Roofing shingles are made from several types of materials. Wood shingles are sawed from red cedar or pine. Modern shingles are cut from new growth trees and must be treated with chemical preservatives to make them last as well as earlier versions that were cut from old growth trees. They must also be chemically treated to achieve a fire resistance rating comparable to other types of shingles; in fact, the highest rating can be attained only by installing them over a special subsurface layer. Aluminum shingles have a long life span, although they are comparatively expensive. Asphalt shingles cover about 80% of the homes in the United States. Their popularity is due to their relatively light weight, comparatively low cost, ease of installation, and low maintenance requirements.
A typical asphalt shingle is a rectangle about 12-18 in (30-46 cm) wide and 36-40 in (91-102 cm) long. Popular styles have several cutouts along one edge to form tabs that simulate smaller, individual shingles. Three tabs are common, but the number may range from two to five. Some styles are made to interlock with adjacent shingles during installation, creating a more wind-resistant surface.
Asphalt has been used as a building material for thousands of years. Ancient Babylonians used it as mortar between clay bricks and as a waterproofing liner in canals. Roll roofing, consisting of long strips of asphalt-coated felt with a finishing layer of finely crushed stone, has been manufactured in the United States since 1893. In 1903, Henry M. Reynolds began marketing asphalt shingles he cut from sheets of roll roofing. By the 1920s, this roofing material had become so popular it was sold through mail-order catalogs. By the 1950s, the typical asphalt shingle looked much as it does today, including the tab-forming cutouts.
Since the late 1950s, manufacturers have sought to develop inorganic base materials as alternatives to the traditional organic felt. Inorganic bases are desirable because they are more fire resistant than an organic base; furthermore, they absorb less asphalt during the manufacturing process, so the resulting shingles weigh less. Asbestos was used in shingle mats until its related health risks became well known. Improvements in fiber-glass matting have made them the most popular asphalt shingle base material in the industry since the late 1970s.
Asphalt shingles are sometimes called composite shingles. Their foundation is a base of either organic felt or fiberglass. Organic felt mats are made of cellulose fibers obtained from recycled waste paper or wood. These fibers are reduced to a water-based pulp, formed into sheets, dried, cut into strips, and wound onto rolls. Thinner, lighter shingles with a higher resistance to fire are made on a base of fiberglass. In a typical process, the fiberglass membrane is made by chopping fine, glass filaments and mixing them with water to form a pulp, which is formed into a sheet. The water is then vacuumed out of the pulp, and a binder is applied to the mat. After curing, the mat is sliced to appropriate widths and rolled.
Asphalt, a very thick hydrocarbon substance, can be obtained either from naturally occurring deposits or, more commonly, as a byproduct of crude oil refining. Before being used in the manufacture of shingles, asphalt must be oxidized by a process called blowing. This is done by bubbling air through heated asphalt to which appropriate catalysts have been added, causing a chemical reaction. The resulting form of asphalt softens the right amount at the right temperatures to make good shingles. To further process the blown asphalt into a proper coating material, a mineral stabilizer such as fly ash or finely ground limestone is added. This makes the material more durable and more resistant to fire and weather.
Various colors of ceramic-coated mineral granules are used as a top coat on shingles to protect them from the sun's ultraviolet rays, increase their resistance to fire, and add an attractive finish. The granules may be small rocks or particles of slag (a byproduct of ore smelting). Shingles designed for use in humid locations may include some copper-containing granules in the top coat to inhibit the growth of algae on the roof. The back surface of the shingles is coated with sand, talc, or fine particles of mica to keep the shingles from sticking together during storage.
Strips or spots of a thermoplastic adhesive are applied to most shingles during the manufacturing process. Once installed on a roof, the shingles are heated by the sun, and this adhesive is activated to bond overlapping shingles together for increased wind resistance.
Asphalt shingles are produced by passing the base material through a machine that successively adds the other components. The same machine can be used to make either shingles or roll roofing.
Quality control begins during the manufacture of the base material. Not only must the material perform well in the final product, but it must also have enough tensile (pulling) strength and tear resistance to withstand the shingle-making process. In addition to these strength characteristics, organic mats are continuously monitored to ensure proper moisture content and absorbency. Fiberglass mats are monitored for proper fiber distribution and uniform weight.
During the manufacturing of the shingles themselves, numerous characteristics are monitored, including uniformity and thickness of the asphalt coating. Application of the mineral coatings is monitored for uniform distribution and proper embedding of the particles. Inspections of the finished shingles verify proper weight, count, size, and color of the product.
In addition to their own quality control measures, shingle manufacturers may also invite third-party, independent testing laboratories to their plants and send them product samples. These testing laboratories verify the product's compliance with ratings defined by Underwriters Laboratories (UL) and the American Society for Testing Materials (ASTM). Such ratings indicate that the product meets specified standards for fire resistance and other performance characteristics like strength and wind resistance.
Asphalt-coated waste from the manufacturing process includes shingle fragments and waste generated by cutting tabs on the shingles. In some cases, this waste is sold for use in making asphalt pavement for roads. For example, the state of North Carolina has allowed such material to be used in asphalt pavement since 1995 and has not encountered any problems with making or placing the paving material. Because of shipping costs, however, this option may not be practical unless a paving asphalt processing plant is located relatively close to the shingle plant.
Gibson, Barbara G., ed. Sunset Books. Roofing & Siding. Lane Publishing Co., 1981.
Herbert, R.D., III. Roofing: Design Criteria, Options, Selection. R.S. Means Company, 1989.
Residential Asphalt Roofing Manual. Asphalt Roofing Manufacturers Association, 1993.
"Frequently Asked Questions." Certain-Teed Corporation. http://www.certainteed.com/roofing/ctroof/faq.html (7 May 1997).
— Loretta Hall