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What is Oriented Strand Board (OSB)?

OSB is a sheathing product manufactured from wood strands glued together with waterproof, heat-cured adhesives. Sheets are assembled in cross-oriented layers, similar to plywood. Many applications where plywood was used now use OSB. APA Voluntary Standard PS 2-92 specifies performance standards for OSB and plywood.

Is Oriented Strand Board acceptable as a part of a tile system?

The most current ANSI standards for tile installation (A108 - 1999) specifically exclude OSB. This is not to say that tile cannot be installed reliably over an OSB subfloor; however, there is significant debate in the tile industry regarding the conditions necessary for a long-lasting successful installation. The allowable ambient moisture level, extent of moisture related swelling before tiling, blocking and bracing and many other issues are still being debated. Each installation materials manufacturer has their own criteria and proprietary methods and requirements. Some backerboard companies will warrant installations using their products over OSB; however, their specific methods must be followed. Note: thinset manufacturers do not recommend installing ceramic tile directly to OSB. Refer to the TCNA Handbook for recommended systems that include OSB, and follow manufacturer's literature for instructions and cautions.

As with all installations, movement joints are necessary to allow for expansion and contraction in the tile layer.