Ice Damming

What is Ice Damming?

Ice damming is a phenomenon that occurs in climates where snow accumulates on the roof surface. Most frequently it forms at the lower edge of a roof but can also occur near party walls (i.e. parapet walls that are shared between separate residences sharing the same roof), at the bottom of roof valleys, and below skylights.

Signs of Ice Damming

Ice damming is a result of snow melting, flowing down the sloped roof, and refreezing at a colder area of the roof surface. Icicles hanging off gutters, between siding boards, from the soffit are a sure sign that ice damming is taking place. Another sign is discolored icicles. Ice damming can be worsened by gutter systems. In some heavy snow areas gutters and downspouts in installed seasonally as the weight of the ice can pull the system away from the building.

Damage from ice damming can be seen along the wall/ceiling intersection line on the interior as stains, dampness, or dripping. Damaged areas may not be located directly in line with the ice damming problem. Water can travel along rafters and joists and manifest itself fairly far from the cause. If you find evidence of water only when it is cold and there are icicles present and not in rainy seasons or drier, warmer weather, you can be confident that ice damming is the cause of the problem.

Causes of Ice Damming

In cold climates the ceiling surface in the living space below the roof/attic needs to be insulated from the cold interior of the space between the ceiling and the roof. This is not just a cost issue (wasted heat) but necessary to keep the roof surface cool. The space below a roof needs to be well ventilated with outside air from the peak to the eave. If heat escapes into the attic and the attic is not sufficiently ventilated, the warm air will cause the snow on the roof to melt, flow down to the colder overhang area of the roof, refreeze, and begin to build up forming a dam. As this process continues, the water “backs up” the roof higher and higher and eventually under the surface of the roof covering. The water finds an entrance point such as the joint between the structural wood surface of the roof, and travels to an interior wall or ceiling.

Tech Tip

Total vent area should be 1/300 of the floor space of the attic. Total vent are equals the sum of the actual vent area provided by the soffit, ridge, roof, and gable vents. At least 50% of the total vent area should be provided by the soffit vents. If the soffit vents provide less than 50% of the total vent area, a negative pressure can occur in the space below the roof and warm, moist air can be drawn in exacerbating the ice damming problem, increasing heating costs, an damaging the underside of the roof structure from moisture.

Example:

The floor space of the attic is measured to be 1200 sq. ft.

Total Required Vent Area: 1/300 X 1200 = 4 sq. ft.

Soffit Vent Area: .5 X 4 = 2 sq. ft.

Combined gable, ridge, and roof Vent Area: .5 X 4 = 2 sq. ft.

If the roof slope is less than 2 in 12, or for cathedral roofs with no attic space a total vent area of 1/150 of the roof area should be used.

Ways to Prevent Ice Damming

• On new roofs or roof replacements install at least 9ft. of ice and water barrier up from the eaves on the roof surface (under the shingles).

• Avoid installing recessed ceiling lighting in any ceiling under an attic.

• Don’t install skylights.

• Make sure that you have 12”-14” of insulation properly installed in the attic.

• Make certain that the access panel to the attic space has as much insulation on it as is installed in the rest of the attic. Use weather stripping to seal the attic hatch.

• Bathroom ceiling fans should have short, insulated ducts. Ducts should be vented properly to the outside.

• Avoid installing any fixtures in the ceiling that will allow the warm air of the interior living space to escape into the attic.

• Check that the soffit baffles are present and not blocked by insulation (located in the attic where the roof meets the attic floor).

• Make sure that your entire roof surface is cold by using adequate ventilation (see Tech Tip above).

• Make sure that some or all of the soffit vents are not dummies. Some siding contractors install vent material under the eaves but fail to cut openings through the soffit. This is more common when house exteriors are resided.

What About Heat Tape?

Don’t waste your time and money on this “fix”. Focus on the root cause of the ice damming problem which is usually poor ventilation, escaping warm air, and/or poor insulation. If you must use heat tape, recognize that the tape must be heated before snow settles on the roof surface. Heat tape should be replaced every 2 years. Between the weather and the ultraviolet light from the sun, heat tape deteriorates quickly. Also, heat tape fasteners can damage your roof.