Ice Dams... What they are, why they occur, and how to prevent them!!

How to stop those ice dams once and for all!! 
(Okay... maybe a slight exaggeration...)

Ice dams can be nightmares, potentially causing extensive water damage to their victim's homes.  In 1993, record breaking snowfalls and an unusually long and cold winter caused tens of thousands of homes throughout New England and elsewhere to suffer ice dam-related damage. And many of these homes' roofs hadn't leaked in the previous 20 to 30 years!

And on any given year we may have a repeat performance!

If anything positive came of this disaster, it was that more people have become aware of what constitutes shoddy roofing practices. We realized that roofers had been getting away with installing minimally efficient roofs for years... all because the relatively mild winters we had been blessed with never put the roofs to the test! Whole housing and condominium developments were the victims of low quality roof installations by builders. And was there ever a price to be paid for this false economy... millions of dollars in interior wall, ceiling, and other property damage!

How ice dams can cause such damage is a mystery to many people.  Let me first explain how they cause the damage that they do. Then, I'll offer some suggestions on how to bulletproof your house against them.  A warning... don't expect a sure cure to be cheap or labor-free!!

When you are done with this article, if you'd like more information please read Q&A from our readers on the topic of ICE DAMS by clicking HERE!

What is an ice dam, and how does it form?

When snow accumulates on a roof, a cycle of melting and refreezing occurs. In a perfect world, the snow would melt off the roof, enter the gutters, and flow harmlessly to the ground. Or the snow would evaporate from the action of the sun, and never really melt off unless the outside temperature rose above the freezing point. However, two key factors interact to cause problems... the outside temperature and the temperature of the inside of your attic.

The warmer your attic is, the more melt off that occurs at the roof surface. This melted snow would normally flow off the edge of the roof. Under certain conditions, though, when air temperature is very low, the water refreezes at the edge of the roof, where the interior roof surface is not being warmed by the attic. This refreezing gradually forms what is fondly known as an "ice dam", a growing heap of ice that blocks path of the melted snow.

Once this dam forms to a certain height, the melted snow that pools up behind it can suddenly leak back under the roof shingles and into your home! On a roof with a low slope, it only takes a small ice dam to cause water backup and leakage.

Contrary to popular opinion, gutters do not cause ice dams. However, an ice dam can extend into a gutter if weather conditions permit.

Can ice dams be prevented or minimized before they occur? The answer... insulation and ventilation!


Since we have determined the main cause of ice dams to be an overly warm attic, a good start in inoculating your home against ice dams is to reduce the attic temperature. Installing additional insulation on the attic floor is as easy as laying additional batts across the existing ones, or having more insulation blown in.

However, there are limits to the usefulness of this procedure... regarding Murphy's Law of diminishing returns.  Once you reach the your area's optimal R-value (a measure of the insulating value of a material), further increases in the amount will not show appreciable decrease in heat loss per dollar spent.  Visit Owens Corning's web site at for more information. They have state by state listings of insulation standards and recommendations... the most thorough listing I have found anywhere.

Install weatherstripping and/or insulation on attic stairways or hatchways, and on attic floor-mounted louvers for whole house ventilation fans. Be careful if there are any exposed recessed light fixtures or vent fans poking through the attic floor. Some of these are not designed to be covered with insulation. Get some information from the manufacturers on the suitability of covering them!


Even with optimal insulation, there is still heat leakage into the attic. This is where the value of ventilation becomes apparent. Without adequate ventilation, heat will build up regardless of the amount of insulation. (As an added plus, ventilation removes water vapor also, which can condense in the attic and cause dry rot on wood and rust on metal items.)

Increasing ventilation can be a major or semi-major project. The usual recommendation for venting is 1 square foot of vent for every 150 feet of attic floor area.  Most older homes don't even come close to meeting this number.  If you have small louvered windows at either end of the attic, known as gable vents, you may be able to replace them with larger ones.  This will take some carpentry skills, but is not a really tough job unless you have difficulty getting to either the inside or outside of the windows.

If your house's roof overhangs the outside walls, add vents into these overhangs (soffits). To complete the ventilation system, add a ridge vent. This is a specialized form of vent that mounts along the length of the peak of the roof.  Cold air entering the soffit vents rises along the inside of the roof and exits through the ridge vent, cooling the roof and removing moisture (that important fringe benefit) at the same time.  This is the best form of ventilation, but cannot be fully utilized without the soffit venting.  However, I would still recommend the addition of ridge venting into any home getting a new roof. Even without the soffit vents, the action of the ridge vent will lower the temperature and reduce moisture in the attic somewhat, in conjunction with the gable vents.

Don't make this attic insulation error!

In an effort to fully insulate the attic floor, people sometimes push the insulation deep into the corner where the roof meets the attic floor. Not good! This causes the lowest part of the roof to be colder than the rest of the roof, setting up the possible formation of an ice dam. Inspect your insulation, and if you see this occurring, pull the insulation back away from the inside of the roof so air can reach it. If you have blown-in, loose insulation, there are styrofoam dams, available at most lumber yards, that can be installed between the floor joists to hold the insulation back from the inside of the roof.

