When snow accumulates on a roof, a cycle of melting and refreezing occurs. In a perfect world, the snow would melt off the roof and flow harmlessly to the ground, or the snow would evaporate from the action of the sun. 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.

How can ice dams be prevented?

INSULATION.  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 attics 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 http://www.owenscorning.com for more information. They have state by state listings of insulation standards and recommendations... the most thorough listing I have found anywhere.

Install weather stripping 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!

VENTILATION.  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.  The majority of 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.

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. This is not good and 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 styarafoam 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.

OTHER OPTIONS.  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. 

Roofing is one of the easiest 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:

• Remove all roofing material from 3 to 6 feet back from the lower edge of the roof.  Repair or replace damaged or rotten plywood underlayment as necessary. 
• Install the self-adhesive ice and water barrier over the plywood.  Since the barrier is three feet wide, install two overlapping courses if you want extra waterproofing or if the slope of your roof is low.
• Reinstall new roofing shingles.

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: 

• Remove roofing material around the flashing adding an additional two or three feet of exposed underlayment.  Do repairs to the plywood underlayment as necessary. 
• Install ice and water barrier across the area to be flashed, and cover all exposed underlayment with it.  Then, install the flashing, followed by the shingles.  The ice and water barrier will act as a diaper under the flashing if the roof cement that seals the flashing fails after a few years. 

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. 

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