Now that the heating season is here I thought it would be good to write about ice dams.
 
There is a lot of misunderstanding about ice dams, their cause and how to cure the problem. Many people, believing that the problem originates because there are gutters on the house, have removed the gutters, only to find the dams continue to form. Another solution frequently suggested is removing the lower courses of asphalt shingles at the eave line and installing sheet metal, commonly referred as ice belts, for the first few feet. This generally works but is expensive, unsightly and usually unnecessary. There is also a belief that having wide overhangs will prevent ice dams. Actually, in some situations the wider overhangs add to the problem, not the solution. Even installing heating cables at the roof edge can make the condition worse instead of better.
 
THE CAUSE:
  Ice dams occur when there is a heavy snowfall, followed by several days or even weeks of very cold weather. But other conditions have to exist. A small amount of surface melting may occur, but just enough to crust over and help to hold the snow in place, keeping the wind from blowing it off the roof. Most of the melting is from below, as heat from the house finds its way into an uninsulated or insufficiently insulated attic. The snow blanket acts as an insulator and the bottom of the snow blankets melts.
   Water from the snowmelt flows down the roof surface under the snow as long as it continues to receive heat from the attic. When it reaches the overhang, where there is no more heat escaping, the water freezes again. At first it is just a thin crust of ice on the roof edge, perhaps with icicles hanging down. As the process continues the ice on the roof edge increases in thickness and stops the water behind it.
   How long it takes to build up enough of a dam to allow water to enter the house depends on several factors.
 
1.     If the roof has a very high pitch the water has to rise very high on the shingle to reach the upper edge and be able to leak inside. For example, at a 12 in 12 pitch (45 degree angle), the water could rise 8 ½ inches before it would reach the top of a 12-inch wide shingle laid along the roof slope. On a 4 in 12 pitch, it could only rise about 2 ½ inches before it reaches the top of the shingle.
2.     If properly installed felt underlayment along with ice and water shield is in place under the shingles, water may be stopped by the underlayment before it reaches the sheathing.
3.     If cold weather persists long enough, a thin layer of water can extend well up on to the roof past the actual dam. As this column or sheet of water rises, it creates its own pressure, due to the weight of the water. The pressure can rise high enough to actually force the water through very small nail holes. Most commonly it will find its way under the shingles and into the house.
 
Once the water enters the roof it can cause the plywood sheathing to delaminate, soak the insulation, damage the walls and ceilings and make the paint peel.
 
Rather than removing gutters and roof shingles consider the cause before you consider the solution. The most obvious solution to the problem of ice damming is to prevent the snow from melting on the bottom by confining the heat to the living portion of the house. A properly insulated attic stays cold because the heat never gets there. It is also important to seal all the penetrations in the attic floor. This includes all light fixtures, wiring, plumbing, attic hatches and pull down stairs. Chimneys should be flashed in the attic with heavy gauge steel flashing and insulated with wool insulation. All bathroom vents should terminate to the exterior.  If the house has a cathedral ceiling or it is a cape with sloping ceilings over part of the second floor, there must be ventilation and a free flow of air above the insulation.
 
A new house built in a region where weather conditions favor the formation of ice dams should have a good pitch. If a low pitch is desired the proper moisture barrier should be installed under the shingles. Above all, every roof should be well insulated, sealed and well ventilated. Some newer designs now call for a completely sealed roof with no ventilation but that’s a topic for another time.