Composition shingles are made of a fiberglass mat at their core, and the exterior is an asphalt coating that is weather resistant. This exterior is embedded with crushed rock. The size and thickness of shingles depends on the manufacturer's own construction. Some brands of composition shingles are very susceptible to algae growth, which may not be very visually appealing, often looking like a light colored streak on a roof. It is, however, possible to buy a shingle with an anti-algae coating, but be aware that this option is more expensive. However, it might improve your roof's appearance.