Some asphalt shingle roofs develop a discoloration or what appear to be dirty streaks. The condition is often caused by wind-borne microscopic algae or mildew spores, which do not degrade or affect the performance of the shingles. It does, however, detract from the overall aesthetic appearance of the roof. The condition can usually be controlled by installing copper or zinc strips across the length of the roof and every few feet down the roof’s slope.
The most common type of roof shingle used in this country is asphalt shingle, made by impregnating mats of fiberglass or organic felt materials such as rags, paper, and wood pulp, with asphalt and covering one surface with mineral granules. The mat is the vehicle for supporting the asphalt, which is a water-resistant material. The granules protect the shingle from damaging sun rays and provide color. When inspecting asphalt shingles, look for loss of granules, missing and torn sections with erosion of the mat. (See FIG. 2-4.) Some fiberglass-mat shingles have failed prematurely because of cracking, which can take the form of horizontal, vertical, and diagonal cracks across the shingles.
A particularly vulnerable location for leakage is the area between the shingle tabs. The granules in this area tend to come loose before those in other sections, exposing the mat to the weather. Although most roofs have a double and triple layer of shingles, a small section of the area between the shingle tabs has only one-shingle coverage. Thus, an eroded mat in this area is very vulnerable to water leakage. Loss of granules and erosion of the mat between shingle tabs is a deficiency that usually occurs on the roof slope with a southerly or southwesterly exposure before other slopes. The condition is usually visible from the ground and can be clearly seen with the aid of your binoculars. When you see such a problem, you should anticipate early replacement of the roof shingles.
Most homes are designed to take three separate layers of shingles, although in some communities only two are allowed. When a new covering is needed on a structure that already has the maximum layers allowed, it is necessary to remove all the layers before installing the new shingles. When reroofing, it is more costly to remove existing layers of shingles than to install a new layer over existing shingles. Therefore, you should try to determine the number of layers. When the roof has an exposed edge, as in the case of a gable roof, look at the thickness of the layers. If you see two to three overlapping shingles, the roof covering is the first layer. In a hip type of roof, since there are no exposed edges, this type of determination cannot be made. In this case, try to find out the age of the house. Asphalt shingles have a projected life of seventeen to twenty-two years. The actual life span of the shingles will depend on the weight of the shingles, the type of mat, and the exposure. Asphalt shingles are classified by weight (pounds per roofing square); a roofing square is 100 square feet. Lightweight shingles, the least costly, weigh about 215 pounds per roofing square. Heavyweight shingles weigh about 350 pounds per square and have a longer life expectancy than lightweight ones.
Fig. 2-4. Deteriorating asphalt roof shingles. Note torn, missing, and brittle shingles with a loss of the granule covering, exposing the roofing mat.