Selecting Sheathing Panels
Sheathing should consist of performance-rated structural panels. These are made from plywood or oriented-strand board (OSB), and should carry a stamp (Figure 2-8) from APA-The Engineered Wood Association (formerly the American Plywood Association).
Panel Use Ratings
For framing, choose only from the following panel grades:
- Sheathing panels are designed for use on roof decks and walls;
- Structural 1 panels are designed for use on shear walls;
For framing, select panels with only the following exposure ratings:
- Exterior panels can be used outdoors;
- Exposure 1 panels can withstand moisture during normal construction delays — up to a full season, according to APA. However, the panels should be covered up as soon as possible.
Other exposure ratings for wood panels include:
- Exposure 2 panels are typically used for protected construction and industrial applications where the potential for high humidity and water leakage may exist for limited times;
- Interior panels are made with water-soluble interior-grade glues and are intended for interior applications only, not for framing.
Panel Span Ratings
Span ratings on sheathing grade stamps may have two numbers such as 24/16. The first indicates the allowable roof span (in this case, 24 in. o.c.) and the second indicates the subflooring span (16 in. o.c.). Span ratings for multipurpose panels are: 24/0, 24/16, 32/16, 40/20, and 48/24.
If a sheathing panel stamp includes only one span number, the panel is for use on walls only.
Plywood and OSB are always 1/32 in. smaller than their nominal sizes. For example, 1/2-in. plywood actually measures only 15/32 in.
Roof Panel Span
Roof panels should be a minimum of 1/2 in. thick for 16-in. o.c. framing, and 5/8 in. thick for 24-in. o.c. framing.
For roofs framed 24-in. o.c., use H-clips to support the edges of the sheathing at mid-span. Clips create the proper gap and help stiffen panel edges.
While the strength and nail-holding ability of OSB and plywood are equal for similarly rated panels, commodity-grade OSB may swell more when wet. Commodity-OSB panel manufacturers sometimes treat panels with a waxy finish to deter water absorption, but this coating deteriorates with exposure to ultraviolet light. The exception, of course, is integrated sheathing (such as Zip System), which includes an integral weather barrier and remains resist to water penetration for the life of the product.