Wood Structural Panel Sheating

Limitations of the Material in this Section

The IRC presents many combinations of materials and installation techniques for installing wood structural panels as sheathing on roofs and as subflooring and underlayment. This section includes common materials and installation techniques used in modern residential construction. Refer to the IRC for information about less common materials and installation techniques. 

Wood Structural Panel Definitions

Combination subflooring:  Combination subflooring is one panel secured directly to floor joists that acts as both the subflooring and underlayment.

Edge supports: Edge supports are approved materials that support structural panel ends or edges between rafters or joists. Edge supports include tongue-and-groove joints, panel edge clips, and wood blocking between rafters or joists. The IRC requires edge support for single panels used as a combination subfloor and underlayment and for some roofsheathing.

Panel span ratings: Panel span ratings appear on the panel as two numbers separated by a slash. The first number is the maximum distance between rafters when the panel is used as roof sheathing. The second number is the maximum distance between floor joists when the panel is used as subflooring. The span is the unsupported distance between the framing members such as floor joists or roof rafters.

Subfloor: Subfloor is the panel secured directly to floor joists.

Underlayment (flooring): Underlayment is at least () ¼ inch thick panel installed over the subfloor, usually as a smooth base for resilient floor coverings such as linoleum.

Wood structural panels: Wood structural panels include plywood, oriented strand board (OSB), and composite panels. OSB is often used in modern residential construction as roof sheathing and as subflooring. Plywood is often used as subflooring and underlayment.

Wood Structural Panel Installation Requirements

1. Install wood floor sheathing panels:

(a) so that they continuously span at least () two framing members, and

(b) with the long dimension perpendicular (90 degree angle) to supports, and

(c) that are at least () 24 inches wide. Panels less than 24 inches wide can deflect or fail under load.

2. Support wood floor sheathing panel edges with solid blocking, tongue-and-groove edges, or other approved means. An additional underlayment layer that is at least () ¹₄ inch or ¾ inch wood floor covering can substitute for edge support in some cases.

Wood Structural Panels Used as Roof Sheathing

1. Use wood roof sheathing panels such as 24/16 rated panels (₁₆ inch nominal thickness) and 32/16 rated panels (15/32 inch and ½ inch nominal thickness). Do not span a 24/16 rated panel more than (>) 24 inches between rafters when used as roof sheathing. Do not span a 32/16 rated panel more than (>) 32 inches between rafters with edge support or more than (>) 28 inches without edge support when used as roof sheathing.

2 You may use panels of other thicknesses as allowed by the IRC.

3. Install panels that are at least () 24 inches wide. Panels less than 24 inches wide can deflect or fail under load.

Wood Structural Panels Used as Combination Subflooring

1. You may use wood floor sheathing panels (such as 23/32 inch and ¾ inch nominal thickness plywood or OSB and ¾ inch sanded plywood) as a combination subfloor and underlayment. Install combination subflooring as described on the previous page. Be aware that while combination subflooring panels comply with the IRC, they may not comply with manufacturer’s installation for some floor coverings such as tile.

2. You may use other, thicker, panels as a combination subfloor and underlayment. You may use two separate panels, one as a subfloor and one as an underlayment.

Wood Structural Panel Installation Best Practice

Wood structural panels can deflect and sag under load when installed at their IRC permitted span limits. This can allow cracks in rigid floor covering materials such as tile and marble. Be aware of manufacturer and industry installation guidelines when selecting, supporting, and installing wood structural panels under all floor coverings, particularly under tile.