Definitions for "HEADLAP"
Keywords:  undermost, ply, overlap, shingle, felt
Is the overlap of the bottom three inches of a slate over the top three inches of the slate in the second row below the first slate, or the top three inches of any slate.
Shortest distance from the butt edge of an overlapping shingle to the upper edge of a shingle in the second course below. The triple coverage portion of the top lap of strip shingles.
In a bituminous assembly, the overlap between the lower edge of the top exposed ply and the upper edge of the bottom ply, which provides a margin which ensures that the membrane is actually a two, three, or four ply membrane.