By Lee Williamson
During the recent extended stretch of bad weather we’ve had in the DFW area, we’re seeing a problem we normally don’t see here, and that’s Ice Damming.
Usually, we have snow or ice and it melts the next day or so and it’s gone. However, during extended periods of sub freezing weather, ice accumulates at the edge of the roof.
This happens because the eve beneath this section of the roof is not warmed by the heat inside of the house like the attic is.
The fact that the attic is warmer than the outside temperature, plus the radiant heat from the sun if there is any, combine to melt the snow and ice and the water travels down the slope of the roof until it hits the “ice dam”.
Over the course of several days, the ice dam is two or three inches tall, or more, and when the final thaw comes, the water backs up behind the ice dam to a point where it spills over into the interior of the house.
It usually becomes evident on an exterior wall in the vicinity of a valley, or where a vertical wall comes up out of the roof.
Acceptable building practices and codes in this area of the country don’t address this phenomenon since it’s occurrence is so rare.
In the northern climes, an “ice and water shield” is applied around the perimeter of the house to a point two feet inside the exterior walls. An “ice and water shield” is a roofing membrane made of modified bitumen, a rubber like material, that swells up around the roofing nails nailed through it to attach the shingles on top of it.
This repels the water until the ice melts and the water can drain off of the roof, unlike standard roofing felt where the water finds a nail hole and makes its way inside.
If you are experiencing ice damming, it may never happen again. After all, 100 hours of below-freezing weather in Dallas is rare.
If you want do some preventative maintenance, we would recommend installation of an ice and water shield in the affected valley and horizontally along the roofs edge for several feet on each side of the valley.