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.

Selecting a Roofing Contractor

There are many roofing contractors-good and bad-honest and dishonest. Some of the shadiest characters claim to be roofers. They taint the trade for the majority of roofing contractors who perform a quality, timely, honest, service oriented business.

• Get recommendations from others and checkout their roofs.
• Ask for references from potential roofing contractors & check them out.
• Ask if the contractor is insured for damage and on-the-job accidents. Ask him to have their insurance company fax or email you proof.
• Don’t pay the roof contractor upfront for the entire job. A reputable contractor has good enough cash flow to order materials. It’s OK to pay the contractor for the materials after they are delivered to your home. Don’t pay the final payment for the job until the work is completed, including the cleanup.
• Get several estimates for the same job (at least 3). Make sure the quotes cover the same materials and workmanship.
• A qualified contractor will help you obtain financing, if you need it.

Before signing a contract make sure that the contract states in detail:

• the work to be completed
• the specific materials to be used
• completion date and penalties
• warrantees & guarantees
• cleanup (removal of materials)
• total price
• method of payment

If you are financing the work make certain that the contract clearly states:

• purchase price
• amount of down payment
• balance owed
• number of installments
• finance charges
• interest rate
  
  
  

Roofing Code and Best Practices

While much of what is written below is from the N.Y. State Residential Roofing Code, it is common sense for any re-roofing job. The code is not stated verbatim to avoid confusion and I have added comments for clarification.

Recovering vs. Replacement

New roof coverings should not be installed without first removing existing (old) roof coverings under the following circumstances:

• Where the existing (old) roof is wood shake, slate, clay, cement or asbestos cement tile

• Where the existing (old) roof is water-soaked, or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing to be installed

• Where the existing roof has two or more applications of an type of roof covering already installed

With asphalt shingles, more than two layers of covering, when combined with the weight of snow, can exceed the weight supporting capability of the roof sub-structure.

Slope

Asphalt 3-in-1 shingles shall not be used in low pitch roofs (lower than 2/12). This type of shingle is not suitable for low pitch roofs as water cannot be easily shed from the surface and wind can blow rain up and under shingles to the unprotected surface of the roof structure.

Ventilation

Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of the roof rafters (virtually all home types) shall have cross ventilation for each separate space by ventilating openings protected against rain or snow. The total net free ventilation area shall not be less than 1/150 of the (floor) space ventilated (if the square footage of the “floor” of the space is 1000 sq. ft. then 6.7 sq. ft. of ventilation is required).

The total ventilation area can be permitted to be reduced to 1/300 of the (floor) space, provided that 50%-80% of the required ventilating area is provided by ventilators located in the upper portion of the to-be-ventilated space, which should be 3 feet above the eave or cornice (gable end vents) vents.

ASHI (American Society of Home Inspectors) recommends that the total vent area be 1/300 of the floor space of the attic for all areas where the roof slope is two in twelve or greater. At least 50% of the vent area should be at the soffits with the remainder at the ridge, roof, or gable vents. If the soffit vents provide less than 50% of the venting, the result can be negative pressure in the attic space causing warm moist air to be sucked out of the living space (through the ceiling, ceiling fixtures, around openings, etc.). Where the slope is less than two in twelve or for a cathedral roof with no attic space, the vent area should be increased to 1/150 of the roof area.

Ice Protection

An ice barrier that consists of at least two layers of underlayment cemented together or a self-adhering polymer modified bitumen sheet (as an ice and water shield) shall be used in lieu of normal underlayment. The ice barrier shall extend up from the eave’s edge to a point at least 24 inches inside the exterior wall line of the building.