Question: We have specifications for a project that specify Exterior Grade sheathing for a roof deck. The contractor used Exposure 1, and insists that it is the appropriate grade for the application.

Our primary concern is that the contractor did not protect the Exposure 1 sheathing, and it was subjected to heavy rains over a period of five to nine days. It is now protected, but is there a possibility of delamination? Prior to covering, we observed some swelling and minor buckling. Would Exterior Grade sheathing have exhibited the same physical characteristics under these conditions? Could you please shed some light on this subject?

Answer: First, we should note that Exposure 1 is the industry standard for roof deck sheathing applications throughout the U.S. In this case, it is likely that Exterior panels would have performed more or less the same as the Exposure 1 panels.

APA trademarked sheathing panels are produced in two exposure durability classifications – Exterior (plywood) and Exposure 1 (plywood and OSB). Exposure durability classification relates to glue bond and, thus, to structural integrity.

Exterior panels have a fully waterproof bond and are designed for applications subject to long term exposure to weather or moisture. Exposure 1 panels are made with the same adhesives used in Exterior panels, have a fully waterproof bond and are designed for applications having temporary exposure to weather, such as where construction delays may be expected prior to providing protection, or where temporary high moisture conditions may be encountered in service. Exposure 1 panels may also be used when exposure to the outdoors is on the underside only, such as at roof overhangs. APA Technical Topic: Bond Classification, Form TT-009, describes durability classifications in further detail.

Although glue bond durability is described by exposure durability classification, the surface and edges of a panel may become uneven and irregular under prolonged moisture exposure. Such weathering of panels is an aesthetic consideration and is not detrimental to structural capacity. In cases of prolonged exposure, panels are likely to look rough but often remain structurally serviceable.

APA recommends covering roof sheathing with roofing underlayment (asphalt-impregnated felt, per ASTM D226, or other product as recommended by the roofing manufacturer) as soon as possible after installation. If this is not possible and the roof sheathing has been exposed to rain or snow, allow the sheathing to dry from the underside to a moisture content of 18 percent before closing in the roof cavity or attic.