Ice dams

In our discussion of roofs back in the summer, we touched upon the problem of ice dams. Fortunately, during the summer, ice dams are not of much concern.

However, this time of the year, ice dams can be a real problem. (As a matter of fact I just came inside from removing snow above the beginnings of ice dams on our office roof.)

Ice dams form naturally in the winter months due to a simple process. Snow in contact with the roof begins to melt due to warm outside air temperatures and/or heat from the house coming up through the roof. This water trickles down the roof surface beneath the insulating blanket of snow on the roof until it comes to the edge of the roof or eave where the water is exposed to the air. Making matters worse, the overhang of the eave does not enjoy the same heat from the house below as the rest of the roof area. Consequently, this water freezes at the edge of the roof. The process continues day after day and slowly the ice builds up between the main roof and the edge. Eventually, either through this continuing process or through a thaw and/or warm, rainy weather, water becomes trapped above this ice dam and forces its way back up beneath the shingles. (As you remember from our discussion of roofing during the summer, shingles are lapped facing downslope on the roof. Therefore, they are very good at resisting water flow running down the roof but are not good at resisting water that is forcing its way up the roof slope.)

Eventually this water can work its way up beneath the roofing and find its way inside the house causing leaks and expensive damage inside the building.

To avoid this problem, the best bet is to remove snow from at least the edges of the roof if you can do so safely. This enables the water to continue running off the roof and not slow down at the edge as it does under the initial snow cover as described above. This also prevents the snow itself at the edge of the roof from turning to ice and causing the dams to build up faster. As previously discussed, one of the keys to an impervious roof is to get the water away as quickly as possible.

Another approach that some people use is heat tapes especially designed for installation along roof edges to melt the snow and ice that might otherwise build up there. If you choose this approach you must select the heat tapes with great care. Some of these products have been found to cause much more serious problems, including fire hazards, than they solve.

While icicles hanging from the edges of our house can add to winter charm, be sure they’re not an indication of ice dams building up beneath the snow on your roof!