Ice Dams 101
In a perfect world, snow will melt off the roof, enter the gutters and flow harmlessly to the ground. However, if you have experienced water stained walls and ceilings after a large snow build up on your roof, you realize that the world is not perfect. Sometimes, after a significant snow fall and a long cold period, we find our homes are water damaged from ice dams.
So, what causes ice dams and what damage can they produce? Often ice dams are caused by the interaction between the cold outside air and temperature inside your attic. The warmer your attic, the more melt off occurs at the roof surface. Under most conditions, melted snow runs off the edge of the roof. However, when the air temperature is very low, water refreezes at the edge of the roof - where the interior roof surface is not being warmed by the attic. Soon an ice dam begins and prevents the water from running off the roof. With repeated thaw and re-freeze cycles, the ice dam problem builds and water is trapped at the eaves or the base of valleys. The water then seeps under shingles and the roof felting, which is supposed to protect the roof system from water intrusion. Eventually the water seepage starts to saturate the wood sheathing around the eaves and wicks upward.
Ice dams can result in water collecting on the ceiling or running down interior walls causing dampness or discoloration. In other cases, water can run through the wall framing and pool under the wood flooring. Sometimes, this process goes unnoticed unless one is checking the roof sheathing from the attic or a member of the household has an allergic reaction and distress caused by microbial growth on the roof sheathing. (Microbial growth occurs when the weather warms up due to direct sunlight or warmer weather, and increased temperature.) Failure to dry moist building material like wet roof sheathing can result in major repair due to progressive microbial germination, amplification and spore dissemination.
Since the main cause of ice dams is an overly warm attic, the best solution is to lower the temperature of the attic. One way to do this is to install additional insulation to the attic. Eventually, you will reach the law of diminishing returns when further increases in the amount of insulation will not show significant decrease in heat loss per dollar spent.
While inspecting the insulation, check to see if someone has pushed insulation deep into the corner where the roof meets the attic floor. This can cause the lowest part of the roof to be colder than the rest of the roof, thus adding to the possibility of an ice dam forming. If this is the situation in your attic, pull the insulation away from the inside of the roof so air can reach it. Also, check to make sure insulation is not blocking the soffit vents. Some homeowners block soffit vents to keep cold air out of attic or garage areas. Unfortunately, this prevents the ventilation and air movement needed to keep roof decking uniformly cool and to evaporate moisture from the decking. It can also cause a stagnant air condition that encourages mold growth and amplification.
Another step toward prevention is to provide adequate ventilation. Without adequate ventilation, heat will build up. The ventilation can also remove water vapor which can condense in the attic and cause dry rot on the wood sheathing. Most building experts recommend venting one square foot of vent for every 150 square feet of attic floor area. If your home has roof overhangs (soffits), add vents into these soffits and, if possible, add a ridge vent – a vent which runs along the peak of the roof. Cold air entering the soffit vents rises along the inside of the roof and vents through the ridge vent. This process cools the roof and removes moisture. Some roofers will also install a three-foot-wide non-porous plastic material, called “ice and water barrier” along the eaves in lieu of traditional roof felting. Typically, this is installed in the valleys and around chimneys and the perimeter of the roof.
Heating panels or cables are sometimes installed around the eaves or other points where snow collects. When activated, these heating devices melt the snow and ice and allow for continuous water runoff. Sometimes these panels and cables can cause shingles to deteriorate faster or may block snow, thus forming an ice dam farther up the roof. Also, one has to remember to make sure they are not left on. Of course, if the electrical power goes out, they will not work.
When installing a new roof, make sure that the roof has the proper flashing, a “ice and water barrier” is installed on all roof edges and over all valleys, rolled asphalt underlayment is installed over the entire roof and quality roofing shingles are installed with the proper overlap.