Question: I specified 1/2-inch roof sheathing for a commercial building. When I was inspecting the project I found that the contractor had installed 15/32-inch. What can be done to correct the situation?

Answer: A wood structural panel that is 15/32 inch thick contains six percent less wood fiber than a 1/2-inch thick panel. The effect on structural properties and characteristics is negligible. In most applications, the 1/32 of an inch difference in thickness of span-rated wood structural panels does not affect performance. In fact, assuring adequate performance in specific applications is at the heart of APA’s rigorous program of quality auditing and testing.

APA members began certifying 15/32-, 19/32- and 23/32-inch panels with the APA trademark in 1980, when the Association implemented APA PRP-108, Performance Standards and Policies for Structural-Use Panels. These “new” panel thicknesses were adopted into model building codes shortly thereafter, first appearing in the Uniform Building Code (UBC) in 1985.

Performance standards allow flexibility in the manufacturing process yet assure that panels meet the demands of the job they're designed for - regardless of thickness. In developing the performance standards, APA engineers and wood scientists discovered that in most cases, 15/32-, 19/32-, and 23/32-inch panels matched the performance required of traditional 1/2-, 5/8- and 3/4-inch panels, respectively.

Span Rating, not thickness, is the best indicator of strength and stiffness when determining which APA trademarked panel is right for subfloor, wall and roof sheathing applications. The Span Ratings listed in the trademarks of APA Performance Rated Panels denote the maximum recommended center-to-center spacing, in inches, of supports over which the panel should be installed. For an overview of the span rating system reference APA Technical Topic TT-037.

The important panel characteristic for horizontal diaphragm and shear wall sheathing is panel thickness. As with other sheathing applications the 1/32-inch reduction in panel thickness has negligible impact on lateral load resistance. 15/32- and 19/32- panels first appeared in model building code shear tables with the publication of the 1985 UBC and continue in the present day International Building Code (IBC).