Roof Sheathing Thickness by Rafter Spacing

Plywood and OSB Thickness Quick Comparison

These are common product associations. Panel grade, ply construction, and manufacturer can affect the exact rating available at a given thickness; always verify with the printed grade stamp.

Important Chart Limitations

This chart assumes qualifying wood structural panels installed according to their grade stamp, span rating, approved framing arrangement, applicable roof loading and locally adopted code. It does not represent the thinnest available panel as a universal recommendation, and it does not replace an engineered design where required.

• Supports roof coverings and construction loads placed on the roof during and after construction.
• Distributes loads from the roof surface to the supporting rafters or trusses.
• Provides a fastening substrate for underlayment, roofing materials and attachments.
• Contributes to diaphragm performance when the panel, fastening and framing are designed and installed for that purpose.

Roof sheathing is distinct from underlayment (a water-resistive layer applied over the sheathing), roof insulation, shingles or other finished roof coverings, and the roof-covering manufacturer’s own installation system.

Roof Sheathing vs. Roof Decking

“Roof decking” is sometimes used broadly to describe any structural roof substrate, including wood panels, solid lumber decking, and metal decking systems. This chart focuses on plywood and OSB wood structural panel sheathing, which is the most common residential roof substrate, with lumber roof decking addressed only briefly for context.

Product grade, thickness, span rating, ply construction and manufacturer availability can vary. Verify the specific product’s grade stamp before purchase.

Use the actual panel grade stamp as the authoritative product identifier. Do not assume every OSB panel of a given thickness carries the same span rating; ratings are product- and manufacturer-specific.

Installation practices, moisture response, panel weight, cost and roof-covering compatibility can differ by product line. Neither material is universally stronger or automatically suitable for every roof; the specific rated product governs.

For example, a panel marked 32/16 has a rated roof-support spacing of 32 inches and a rated subfloor-support spacing of 16 inches within its specified conditions. Span rating is not the same thing as panel thickness: two panels of similar thickness can carry different span ratings depending on their internal construction and grade.

Explore related span data on the Roof Rafter Span Chart and Roof Truss Span Chart.

Roof Sheathing Span Rating vs. Actual Rafter Spacing

Compare the roof-span number in the grade stamp (the first number in the rating) with the actual rafter or truss spacing on the project, whether that is 16, 19.2 or 24 inches. A rated maximum span does not by itself demonstrate adequate performance for unusually heavy roof coverings, high snow loads, concentrated loads or nonstandard installations; those conditions require separate verification.

Do not assume all panels of the same nominal thickness are interchangeable; verify the actual grade stamp for every product used.

A thicker or higher-rated panel may be selected for higher roof loading, improved stiffness, particular roof-covering systems, additional construction loads, or project-specific deflection requirements. Whether panel edge support is required at this spacing depends on the specific span rating, actual spacing and applicable code, not on thickness alone.

• Panel thickness and stiffness become more significant as spacing widens.
• Edge support requirements are more likely to apply.
• Correct strength-axis orientation is essential.
• Roof loading and structural design should be verified directly rather than assumed.

There is no universal rule for all wider spans; always confirm the specific panel rating and design against the actual framing layout. See the Roof Purlin Size Chart and Purlin Spacing Chart for related wide-spacing framing, and the Metal Roof Panel Span Chart for metal panel systems.

Installing a panel with its strength axis parallel to the rafters instead of perpendicular can significantly reduce its effective span capability, even though the panel’s thickness and rating remain unchanged on paper.

A span rating by itself is not a complete structural load calculation. Distributed loading and localized/concentrated loads are evaluated differently.

There is no universal thickness-by-snow-depth conversion; always use the applicable design snow load and span tables. See the Roof Rafter Span Chart, Roof Truss Span Chart, and the Snow Load Calculator.

• Wind exposure category and design wind speed affect required fastening.
• Roof edge and corner zones often require closer fastener spacing than the field of the roof.
• Ring-shank nails may be specified where required by the applicable fastening schedule.
• Local high-wind provisions can differ from the ordinary baseline installation schedule.
• Roof-diaphragm design ties sheathing, fastening and framing together as a system.

