Subfloor sheathing are structural boards that span across floor joists to provide both a diaghram and a base for the finish flooring material. When sheathing a subfloor, plywood or OSB may be used. OSB currently has the market share over plywood in general and is most often used for subfloor material due to it’s often more economical price and adequate load-bearing characteristics. 1/2 – 5/8″ thick boards are commonly used for a 16″ joist span and 3/4 – 7/8″ is commonly used for 20″ joist spans.  As to the question of which product is superior, many experts agree that plywood and OSB are virtually equal in structural performance.
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OSB is a resource friendly material and has a consistent density though it is typically slightl heavier than plywood of equal thickness. Plywood on the other hand is often preferred by tile and resilient floor covering installers due to it’s smoothness and resistance to swollen edges vs. OSB. Many feel that plywood will resist water better than OSB but when it does become wet, will take longer to dry out. On a sheet of plywood or OSB, a gradestamp will provide various bits of information, one of which being it’s allowable span for it’s thickness when placed on joists or rafters.
    The subfloor installation is typically started panels at a corner, staggering panels at least 2 supports and minimum of 1/2″ bearing on joists or rafters, and an 1/8″ gap at panel ends. Fasteners typically used are wood screws (reduce eventual creaking), common or deformed shank nails, or power driven fasteners. 6 or 8d ring shank or common nails are typically used for panels up to 3/4″ thick. Also glue may be applied via a bead on the joists or rafters, which helps decrease eventual floor squeaks and also adds to the structural rigidity.
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Panels should be fastened every 6″ on edges and every 12″ along joists. No fastener should be within 3/8″ of the edge of the panel. AFter installation any uneven joints should be sanded flush before underlayment or finish flooring is applied.
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