Do not price an overlay you have not opened
A Marrero ranch from the 1960s or 1970s sometimes has a second layer of shingles. The scope starts by knowing whether the deck is sound. New shingles on rotten plywood fail at the eaves first. We say if the tear-off has to include the deck.
Estelle and the later subdivisions are cleaner: one layer, a hip, a ridge, a set of boots. Those fail in the details. We do not replace a field of sound shingles because the ridge vent cracked.