When ice forms at the roof's edge

House Doctor | Gary Dymski

March 3, 2005

The Jan. 23 snowstorm prompted dozens of e-mails and letters from readers about "ice damming" on rooftops. From all their questions, I'm not sure if readers understand that there can be several reasons for this problem, including inadequate ventilation and insulation in the attic and poorly designed gutters.

Ice damming is caused when a mass of ice forms at an overhang or near the edge of a roof. This formation often is caused by a significantly colder temperature on the surface of these areas than at the middle of the roof. When water from on top of the roof reaches the ice mass, it can back up under the roofing materials.

In one e-mail, a reader said he purchased an expensive, wide, helmeted gutter system, which is enclosed to eliminate gutter cleaning, in the hope it would divert more water than the average system and prevent ice damming. However, he also said he recently added a second layer of asphalt shingles to his roof.

Duh. By adding a layer, he did not examine two possible causes of ice damming - ventilation and insulation. Also, his roofer likely could not tell if any wood sheathing beneath the first layer of shingles had rotted over the years and needed replacing.

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Covering. The shingles or tiles and the underlayment that protect the sheathing from weather.
Sheathing. Boards or sheet material (plywood) fastened to roof rafters.
Structure. Rafters and trusses built to support the sheathing.
Flashing. Metal or other material installed in a roof system's various joints and valleys to prevent water seepage.
Drainage. A roof's design features, such as shape, slope and layout, that help it shed water.

The National Roofing Contractors Association (www.nrca.net) has an excellent Web site that explains the components and even advises how to visually inspect a roof.

In addition, several other factors are critical in the roof's ability to shed ice, snow and rainwater. Ventilation and insulation help control air temperature in the attic. In winter months, these two conditions help keep the attic air temperature within one degree of the outdoor temperature, which is a major point in preventing ice damming. Other elements include gutters and downspouts, and soffit vents.

Often, there is not just one reason for a roofing problem. For example, one reader figured he had all the bases covered, except his soffits did not have vents. He identified his overhang as a problem and even sent pictures of his roof. Just one look at the photo and the lack of soffit vents was obvious.

Also, it's amazing how many homeowners simply add a second layer of shingles to fix roofs that leak or have ice damming. My rule is always remove the old layer before adding a new one.

A roof is a 25- to 30-year investment. When considering a new one, get two or three estimates from reputable roofing contractors. Ask about calculating ventilation needs - there is a formula - and whether adding attic insulation is necessary. Ask about soffit ventilation and valleys that could develop leaks. Spend more time and, if necessary, more money to make sure the job is thoroughly evaluated and then completed correctly.