Technical Questions

A new roof typically requires a building permit and must adhere to local codes. In most cases, that means using roofing materials that have at least a Class C fire rating. These ratings are issued by independent testing agencies, such as Underwriters Laboratories. A Class C rating signifies that the material will not ignite in the presence of a small fire or burning embers. The top rating is Class A, which means the roofing material will not combust even in the presence of a large fire or help spread a fire over the roof area. You'll find fire ratings listed in product literature. Generally, materials that don't burn, like slate, have a Class A rating. Asphalt shingles-the most common roofing material-can have a Class C or Class A rating. Check requirements in your area and review your homeowner's policy to see if the higher rating is necessary. The weight of the roofing material can also be a concern. Products range from about 250 lbs. per square (100 square feet) for an economy line of asphalt shingles to 2,000 lbs. per square for some types of slate. Anything over 600 lbs. per square typically requires you to beef up standard roof framing. Check with your local building inspector or a structural engineer to determine what's involved for your roof. Perhaps the question asked most often is whether the old roof has to come off. If it's slate, tile or wood shakes, the answer is yes; it's impossible to apply new roofing directly over these materials. However, a layer of asphalt shingles or thin wood shingles can have new roofing applied over them. Most municipalities allow for two layers of roofing in place at a time. But there are some good reasons to remove even a single layer of old roofing. Maybe most important, it allows the contractor to inspect and repair damage on the roof deck. What's more, the warranty backing the new materials sometimes requires stripping away the old ones. In fact, the argument for leaving the old roof in place is a strictly financial one: A tear-off adds $25 to $45 per square to the overall cost of the project.
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