Reading Span Ratings

Span ratings are based on the panel’s recommended end use, which may be as roof sheathing, floor sheathing, wall sheathing, or siding panels.

The Fractional Number Index
Roof and subfloor span ratings are actually two numbers, displayed as a fraction. The roof index is on the top (or left) and the subfloor index is on the bottom (or right). Panels for wall sheathing also use the top (or left) number for the maximum stud center-to-center spacing in inches. The span ratings applicable to roof, subfloor and wall sheathing include 12/0, 16/0, 20/0, 24/0, 24/16, 32/16, 40/20, 48/24, and 60/32.

For example, a span rating of 24/16 indicates that the sheathing panel may be applied over roof supports 24 inches on center, wall studs 24 inches on center, or floor joists 16 inches on center. See the illustration below for the span rating for floor sheathing.

The Single-Number Index
A single number is used in the trademark stamp for single-floor panels and siding panels.

Sturdi-Floor panel span rating examples include 16, 20, 24, 32, and 48 oc. This number denotes the maximum recommended center-to-center spacing between floor joists with panels laid with the long dimension across three or more supports.

Siding panels are usually labeled with the index numbers 16 or 24 in the trademark stamp. This means the stud framing in the wall must be either 16 or 24 inches on center.

Panel Orientation
It is important to install the long dimension of roof or floor sheathing across the rafters, trusses or joists unless the strength axis is indicated to be across the narrow dimension of the panel.

APA-Rated wall sheathing may be installed vertically or horizontally. For best panel performance, apply continuously over at least three supports.

APA Rating siding panels are installed vertically over studs according to the index number (16 or 24). Any panel can be installed horizontally on either 16 or 24-inch on center studs as long as the panel edges are blocked and nailed.

Don’t Forget
Design requirements for roof or floor loads could require a stiffer panel to support a greater-than-average load.

Shear requirements may require a stiffer panel anywhere sheathing is used for shear in the building.

Always consult local code requirements before choosing a panel for a specific application.

The 2006 International Residential Code (IRC) section R503.2 has charts to assist in choosing the right panel thickness and span rating for the job. This information is also available in the 2006 International Building Code (IBC), section 2304.7.

An easy way to remember how to read a fractional span rating is to realize the index represents “roof or wall over floor” (Roof, Wall / Floor).