Ice dam basics: How to prevent them and avoid a lot of costly water damage

You don’t want them this winter.

You don’t want them this winter. John Tlumacki / Globe Staff

December 21, 2015

When it snows, you’re going to need to make sure your house is prepared so you don’t end up going through spring doing repair after repair.

One way to do that is to make sure that you don’t get ice dams this year. We talked to Ben Weil, extension professor of building energy at the University of Massachusetts, about how to prevent ice dams and what to do if you get one.

Here’s what you need to know.

What is an ice dam?

“An ice dam happens when there is snow on a roof and there is some differential heating on the roof,’’ Weil said. “Usually the heat is coming from inside, and it melts the snow and the liquid runs down the roof toward the eve where there is no longer enough heat and then the water refreezes.’’

This leads to icicles coming off your roof, which – though they look really cool – can cause a lot of damage.

How does it happen?

Weil said there are two ways for the snow to melt, the first way being through heat loss from the house.

Dmitry Lipinskiy uses steam to break up icicles on a Carlisle home last winter. He owns Icedam Liquidators, a Minnesota-based company, which drove a crew to the Boston area for a couple of weeks to help people clear their roofs and clear out their ice dams. – Joanne Rathe / The Boston Globe

“The temperature from the indoor air is like 70 degrees and it comes in contact with the roof deck’’ if there is a leak in the house, Weil said. “That’s why often you can look at roofs and see the pattern of air leakage showing up in the snow.’’

The second way is through letting heavy snow pile up on your roof.

“Like the Eskimos knew, snow is not a terrible insulator,’’ Weil said. “If you pile up two feet of snow, you have more insulation than by code for a wall.’’

The snow closer to the roof is going to be closer to the inside temperature because of the snow’s insulation. Weil said the only way to counteract this is to have more insulation in the attic.

So what’s the big deal? Icicles are cool.

Well they do look cool, but they can cause all sorts of problems. As The Boston Globe reports, once the icicles are formed from the melting and refreezing, they can block other water still on the roof from dripping off. The Globe writes, “The draining water hits the ice dam, backs up under the roof shingles, finds cracks and openings in the roofing, and eventually manifests itself as an interior water leak.’’

Construction workers try to fix a home with serious ice dam damage last winter. – Suzanne Kreiter / Globe staff/File

Weil said that just last winter (which we know was particularly bad) he saw many people that needed to have their entire drywall replaced on the second floor or needed their entire electrical system to be redone.

“We are talking lots and lots of money,’’ Weil said.

How can you prevent this from happening?

“The most important thing is preventing air leakage from inside the house from anywhere into the roof system,’’ Weil said. Before it is too late and winter is upon us, you should get your roof checked to see if there are leaks – a group like MassSave could help you with that.

Or, if you go outside and look at your roof when it has frost on it, take a picture and look where there might be strange lines where there isn’t frost. Weil says, “something is causing the frost to melt and that’s an indication of where the snow will melt.’’

A second thing you can do is to get more insulation in the attic. He said this is pretty easy if you have a flat attic and more difficult if you have a cathedral ceiling.

“A third strategy, NOT of equal value,’’ Weil emphasizes, “is proper roof venting. It makes it so that if you have enough outside air travelling up the underside of a roof deck the temperature of either side of a snow roof assembly will be what the outside temperature is and you won’t have melting unless it is a warm day.’’

This will make your problems worse, however, if you haven’t already blocked air leakage nor proper insulation.

How can you fix the problem if you already have it?

“There are a number of solutions that are really Band-Aids to an ice dam problem,’’ Weil said. “Very commonly all over the Northeast you see metal edging, the bottom two feet of a eve has metal roofing. The idea is that the snow will slide off, and often times it doesn’t. The metal can help but it’s not treating the problem.’’

Ice dam in Newton, Massachusetts in Feb. 2015. – Heather Hopp-Bruce/Globe staff

The only thing that is worse than this to Weil are heater strips that turn on when the temperature gets blow a certain number, which melts the snow on the roof.

“They might help get you through a winter but is not a solution,’’ Weil said.

In conclusion

Make sure you home doesn’t have air leaks and make sure you have the proper insulation and keep an eye on your roof throughout the winter months.

“Energy is relatively cheap so it is a hard sell to get people to air seal and insulate as an energy saving tool, but as a maintenance cost avoidance budget this is one place where you don’t have to pay for it with energy savings you can pay for it for avoiding ice dams.’’

Related: Here are the 50 safest towns in Mass.