A. There are no problems with adding a second layer of shingles over conventional asphalt shingles, if they are properly installed. To keep the second layer flat and avoid a wavy pattern in the new roof, cut the tabs off the new shingles for the starter course, as shown below. Nails, not staples, should be used to fasten the new shingles.

I would not recommend more than two layers of shingles, although many codes permit three. There are three reasons for not going past two layers: 1) the combined weight of the layered shingles would be very heavy and might exceed the design load of the roof structure; 2) the deep layers would give inadequate lateral support for the nails and there would be a greater chance of a blowoff; and 3) the roof would retain more heat, which would reduce the life expectancy of the shingles.

I would also not recommend a second layer over textured (architectural-style) asphalt roof shingles, because of the added weight and the difficulty of producing an even surface.