This installation technique overlaps shingles or rolled roofing so that the lapped portion exceeds the exposed portion by at least two inches. The result is a continuous double layer of weather protection across the roof surface. It is often required on low-slope roofs to prevent standing water from backing up under the courses.
Heard on the job
Check the spec for the low slope section, we need double coverage on the underlayment before the shingles go on.