Asphalt shingles were historically classified by weight. The most common type of shingles used today weigh two hundred and ten pounds per square. They have an average life expectancy of twelve to fifteen years. Heavier asphalt shingles such as 225’s (two hundred and twenty-five pounds per square) 235’s and even 320’s are available. 225’s and 235’s have an average life expectancy of fifteen to twenty years, while 320’s have a life expectancy in excess of twenty-five years.

Today, asphalt shingles are classified by the warranty offered by the manufacturer. They would now be known as 10 year, 15 year, 20 year, 25 year, 30 year or 35 year shingles. The reason for this change was the use of lighter fiber glass matting. Modern shingles are also available in various textures and edge patterns.

Since the mid 1960’s, most asphalt shingles have been of the self sealing type. A strip of tar is put on the surface of the shingles by the manufacturer. This strip is covered by the shingle installed immediately above. When the sun warms the roof surface, the two shingles stick together. This helps prevent the shingles from being blown off in a wind storm. (Shingles installed in the late fall and winter often do not seal themselves until the next spring.) On older, non-sealing asphalt shingles, a wind storm is often the final blow (no pun intended). The shingles, brittle with age, simply tear off and land in the garden. 

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Conventional asphalt shingles can be used on a slope as low as four in twelve, using normal techniques. Some roofers use these shingles down to a pitch of two in twelve if the roof is first covered with non-perforated, saturated felt papers. The felt papers must be overlapped by fifty percent and the section at the eaves (from the bottom edge up to twenty-four inches beyond the interior of the exterior wall) must be cemented in place to provide extra protection. Unfortunately, it is impossible to determine, during a visual examination, whether this procedure was undertaken. Many roofing experts feel this is not an ideal approach.

There are also special low slope shingles which are designed for pitches down to two in twelve. With these shingles, only one third of the shingle is exposed to the weather (as opposed to half of the shingle on a conventional installation) and the shingles are individually cemented in place.

Regardless of the type of asphalt shingle used, there are two general rules with regard to wear. 1) Sunlight is one of the biggest enemies of asphalt roofs and consequently in many areas the south and west exposures wear out the fastest. (Lighter colored shingles reflect more light and, consequently, last slightly longer.) 2) The steeper the pitch of the roof, the longer the shingles will last.

As asphalt shingles wear, they lose their granular covering. The granular material protects the shingles from ultra-violet light. As it is worn off, the shingles dry out and become brittle. They crack, buckle, and curl. Areas where the granular material has eroded the fastest, wear out first. These may be areas where there is heavy foot traffic, abrasion from tree branches, or erosion from downspouts discharging onto the roof surface.

Occasionally, shingles will wear out prematurely due to a manufacturer’s defect. Blisters, approximately the size of a dime, form underneath the granular surface and cause raised sections in the shingles. While these are not aesthetically pleasing, they do not affect performance until the granular material wears off in these areas.

In some instances, premature cracking and splitting of fiberglass reinforced shingles occurs. This cracking is sometimes noted as early as 6 months into the shingle life. Cracks on the shingles can be in any direction. Cracks lead to reduced shingle life.

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