Double-coursing uses two layers of shingles (or one layer of shakes over a layer of No. 3 or No. 4 shingle). This application allows for wider weather exposures, and creates deep shadow lines at the butt ends (Double-Coursing, below).
Begin with a double underlay course or a single underlay course shimmed out the thickness of a shingle butt with wood lath.
Double-course applications require face-nailing. Face-nail outer shingles with two nails placed about 2 in. above the butt line and 3/4 inch in from each end. The outer course drops about 1/2 in. below the undercourse, providing an effective drip edge that limits absorption of water by capillarity.
For more on installing shingle siding, visit the JLC Field Guide
