The term plywood refers to sheet goods composed of nearly solid layers, called plys or veneers. Most manufacturers literally peel the solid layers of wood from logs and laminate successive layers with adhesive to create a 4-by-8-foot sheet. Although the required thickness of plywood for roof sheathing varies according to the span between rafters, most municipalities require 5/8 inch or greater plywood. Additionally, plywood for roof sheathing applications must have an exterior rating to withstand potential exposure to moisture and temperatures.
Oriented Strand Board (OSB)
Oriented strand board (OSB) consists of a mass of tightly bound wood chips. Although OSB's strands appear randomly placed, the arrangement of the board's chips enhance the sheet's overall strength and stability. Depending upon local codes and engineering requirements, OSB is suitable for use as roof sheathing. OSB is generally less expensive than plywood and exhibits similar strength. However, OSB is heavier and tends to suffer water damage more easily than plywood.
Radiant Barrier Sheathing
The term radiant barrier refers to a foil-like coating that blocks the heat from entering an attic space and reduces a structure's overall cooling costs. Both plywood and OSB roof sheathing are available with a radiant barrier coating. Radiant barrier covers only one side of the sheathing, and roofers install the radiant barrier side of the sheathing facing the structure's interior. Although slightly more costly than standard sheathing, radiant barrier sheathing significantly reduces attic temperatures and cooling loads in hot climates.
Roof sheathing has specific nailing and installation pattern requirements. Although nail size and placement vary according to engineering specifications and local codes, roof sheathing typically requires 8d or larger nails spaced 6 inches on center around the sheet's perimeter and 12 on center throughout the sheet's field. Additionally, builders must install roof sheathing in a staggered pattern. In other words, the butt joints between two sheets of sheathing cannot line up with the butt joints between adjacent rows of sheathing, similar to the pattern created by the mortar joints of traditional block or brick walls.