When snow accumulates on your roof, it can lead to water leakage into your house, even if your roof is new. The culprit is ice damming, the insidious snow-melting phenomenon that is all too familiar to many of us.
Ice dams can form on the roof in areas where snow covers the transition between interior and exterior spaces. If there is sufficient heat loss from the heated space to melt the first few inches of snow on the shingles, water will form. The water runs down the roof until it encounters un-melted snow over the unheated space where it stops, and re-freezes. After a while, a dam of ice forms on the roof. The dam can cause the water to form a small pool.
Shingle-type roofs cannot stop pooling water, and the water may back up under the edges of the shingles. If left uncorrected, water damage will occur to the ceiling or wall finishes, and at worst, structural rot can occur.
If a house has little insulation, heat loss will cause snow to melt. If the roof can be kept cold, little melting will occur which will avoid ice dams. Luckily, upgrading insulation in attics and roof spaces is often easily done. The goal is to keep the attic cold in winter with good ventilation. However, this does not work in all cases. The reason lies in the often neglected problem of air leakage or air loss, which is the leaking of warm, moist air from the house into roof spaces. Air loss occurs in every house. Warm air is light and buoyant, so it always rises.
Remember that intact drywall or plaster ceiling is a good air barrier, so the most typical air leak paths are places where the ceiling is discontinuous. Here are some common problem spots that you can address from inside the house: HVAC registers in the ceiling - the joint between the ducting and the ceiling should be well air-sealed and insulated. Pot lights and bathroom fans - should be sealed at their boxes. Attic access hatch - an attic hatch or door needs to be both insulated and weather-stripped.
The more difficult air paths to seal are those accessible only from the attic. In these areas, the attic floor is discontinuous. Common problem areas are: Attic mounted ductwork - ductwork in general is usually poorly sealed at joints, and leaks air readily. Plumbing stacks and chimneys - large chases running the entire height of the house, housing smallish pipes…great paths for heat loss.
Once the warm air is kept inside the rooms where it belongs, the existing level of attic ventilation will often be sufficient to keep the attic and roof spaces cold enough to prevent ice dams from forming.