How Do I Know if I Have an Ice Dam?

By Rob Robillard

What causes ice dams?

Ice dams are ridges of ice that form at the edge of a roof and prevent melting snow from draining. Ice dam problems usually occur after heavy snowfall and several days of freezing temperatures. The past few weeks in Massachusetts have been a perfect storm for ice dams.

Public domain graphic via  NOAA/National Weather Service.

Public domain graphic via NOAA/National Weather Service.

Ice dams form as a result of warm air inside your home leaking into the attic and warming the underside of the roof, causing snow and ice to melt. The melted water will drain along the roof, under the snow, until it reaches the cold overhang. The overhang tends to be at the same temperature (below 32 degrees) as the outdoors, and the melted water will refreeze and form icicles and possibly an ice dam.

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.

Do those giant icicles on my house mean I have an ice dam?

Should people be worried about icicles? They are a part of winter, and pretty, but when the icicles hanging from a house are large (2 to 3 inches or more in diameter), that is a sure sign that ice dams are forming. By the way, large icicles are a byproduct of ice damming, and banging them off your house does little other than risking window breakage and personal injury. Leave them alone unless they are threatening people and property below. Gutters and the guards that keep leaves out of them have nothing to do with ice dams.

What should I do if I have an ice dam?

Using a roof rake to clear the snow from the first three to four feet along the roof edge is a great way to prevent ice dams. This should be done immediately after it snows. Roof raking eliminates one of the ingredients necessary in an ice dam recipe and is BEST done before the ice dam forms.

Roof raking allows the sun to warm the roof edge and melting snow to drain.

What should I do if I have an ice dam leak?

I prefer prevention, but in an emergency situation where water is flowing into your house, you need to remove the ice dams. Steam is by far the safest method and poses the least risk of further damage. Several roofing companies use a steamer method, and, if ice dams are a common problem for you, you will want to make sure you are on their “dance card” early in the season. They book fast.

A steamer uses a home’s water supply, heating it to 300 degrees. The steam is forced through a hose and wand, where it is delivered in a low-pressure stream used to cut through ice. The most efficient way to steam ice off a roof is to cut it into chunks and throw the chunks from the roof.

A true emergency sometimes means you cannot wait for a contractor to use a steamer. In this case, I have cut channels through the ice dam to allow the water behind it to drain off the roof. In these situations, I have used a hammer claw or a large chisel to carve the channels.

Your goal is to allow the roof to drain, and you need to be super careful not to damage the roofing material underneath. Stop chipping when you get close to the roof's surface. I suggest cutting channels every 2 to 3 feet.

Hammers and other impact instruments are commonplace in ice dam removal BUT can result in costly damage. Using a power washer can also do significant damage, so I advise against that.

TIP: You can use snow melt or rock salt to keep these channels open. I have even filled the legs of nylons with rock salt and placed them in the channels. I have seen guys tie long strings to the nylons, so they can retrieve them later without having to climb a ladder. I have to warn you that these chemicals can damage aluminum, copper gutters, flashing, and plants.


Unless you enjoy and are good about roof raking, you should look closer at your building to help prevent future issues. Here are a few suggestions for prevention of ice dams:

1. Install a rubberized roofing underlayment under your shingles 6 feet up along the edges, all valleys, and covering all low-pitch areas. This is best done when reroofing.

2. Increase the attic insulation to cut down on heat loss by conduction. State code requires an R-value of 38 above the ceiling for new homes. In narrow spaces, use insulation products with a high R-value. If your house is older, you can add furring strips to the rafters to create a deeper space for more insulation.

3. Ensure you have adequate gable or soffit-to-ridge ventilation.

4. Reduce heat sources in your attic by sealing the air leaks into it. Sealing air leaks is a topic for another article. Many people think that insulation alone does the job effectively. The truth is that, in order to improve the energy efficiency of your insulation, you need to seal air leaks to prevent heat loss. Insulation works best when air is not moving through or around it. Sealing air leaks is not hard; you just need to know where to look.

5. Avoid complicated roof designs in snowy climates. Complicated roof lines are sexy, but create intrinsic problems with ventilation and drainage.

6. A deicing system efficiently reduces ice dam formation along roof edges and in gutters, providing a path for snow melt to flow off the roof edge or through the downspout. I suggest this to my clients as a last resort and for complicated roof designs and hard-to-rake areas. Roof and gutter deicing cables, or “heating cables,” provide an effective and economical method for draining the roof of snow melt. I also advise my clients against using cheap hardware-store versions. These cables are not “smart systems” and turn on when temperatures reach a certain point, snow or not. To combat this, many people install a light switch to control the cables, and then forget to turn it on or off. Additionally, these cables have a short life span and often burn out. Instead, I use a commercial-grade self-regulating heating cable that has sensors to detect snow and ice dam conditions. The system is “smart”; it turns on when it snows and off a few hours after it stops.

7. Move to Florida.

Text © 2015 by Rob Robillard

Rob Robillard is a general contractor, carpenter, and editor of, which specializes in problem solving for home maintenance. He writes a Q&A column in the Boston Sunday Globe. The author enjoys using his knowledge and experience to help and educate on best practices in the remodeling industry. To learn more, visit his two websites, and

This article originally appeared in the Globe on February 11, 2015. It is republished by Boston Building Resources with the author's permission.