Thicker sheathing alone is not the standard response to high wind; fastening schedule, edge/corner detailing and connections typically matter more. Use the Wind Load Calculator and confirm requirements with the locally adopted high-wind provisions.

Roofing-material compatibility and total loading can change the sheathing specification beyond the base span rating; always check the specific manufacturer’s requirements.

Roof Sheathing for Low-Slope Roofs

Certain low-slope assemblies, including built-up roofing, single-ply membranes and modified bitumen, can require stiffer decking than ordinary pitched residential roofs due to surface-smoothness, deflection and membrane-adhesion requirements. Detailed membrane and drainage design is outside the scope of this chart.

The attachment design and load path for solar mounting hardware, equipment stands and similar items must transfer loads appropriately into the roof framing. There is no generic sheathing thickness that automatically supports every solar or equipment installation; each attachment layout should be reviewed against its own structural requirements.

These are exact conversions of the labeled thickness categories, not guarantees of the precise measured thickness of every manufactured panel.

• Manufacturer or mill identification
• Grade
• Span rating
• Performance category
• Bond classification
• Applicable product standard
• Special structural designation, where applicable

Structural plywood is commonly manufactured to DOC PS 1, while performance-rated wood structural panels, including many OSB products, are commonly manufactured to DOC PS 2, alongside other standards recognized by the locally adopted code.

Neither classification means roof sheathing should be left unprotected indefinitely, and bond classification is separate from roofing underlayment and the finished roof-covering’s own weather protection.

This is APA’s stated ordinary example, not a universal fastening rule. Adopted code, engineered diaphragm design, manufacturer instructions and high-wind provisions may specify different fasteners or closer spacing. See the Nail Size Chart for detailed nail dimensions and the Wood Screw Size Chart for related fastener data.

Panels can expand after absorbing moisture. Installing panels tightly against each other without an adequate gap can contribute to panel-edge buckling or ridging as the material expands.

• Check the manufacturer’s specific gap recommendation before installation.
• Maintain consistent gaps across all panel joints.
• Avoid installing panels in wet conditions without appropriate precautions.
• Store panels properly to limit moisture absorption before installation.

1. Check framing alignment, spacing and squareness before laying panels.
2. Confirm panel grade, span rating and performance category from the grade stamp.
3. Install panels with the specified strength-axis orientation.
4. Support panel end joints over framing members.
5. Maintain the recommended panel gaps at edges and ends.
6. Use the correct nail type and fastening schedule for the project.
7. Protect panels from excessive moisture before and during installation.
8. Install the appropriate roof underlayment promptly after sheathing.

• Identify the existing sheathing material (plywood, OSB, or lumber decking).
• Measure panel thickness directly where accessible.
• Check for deterioration, delamination, or moisture damage.
• Verify the actual rafter or truss spacing.
• Replace damaged or delaminated panels rather than covering over them.
• Check remaining nail-holding capacity in older panels.
• Follow local reroofing code requirements for sheathing inspection and replacement.

Existing panel thickness alone is not sufficient to determine whether an older roof meets current span, loading, or code requirements.

Use the Roofing Calculator for full material planning and the Roof Pitch Calculator to account for slope-adjusted area, plus the Roof Pitch Chart for slope reference.

1. Identify the locally adopted building code.
2. Determine the actual roof framing spacing.
3. Calculate the applicable roof loads, including snow and wind where relevant.
4. Identify the roof-covering system planned for the project.
5. Check the panel’s actual APA (or equivalent) span rating from its grade stamp.
6. Verify the performance category and bond classification.
7. Confirm the correct panel orientation.
8. Determine whether edge-support hardware is required.
9. Check fastening and wind-uplift requirements for the region.
10. Check the roof-covering manufacturer’s own sheathing requirements.
11. Review construction and concentrated loads, including solar or equipment attachments.
12. Follow engineered design where the prescriptive tables do not cover the condition.

For related framing dimension and span references, see the Lumber Size Chart, Lumber Span Chart and Roof Rafter Size Chart. For framing takeoffs, use the Framing Calculator and Lumber Calculator.