Eliminating rooftop ice

As I was driving my van last winter, I passed a man in the throws of a particularly Canadian form of desperation. He was at the top of a ladder leaning against his house, using a full-size axe to attack a massive berm of ice that had formed around the eaves. He was going at it with both hands, chips flying. Never mind that it was absurdly dangerous up there, and never mind that it would take a miracle to shield the shingles from damage. The ice had to go at any cost, probably because this guy knew what it would cause later if it remained in place as the weather warmed up.

Long stretches of cold weather always lead to rooftop ice buildup on some homes, and this situation shouldn't be ignored. Ice is beautiful, but more than a little on your roof is a bad thing. Besides the danger of it falling off and hurting someone, rooftop ice often causes leaks in otherwise sound, shingled roofs by preventing the downward flow of water when the weather warms up. Many roof surfaces have almost no ability to seal out pools of water since they're only good at keeping things dry when gravity is actively propelling water downwards. And the fact that rooftop ice stops this flow is a big problem.

Ice forms on roofs for one reason: the roof surface is too warm. This may be caused by inadequate attic insulation, by less-than-enough attic ventilation, or a combination of the two. Either way, the result is the same. Excess rooftop heat causes snow to melt on the main part of the roof, even when the surrounding air is below freezing. As melt water trickles downwards, it eventually comes to the eaves which are still below freezing. The melt water freezes and forms a small ridge of ice or icicles. These won't be noticeable at first, but they're the seeds of trouble. As the melting-and-freezing process continues, the ice buildup gets larger and larger until it's high enough to hold back pools of water on days when the temperature rises above freezing. And if the downward flow is stopped like this, water will find an alternate route between your shingles and down through your ceiling.

The worst thing about ice-caused roof leaks is that there's no permanent fix you can apply until spring. Adding more insulation and ventilation will make your roof surface colder, and that's good, but it won't get rid of existing ice build up and the leaks it may cause later in the spring. Another problem is that upgrading insulation and ventilation isn't easy with cathedral ceilings, which are usually associated with the heaviest ice build up. There's simply no way to get into the attic space to work. In these cases, the best you can do is add 2 inches of rigid foam insulation to the underside of the ceiling, then drywall over it. But even then, it may be necessary to install electric rooftop heating cables along the eaves if the foam isn't enough to keep the roof cold. You'll only know for sure next winter.

Another remedial approach is installation of a seamless rooftop membrane all around the eaves. By running the membrane up several feet higher than the area of ice build up, any standing water is prevented from trickling down through the shingles. This fix doesn't look great when it's applied over an existing roof, but it's better than brown water marks on the ceiling. Installation of a roofing membrane called ice and water shield (www.graceconstruction.com) under new shingles during the next re-roofing job does the same thing invisibly.