Types of Roof Sheathing
Planks and panels are the two types of roof sheathing commonly used in residential construction. Planks are nominally 1 inch thick and vary in width, ranging from 4 inches to 8 inches. Planks also vary in edge finish, being either straight-edged or interlocking in design with a tongue-and-groove system or shiplap edging. Panels are made from plywood, oriented strand board (OSB) or wafer board, although plywood is the most common.
Open and Closed Roof Sheathing
Plank roof sheathing can be applied in either an open or a closed manner. Open sheathing is installed on the roof with gaps between runs, or rows, of planks. Closed sheathing rows are laid butting against the adjacent planks or are interlocked with adjacent planks when using tongue-and-groove or shiplap. Open-style roof sheathing may be used only in areas where blizzards do not occur or in areas of high humidity, and generally it only allows for lightweight roof covering such as wood shingles. Closed-roof sheathing is required for heavier roof coverings, such as tiles and asphalt shingles, and is required in areas where snow creates a heavy load on a roof.
Plywood Roof Sheathing Installation
Panels are installed in a closed style, with minimal gaps between them. A gap of 1/8 inch is needed between adjacent rows and a 1/16-inch gap is needed between ends of panels in one row, with H-clips installed to hold these gaps. This allows for expansion of the wood from moisture. The thickness of the panels is determined, in part, by the spacing between rafters. A 16-inch spacing requires a minimum 3/8-inch panel thickness, and a 24-inch spacing requires a minimum 1/2-inch thickness.
Important Installation Tips
Start the first row of roof sheathing at the left or right bottom corner at the eaves. Always make sure the end of each plank or panel ends at the middle of a rafter, so that both it and the next in the row can be nailed to the rafter. Each plank or panel must span at least 2 rafters as well, for stability. It is also recommended that you stagger the vertical joints from one row to the next as you work toward the ridge at the top. This can be done randomly or in a repeating pattern.