Roofs that are not flat have pitches, or slopes. The standard way to describe the slope is by the number of feet the roof rises vertically (called the rise) for every 12 horizontal feet (called the run). If a roof has a 4-ft rise in a 12-ft run, it has a slope of 4 in 12.
Roofing materials comes in various, and unequal, types. Some are suitable only for steep roofs (with slopes, say, of 7 in 12). No matter what the roofing material, do not install a material on a roof with a slope lower than the minimum specified by the manufacturer.
The most common pitched roof material is asphalt shingles, which are suitable for roofs down to a slope of 4 in 12, or 2 in 12 using special procedures prescribed by the manufacturer. Older asphalt shingles are made by forming wood pulp and paper into mat, saturating it with asphalt, and then covering the top with mineral granules. The granules adds weight and body, prevent abrasions and, most importantly, keep sunlight from degrading the mat.
These wood pulp based shingles now have been all but replaced by shingles made with a fiberglass mat. These shingles are lighter and last longer. Because fiberglass deteriorate much slower than organic materials, fiberglass based asphalt shingles should last 25 to 40 years, rather than the 15 to 30 years that the older, wood based type usually lasted.Generally both types of shingles last longer than their guaranteed life. Fiberglass based asphalt shingles are a bit more expensive than the old kind, but more than pay for themselves over their longer lives.
Of course, there are other suitable roofing materials that may be more appropriate than asphalt shingles in certain locations or on certain buildings.