Asphalt shingle roofs are the most popular roof material. They cover approximately 70-80 percent of all residential construction. Asphalt shingles are relatively inexpensive, easy to install and requires little maintenance than other more complicated roof coverings. The shingles come in a multitude of colors and designs and last from 15 to 30 years typically. Another advantage of asphalt shingles is normally a second layer that can be applied after the first has worn out. A common practice of installing a third layer to save expense by homeowners and roofers is often undertaken. This usually is a problem because it puts too much weight on the roof, shingles wear rapidly and the roof surface becomes uneven and is prone to wind damage. Asphalt shingles are sometimes improperly installed over old wood shingled roofs and sometimes even old slate roofs.