What Are Ice Dams?
Many homeowners in the northern United States are all too familiar with ice dams. These are thick accumulations of ice that form over the eaves of a house. Water then collects behind the dam and gradually works its way beneath roof shingles through a cycle of freezing and thawing. The result can be leakage into the living areas of the home, which in turn can produce sagging plaster, stains, and other damage.
Low-pitched roofs are the most likely to be affected, but the cause is a warm roof. Heat escapes the living areas of the home and rises, warming the blanket of snow on the roof. As the snow melts, it flows down the slope of the roof only to refreeze on top of the unheated roof overhang. The ice builds up, the thawing and freezing cycle continues, and the flow begins.
Ice dams are preventable. If new roofing is being added at your home, make sure that proper precautions are being taken. The keys are these:
1) Ensure adequate ventilation.
The roof needs to be vented both at the eaves (usually in the soffits) and at the peak, either in the roof itself or via vents in the end walls of the house. An air space above the insulation in the ceiling or attic then allows cold air to move freely, keeping the roof cold and preventing the snow cover from melting. When a roof is being constructed, inexpensive Styrofoam baffles may be installed to ensure that there is a passageway for cold air from the eaves to the peak.
2) Seal off the house.
Proper insulation of the attic rafters or ceiling joists is another part of the solution. So is a tight vapor barrier to prevent moisture from passing from the living areas into and through the insulation.
3) Install a snow and ice shield.
There are a number of products on the market that, when installed immediately on top of the subroof and beneath the shingles covering the overhang of the roof, will prevent water from working its way into the home. A snow and ice shield consists of a bituminous membrane that seals the roof, forming a continuous barrier to water.