Span Rating
A span rating is the pair of numbers stamped on structural panels (like 32/16) indicating the maximum recommended center-to-center spacing of supports — the first number for roof rafters and the second for floor joists, both in inches.
In Detail
Understanding Span Ratings
Span ratings appear on every APA-rated structural panel and are one of the most important numbers a contractor reads on the jobsite. The format is always two numbers separated by a slash — for example, 32/16, 48/24, or 24/0.
The first number indicates the maximum center-to-center spacing (in inches) of supports when the panel is used as roof sheathing with the long dimension across three or more supports. The second number is the maximum spacing when used as subflooring.
A rating of 24/0 means the panel is approved for roof sheathing over supports spaced up to 24 inches on center but is not rated for subfloor use. A 48/24 panel can span 48-inch rafter spacing for roofs and 24-inch joist spacing for floors.
Key Points
- Span ratings are determined by panel thickness, wood species, and bond classification.
- They are NOT the same as the panel thickness — a thicker panel of weaker species may have the same span rating as a thinner panel of stronger species.
- Always verify local building codes, as some jurisdictions impose additional requirements beyond APA ratings.
- Panels installed with the strength axis perpendicular to supports achieve the rated span. Rotating 90° reduces capacity significantly.
Related Terms
APA Rating
An APA rating is the performance certification stamped on structural wood panels by APA — The Engineered Wood Association, indicating the panel's span rating, exposure durability, species group, and manufacturing standard compliance.
Structural I
Structural I is a premium APA panel designation indicating the panel is made exclusively from Group 1 wood species (strongest species group) and has enhanced cross-panel properties, providing higher shear and racking resistance for demanding structural applications.
Exposure 1
Exposure 1 is a plywood adhesive classification indicating the panel uses waterproof glue that will not fail from moisture, but the panel is designed for temporary construction moisture exposure — not permanent outdoor use.
Deflection Limit
A deflection limit is the maximum amount a structural member (beam, joist, or rafter) is allowed to bend under load, expressed as a fraction of the span length. Common limits are L/360 for floors (1 inch of deflection per 360 inches of span) and L/240 for roofs. Deflection limits prevent bouncy floors and cracked finishes.
Related Materials
CDX Plywood
This is a test description for CDX plywood as we work on the technical backend of the website.
OSB (Oriented Strand Board)
Oriented strand board (OSB) is an engineered wood panel made from compressed wood strands bonded with waterproof resin. It is the most widely used structural sheathing panel in North American residential construction, offering uniform strength properties and lower cost than plywood.