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ADVANCED RESIDENTIAL FINISHES AND DECKS

Structural Wood Panels Historically, boards were used for roof, floor, and wall sheathing; in the last 30 years, however, structural wood panel products have come to dominate the sheathing market. Structural wood panel products are more economical and efficient and can be stronger than traditional board sheathing. Structural wood panel products primarily include plywood and oriented strand board (OSB). Plywood is manufactured from wood veneers glued together under high temperature and pressure. Each veneer or ply is placed with its grain perpendicular to the grain of the previous layer. The outer layers are placed with their grain parallel to the longer dimension of the panel. Thus, plywood is stronger in bending along the long direction and should be placed with the long dimension spanning floor and roof framing members. The number of plies typically ranges from 3 to 5. Oriented strand board is manufactured from thin wood strands glued together under high temperature and pressure. The strands are layered and oriented to produce strength properties similar to plywood; therefore, the material is used for the same applications as plywood. The designer should specify the grade and span rating of structural wood panels to meet the required application and loading condition (i.e., roof, wall or floor). The most common panel size is 4×8 feet panels, with thicknesses typically ranging from 3/8-inch to more than 1 inch. Panels can be ordered in longer lengths for special applications. Residential Structural Design Guide 5-7 Chapter 5 – Design of Wood Framing Plywood is performance-rated according to the provisions of USDOC PS1 for industrial and construction plywood (NIST, 1995). OSB products are performance-rated according to the provisions of USDOC PS-2 (NIST, 1992). However, these standards are voluntary and not all wood-based panel products are rated accordingly. The APA–Engineered Wood Association’s (formerly American Plywood Association) rating system for structural wood panel sheathing products and those used by other structural panel trademarking organizations are based on the U.S. Department of Commerce voluntary product standards. The veneer grade of plywood is associated with the veneers used on the exposed faces of a panel as follows: GradeA: The highest-quality veneer grade, which is intended for cabinet or furniture use. Grade B: A high-quality veneer grade, which is intended for cabinet or furniture use with all defects repaired. Grade C: The minimum veneer grade, which is intended for exterior use. Grade D: The lowest-quality veneer grade, which is intended for interior use or where protected from exposure to weather. The wood strands or veneer layers used in wood structural panels are bonded with adhesives and they vary in moisture resistance. Therefore, wood structural panels are also classified with respect to end-use exposure as follows: • Exterior panels are designed for applications with permanent exposure to the weather or moisture. • Exposure 1 panels are designed for applications where temporary exposure to the weather due to construction sequence may be expected. • Exposure 2 panels are designed for applications with a potential for high humidity or wetting but are generally protected during construction. • Interior panels are designed for interior applications only.