If you have soffit vents, the same holds true. Insulation should not block the flow of cool air up from the soffit to the ridge vent.

What about roof mounted heating cables?

Heating cables mounted on the roof are designed to form a path for melted snow to travel through an ice dam. The will not work if an ice dam forms above them. They will also not work if you forget to turn them on.

The biggest downside to these is that you cannot leave them running all the time, or they will quickly burn out. So you must remember to turn them on and off. And if the electrical power goes out... forget it!

Most homes do not have electrical outlets located outside the house at the roof line to plug the heating cables into. Whether you do the job yourself or have an electrician do it for you, be sure to put a shutoff that controls all the cables at a convenient location. Also, be sure that the circuit is protected with a GFCI (ground fault circuit interrupter).

Having a properly insulated and ventilated attic is a better solution, because it requires no ongoing involvement from you! So weigh the comparative costs of each solution before choosing one or the other!

Can anything be done to the outside of the roof to stop ice dams?

Not to the roofing itself.  But a properly installed roof can eliminate much of the damage they cause. Even if you did everything recommended above, ice dams might still form under severe conditions. Modification or replacement of your roof is the only sure way to permanently stop the leakage.  The trick is to get your roofer to do the job right and not cut corners.  Ditto if you do it yourself!

Roofing installation is one of the least technical parts of building a house, but is labor intensive and potentially dangerous.  It is also a place where contractors tend to skimp, because the effects of a poorly installed roof may not become apparent for years... long after the check clears! A quality roof installation in the snow belt should have four components:  proper flashing, ice and water barrier installed on all roof edges and over all valleys (places where two roof lines meet), rolled asphalt underlayment over the entire roof, and quality roofing shingles with the proper overlap.

If you have one specific problem area, though, and your roof is otherwise sound, you can do the following repair procedure to fix the local problem:

If there is flashing in the leaking area, such as in a valley, and you suspect a problem with it, the basics of the repair are as follows:

In this sort of repair, there is no need to install an asphalt felt underlayment.  However, if you were to decide to replace the entire roof, you should cover the entire roof, including the ice and water barrier, with the asphalt-impregnated felt.  CAUTION:  Be sure that the ice and water barrier will not chemically react with the asphalt!!

Other methods for ice dam removal and prevention...

Mechanical removal of an ice dam could well destroy your roof.  However there are a few ways to improve the situation...

1) Removing the snow from the roof can help slow down dam growth.  On very low-sloped or flat roofs, some folks shovel off ALL the snow!  On a roof with a greater pitch, removing three or four feet of snow above the roof line will slow down the growth of the dam.  There is a special tool for removing snow called a roof rake (see graphic left). Here's a shot of NH doing a little snow removal. Roof rakes are great to use if the show is light and not very crusty.  They also are somewhat dangerous to use on ladders.  So be warned! (Yes, I know, a sometimes a man's gotta do what a man's gotta do!)  During the worst of years, I leave the ladder up throughout the winter on a particularly difficult area.  Since the rungs can become ice-covered, be sure the ladder is completely stable and wear ice cleats on your boots!

2)  Chisel grooves into the dam to allow the water behind it to drain off.  This is a good emergency measure... especially if rain or a sudden thaw is coming!  Using a cordless drill with a spade bit (larger is better) can speed this job along.  BE CAREFUL NOT TO DAMAGE THE ROOF!!

3)  Suggested by a reader... fill an old pair of your pantyhose (or, if you don't own any, borrow some) with calcium chloride snow melt and lay it across the dam.  If will help to melt the dam and also keep that area of the roof clear.  DO NOT USE ROCK SALT... it will stain the roof and siding!  BE CAREFUL WITH THIS ONE 'cause if the dam is too big you may increase the pool of water behind it!  Best for small dams or prevention.  Also a good idea to scrape the snow off the roof first.

4)  One reader suggested using a torpedo-style heater strapped to the plumbing vent pipe!  He turns it on and off from inside to keep that troublesome area of his roof clear.  He doesn't think it is a fire hazard, since these heaters are considered safe. OK... I leave that judgment to you!  To quote...

"I don't mess around.  By putting a kerosene "torpedo" type heater up there I can guarantee the ice will melt.  So what if the neighbors think you've got a jet engine after-burner on the roof!  It works like a charm, lasting up to 20 hours and you use the extension cord to hang it from the "stink" pipe.  Plus you can turn it on and off from inside the house."  Fritz from Detroit

5) Heating the attic may help for the short term.  It may seem counterintuitive, but this is one way to lessen the snow load on the roof, too!  As with using ice melt on the roof, be sure there is a path (other than inside your home) for the melting snow to runoff!!

Read Q&A from our readers on the topic of ICE DAMS by clicking HERE!