Headlap | Article about headlap by The Free Dictionary
In the lapping of roofing shingles, the shortest distance between (a) the lower edge of an overlapping shingle and (b) the upper edge of the lapped unit in the second course below.
References in periodicals archive
"But it's primarily used as headlap
-- the section of the shingle underneath the overlap of the shingle above it -- because it's less expensive than the colored granules that make up the wide variety of colors you can put on your roof."