A.The differences between organic and fiberglass shingles are considerable and affect the quality rating of each material.

Organic shingles are composed of five components. The mat is made from wood fibers and recycled paper. This mat is saturated with a special saturant-grade asphalt. It is then coated on both sides with a harder, stabilized coating asphalt. A surfacing of mineral granules, usually ceramic-coated crushed rock, is added to protect the asphalt from the ultraviolet rays of the sun and to provide a decorative surface. Finally, a backcoating of particulate material, usually sand, slag, talc, or limestone, is added to keep the shingles from sticking together and from staining during storage.

Fiberglass shingles use only the coating-grade asphalt, which saturates a fiberglass mat and holds the surfacing. The granules and the backcoating are the same as for organic shingles.

In the past, when all asphalt shingles were organic, the quality of a particular shingle was judged by its weight and the length of its warranty. This is no longer considered an accurate basis. There are considerable differences in both the quality of the asphalt coating and the weight of the mats for both organic and fiberglass. Some tests have shown that a mid-weight shingle with a strong mat is more durable than a heavy shingle with a weak mat.

Despite warranty claims ranging from 15 to 25 years, there is evidence of premature failures of both types of shingles. The best assurance of quality is tear resistance.

Minimally, roofers and builders should look for compliance with the ASTM Standard D3462 for fiberglass shingles, which calls for a minimum tear strength of 1,700 grams, using an Elmendorf Tear Tester. There is no standard for organic shingles, but most will exceed 2,000 grams of tear resistance.