eaves.
Once at the eaves, the water is past the end of the attic, so no more heat is being lost from the house to keep the water warm enough to remain a liquid, allowing it to re-freeze into a solid dam of ice along the eaves. If the snow remains on the roof and the outside temperatures remain below freezing, the process will continue to repeat itself, and here is where the real danger starts.
As water keeps running down the roof and hitting the ice dam, it has no where to go but back up the roof, where it can work up under the shingles and lift and damage them as it freezes. When the outside temperatures eventually rise again, or when the ice dam gets over the heated portion of the attic once more, it melts. The water now has the opportunity to get inside the house, where it can cause a considerable amount of damage to the attic framing, the insulation and the drywall below.
WHAT TO LOOK FOR
Luckily, there are warning signs the alert you to the formation of an ice dam. One of the most obvious is icicles hanging over the edge of the roof, which indicate the melting and freezing cycle of the snow on the roof and are often an indicator that an ice dam is forming. The larger the icicles, the larger the ice dam is above them.
You may also see smaller icicles coming out of eave vents, or even behind siding boards. This is an indication that water has now gotten into the attic or has dripped down behind the siding, which typically indicates a more severe problem. The appearance of water stains along the corner between the ceiling and an exterior wall is another indicator that water has gotten into the attic, wetting the insulation and making its way down to the drywall.
HOW TO PREVENT AN ICE DAM
Ice damming requires below-freezing outside temperatures, a layer of snow on the roof, and heat being lost from inside the house. There’s nothing you can do about the temperature, but you can do something about both the snow and the heat loss.
The first and most obvious solution is to be sure your attic is well insulated, so that heat remains inside the living space where it belongs. Be sure that your attic is insulated to at least R-38 – about 14 to 17 inches of blown fiberglass insulation – and that vaulted ceilings with no attic above them are at or above R-30. Don’t overlook heating ducts running through the attic either. They can result in substantial heat loss, so make sure they are insulated to R-11 or better.
Even with good insulation, a certain amount of heat is still lost into the attic, so once it’s there you need to get rid of it. The most effective way of doing that, just like in the summer, is through good attic ventilation. A combination of low vents under the eaves and high vents in the gable ends or along the ridge of the roof keeps a continuous flow of air moving through the attic, dissipating the waste heat before it can warm the roof sheathing and melt the snow above. Also, never intentionally block off attic vents, and be sure they remain clear when insulating.
Removing the snow layer also helps, since it allows sunlight to work on the ice layer below and melt it off. A snow rake, which is a lightweight plastic or metal rake head attached to a series of long poles, allows you to drag snow off the lower portions of the roof from below, without the danger of getting onto the roof to shovel it. Snow rakes are not effective on ice or hard snow, so rake it off while the snow is still soft and try to clear an area from the eaves up to about three feet past where the roof meets the exterior walls.
Heat cables installed on the roof near the eaves, once a fairly common solution for melting off snow on the lower portion of the roof, has been proven to be ineffective. The cables are expensive to install and operate, they can present a fire hazard, and the snow and ice will simply re-freeze a short distance past the cables, leaving the ice-damming problems still in place.
When it’s time to re-roof your house, keep ice damming in mind. Shingles that are flat, uniform and adhere well to each other, such as composition shingles, form a better barrier against water intrusion than irregular roofing such as wood shakes. Metal roofing, which has no horizontal seams, is virtually imperious to water intrusion from ice damming. Also, install a layer of rubberized ice and water barrier from the eaves up past the line where the exterior wall meets the roof. Ice and water barriers seal around the fasteners and are very effective at keeping water out.
Remodeling and repair questions? E-mail Paul at firstname.lastname@example.org.