A. Bryan Readling, P.E., an engineered-wood specialist for APA, responds: For APA-rated structural panels - both plywood and OSB - there are two exposure durability grades: Exterior and Exposure 1. These are also known as bond classifications, because they relate to the moisture resistance of the glue bond, and thus to the structural integrity of the panel. Bond classification does not relate to the panel's physical resistance to, say, erosion or ultraviolet degradation, or to its biological resistance to mold, fungal decay, insect damage, and the like.
Although both panel grades use the same moisture-resistant adhesive, only Exterior-grade panels (such as sanded A-C and B-C plywood, T-111 siding, and overlaid panels like MDO) are intended for long-term exposure to weather, and even then they must be properly finished and maintained.
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