Sheathing is the term used to describe the material placed over the framework of a flat or pitched roof. The most commonly used sheathing is plywood or another type of manufactured wood called OSD. It is nailed into the roof joists or rafters and provides the base upon which to apply shingles or other types of roof covering.
The benefits of plywood sheathing include how quickly it is installed and the solid base it provides with fewer joints. There is little waste and it can be used under any type of shingle. The sheets measure 4 feet by 8 feet and are processed in several thicknesses and grades. The larger the space between the rafters, the thicker the plywood should be to provide the proper amount of strength and stability and minimize buckling. Thicker sheathing is also called for on buildings that experience heavy loads of snow and ice. Plywood is graded on its smoothness and strength and one side is usually better than the other. The lowest quality has cracks and missing knots, while the best quality is smooth and has no knots.
Wood boards from fir, hemlock or spruce are the standard sheathing for roofs finished with wood, clay or asphalt shingles. They are also used to provide the extra stability needed on a flat roof where a flat roof expanse will be used as a balcony or deck. The boards must be seasoned to minimize shrinking, warping and buckling. They can be laid out diagonally or horizontally and with or without spacing, if laid horizontally. On buildings frequently exposed to blizzards, no spacing is recommended.
Other Sheathing Types
Wood fiber sheathing is comprised of small pieces of wood laid in a crisscrossing pattern across each other and sealed with water-resistant glue. Also called OSB, Oriental Strand Board, it is manufactured in 4-foot by 8-foot sections and can be used interchangeably with plywood. It's less expensive, has a slip-resistant side, but some believe it is less desirable than plywood because it doesn't hold nails well and is not as strong. Foil-backed roof sheathing creates a barrier against the hot, broiling sun and is a popular choice on homes located in southern parts of the U.S. The sheathing is installed on new construction with the foil side facing down into the attic, keeping that space significantly cooler and minimizing the transfer of heat into the living area. On existing construction, the sheathing is installed against the trusses or exposed rafters from the attic side.
Sheathing Installation Tips
The roof of a building takes a particular beating from the elements and from constant exposure to sunlight. With time, roof sheathing may warp and buckle, a problem that the proper placement and spacing of materials can minimize. Plywood and particle board expand in the heat and contract in the cold and need breathing room. Adequate spacing means leaving a 1/16th- to 1/8th-inch gap between each section. Plywood sheathing joints must connect over a rafter; thus, the grain should be placed perpendicular to the rafters, according to the University of Florida Extension. Particle board can be installed perpendicular or parallel to the roof rafters and should be nailed at least 3/8 inch from the edges.
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