If the roof is in good condition and only has two layers of shingles in place, the new shingles can go over the existing roofing. If three layers are already in place, they need to be removed first. A layer of felt goes down over the plywood sheathing when shingles are installed on a bare roof.
The installer starts by snapping a vertical chalk line at the center of the roof to serve as a guide for installation. A starter row of shingles with the tabs cut off goes along the bottom edge of the roof with a 1/2-inch hangover.
Additional chalk lines help install the shingles in straight rows. The shingles should overlap so only the tabs of the previous shingle show. Each row should be offset by about 6 inches from the previous row so the tabs don't line up.
When both sides of the roof are shingled, the installer needs to install ridge shingles. Homeowners can purchase special shingles for the job or cut the tabs from regular shingles. The shingles go over the ridge, so part is on each side before being nailed in place.Learn more about Building Materials