double coverage

Double coverage is a roofing installation method where shingles or roll material overlap so that at least two full layers of material sit above the deck across the entire roof surface. The lap, or overlap, must be at least two inches wider than the exposed portion that faces the weather. This means if a shingle shows 4 inches to the weather, the overlap beneath it is at least 6 inches, creating a 10-inch total shingle width.

The method applies to three material types: asphalt shingles, slate tiles, and wood shakes. Asphalt shingles are most common; these three-tab and architectural shingles are installed starting from the eave and work upward in overlapping rows, with fasteners driven through both the exposed shingle and the one beneath it. Slate and wood shakes follow the same principle but demand more precision in lap width due to their weight and cost. The extra overlap adds significant material volume and labor time compared to standard single-coverage installation.

Double coverage serves several purposes. The doubled layer provides backup protection if water penetrates the outer shingle during heavy rain or wind-driven moisture. It distributes loads better on the underlying structure and extends roof life in high-traffic or severe-weather zones. Building codes in some regions specify double coverage for roofs on steep pitches, for buildings in snow loads above certain thresholds, or in high-wind areas. It is also common practice in renovation work when new shingles must lap and bond securely over existing single-coverage roofing.

Double coverage fails when the lap width is undersized, leaving portions of the deck exposed to direct weather rather than backed by two layers. This occurs if shingles are installed too far down the roof or with insufficient starter courses at the eave. Wind can lift poorly fastened overlaps, allowing moisture behind the lap. In wood shakes, splitting and cupping in the overlap zone creates gaps where water enters vertically. Slate and tile double coverage requires careful alignment; misalignment causes the overlap to miss the coverage zone entirely, negating the protection benefit.

The term "double coverage" distinguishes this practice from "single coverage," where shingles overlap only enough to protect the fastener zone but not the full width. Single coverage reduces material and labor costs but provides less weather redundancy. Double coverage appears in specifications as either a requirement or an option; when specified, it increases the roofing material takeoff and labor estimate by roughly 30 to 50 percent depending on pitch and existing conditions.