Source: Lynch Building Inspection Services Ltd. of Nelson, BC.
Originally appeared in The Nelson Express.
Every winter during the colder stretches the eaves on the roof of our house get a thick layer of ice build-up. This year I noticed water stains on the inside of our living room wall right where the wall meets the ceiling. I suspect it is related to the ice. Do you have any ideas or suggestions about this ice and the water stains?
From your description this sounds like a common problem caused by ice damming. In the industry your roof “makes ice”. The most common reason for this problem is a lack of attic ventilation at or near the junction of the rafters or trusses to the outside walls of your house. Often attic insulation has been carelessly pushed up against the underside of the roof deck in this triangular shaped area within a foot or two of the outside building walls. The insulation is warmer than the snow on the outside roof shingles. Because the insulation is in contact with the underside of the roof deck, the snow on the other side of the roof deck melts along this insulated area for a foot or two near the eaves. As the melting snow (water) travels down beyond the outside walls to the unheated eaves, the water freezes. Ice dams form along these eaves and the dammed water problem increases as the ice dams build in thickness. (This problem can also occur in uninsulated attics.)
When the liquid behind the ice dam backs up the water often find its way under the shingles. This usually happens directly above the outside walls because the warm air escaping from your attic keeps the melting snow water in a liquid state. This water can now find its way down into the outside wall cavities. In turn, the water will often appear as stains near the top of exterior walls on either the inside or outside of the walls.
Some folks resort to a quick and simple answer to the problem. They install heat tape along the eaves of the roof. This melts the ice dams eliminating the potential for water build-up behind the ice dams. But this solution does not address the fundamental problems. Here are some suggestions for eliminating the problem.
First, go up in your attic and remove any insulation in contact with the underside of the roof deck. You can also install insulation baffles that will help hold the insulation down and away from the deck of the roof at the rafter junctions with the outside walls. If you have little or no attic ventilation through your soffits, gable ends and at the ridge of your roof, definitely install more of each. Then go outside. With a ladder on your roof eaves lift up the first course of shingles at the eaves. There should be one or two layers of felt paper under the shingles (dampproofing). This dampproof layer is the last line of defense against ice damming. It is installed to at least one foot above the outside wall of your home and its purpose is to help carry off water that may build up under shingles from ice damming. If you don’t have a layer of dampproofing felt you don’t have the final line of defense. Correct the insulation and ventilation problems first and the next time you install new shingles install the dampproof course at all the building eaves.