Monday, December 22, 2008

Ice Dam Nation

It happens every ten years or so. We had at least a foot of snow this past weekend, and now it is bitterly cold. It barely made it out of the teens (degrees F) today and we may see single digits tonight. These are perfect conditions for ice dams.

An ice dam is a phenomenon that occurs on the roof of a house after a deep snow in cold temperatures. Heat escaping from the house melts snow on the roof surface. This water then flows downhill under the insulating blanket of snow until it hits cold surfaces at the eaves or gutters (eaves troughs, for my Canadian friends) where it refreezes. This freezing water builds up until a dam of ice forms. When the dam gets high enough, a puddle of water forms behind it. When this puddle gets deep enough, the water may back up under the roof shingles and leak into the house. The problem is often worse on flatter roofs because the water doesn't have to be very deep before it gets under the shingles. This water may show up inside windows or dripping from ceilings near exterior walls. Sometimes it runs down inside walls and shows up downstairs. In some cases the damage can be extensive and expensive with damaged drywall, paint, plaster and woodwork.

Ice dams are a symptom and not a disease. The disease is heat loss. Warmth from the house escapes into the attic or the space above a cathedral ceiling because of poor insulation and/or air leaks. The effect can be exacerbated by inadequate ventilation of the attic or cathedral ceiling system. A house that is well insulated and ventilated will not have ice dams. A roof should be cold.

It is easy to spot the houses that are likely to have ice dam problems. As I drive and walk around town I always notice the homes with the prettiest and most dramatic icicles. Those are the houses that are wasting energy and probably having leaks inside. As a comparison, one can look at an unheated shed or garage and see no ice at all.

You have plenty of insulation, you say, but still get ice dams? In that case, there may be other sources of heat in the attic. These might be uninsulated recessed lights or leaky HVAC ducts. The worst situation is where the actual hot air furnace is located in the attic. All of these things represent poor design and unfortunate energy waste.

What to do when confronted with ice dams? Well, when water is actively leaking into the house, there's only one thing that can be done, and that is to get the snow off the roof. If the water source is eliminated, the leaks will stop. The best way, if the house is not too high, is to use a roof rake. A roof rake is a wide blade on a long pole that is used to pull snow off the roof while standing safely on the ground. If the roof is not too steep, it might be possible to get up there with a snow shovel. I've also had some luck with chipping channels through the dams with a hatchet to allow the water to escape, but this must be done carefully to avoid damaging the roof and gutters. Needless to say, climbing around on icy ladders and roofs is dangerous business, so Kids, don't try this at home.

Longer term, there are other ways to treat the symptom without curing the disease. These include heat cables to melt channels through the dams (That's a favorite American solution because its' just throwing more wasted energy at the problem.), rubberized membranes under the roof shingles, and strips of sheet metal along the edge of the roof.

But really, especially in this age of climate change, dwindling energy supplies and global conflict over fossil fuels, we just need to make our houses better. Install as much insulation as practical, seal air leaks, and eliminate heat sources in the attic. You'll save energy, be more comfortable and avoid unnecessary damage to your home.


SimplyTim said...

Hi MojoMan,

I've been there with the ice dams....maybe I should say the damn ice dams.

At the worst of it a number of years ago, we had a leak into our living room. Thankfully it was covered by home owners insurance - but that led us to another development where people told us to be careful about putting in other claims for whatever reason for the following few years because they said that the rates would go up.

Anyway, we eventually got a channel in the dam so the accumulated water and slush could flow down and away. Once I saw how that happened, I understood instantly how the leak could happen even at a distance from the source.

The roofer who worked with us put down a waterproof membrane for several feet in either direction from the valley on the roof and that worked very well.

The biggest danger is in trying to solve the problem by by pouring boiling water on the surface of the ice, or chipping away with a hammer while perched on a ladder and your fingers are freezing and you're tired, etc., etc.

Maybe that's natures way of culling the herd. I was just lucky that time...but doubly lucky to not try it again.


MojoMan said...


Yes, the membranes (ice and water shield) can be a big help, but as I tried to say, they are a bandaid on the symptom. Valleys can be tricky, but try to find out why that snow is melting in the first place.

John said...

Yes, even our family in the past has climbed all over the guttered part of our old home and use a dull axe, too. Spring came soon and were had a need for a new roof. Almost all the new roofs in this area of the UP are becoming METAL & STEEP.

roof de icing

MojoMan said...

Thanks for the link, John, but these are just the sort of systems that throw more wasted energy at the problem rather than fixing the root cause of improper insulation and air-sealing.