Understanding and Preventing Ice Damming

"Ice damming, what in the world was that?" Peter heard what the contractor was telling him, but the words didn't make any sense. There was water damage to the drywall and carpet in his living room and this sounded like a contrived excuse for someone to charge him a lot of money.

One of the least known and mis-understood forms of water damage comes from ice damming. It occurs when climatic conditions are just right. Heavy snow accumulation, combined with warmer days (35-40 degrees F) and freezing nights (20-30 degrees F) precipitate the formation of ice dams.

During the day, heat rising through the insulation from the interior of the house and warmer temperatures outside, begin to melt the snow. Water runs down the roof, under the snow, to the edge of the eve. Because there is no heat under the eve, the water freezes in the last foot or two, creating an ice dam. Day after day, it continues to increase the height of the dam. More and more water fills the reservoir. It begins to seep under the shingles because of capillary action. Water can wick up hill to a height of two to three vertical feet. Eventually, it finds a breach in the roofing and enters the house.

The process to fix the problem has two parts. First, the wet drywall and insulation must be removed and discarded. If left in place, significant mold growth is inevitable. Often some of the drywall on the ceiling will need to be removed. Plastic may need to be stapled up on the ceiling to separate the warm air in the house from the freezing temperatures in the attic.

Water will continue to seep in as long as the snow is there, so the cavity needs to remain open until the snow has melted from the roof. This will allow the owner to monitor progress and know when the repairs can occur. One to three months is not uncommon to wait until the repairs can confidently be completed.

The second step is to remove the ice dam from the roof. There are a couple of options available:

1. Physically remove the snow and break the ice dam. This process is practical only on flat to lower pitched roofs. Even then, there is a great possibility for damage to the roof as well as the snow remover.
2. Lay heat cable in a W pattern from the eve to at least one foot over onto the heated space. This will melt channels through the dam enabling the water to flow out and stop the damage. Check to ensure that the cable does not overlap or lay over on itself, as some cables will short out and stop working if this happens. Fires have also been known to occur from these situations.
   

The prevention of ice damming requires proper drainage of the melting snow. This is usually best done with heat cables run in a W pattern along the eve, up valleys and in gutters and downspouts. The key is to make it possible for the water to run clear of the house, even in the most extreme conditions.