The outside temperature is below freezing.
The temperature inside your attic is above freezing.
Because of the different temperatures the snow on top of the roof starts to melt and the melted snow starts to run down the roof in an attempt to drain off the roof. As soon as the water reaches the overhang of the roof (unheated space) it gets exposed to the freezing air. The water freezes along the edges of your roof and the gutters.
As more and more water runs down your roof, it continues to freeze behind and on top of the previously frozen run off, forming a ice dam. Now, all the melting water is dammed and has nowhere to drain to, so it builds up. This water backs up refreezes and is forced under your roofing material actually lifting the shingles. When it reaches the heated space again, it remelts and drips into your living area, causing staining and damage
Keep the entire roof cold. This can be accomplished by implementing the following measures:
Seal all air leaks in the attic floor, such as those surrounding wire and plumbing penetrations, attic hatches, and ceiling light fixtures leading to the attic from the living space below;
Increase the thickness of insulation on the attic floor, duct work, and chimneys that pass through the attic;
Increase the attic’s ventilation;
Remove snow from the roof. This can be accomplished safely using a roof rake from the ground. Be careful not to harm roofing materials or to dislodge dangerous icicles.
If you’re in the market for a new roof, install a metal roof. Ice formations may occur on metal roofs, but the design of the roof will not allow the melting water to penetrate the roof’s surface. Also, snow and ice are more likely to slide off of a smooth, metal surface than asphalt shingles.
House Doc is written by Bob Weinman, a NYS Licensed Home Inspector and owner of Hudson Home Inspections. He can be reached by calling 518-821-7547 or firstname.lastname@example.org.