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Sheathing Thickness and Span Ratings

Roof sheathing panels carry a span rating stamped on each sheet, such as 24/16 or 32/16. The first number indicates maximum spacing in inches for roof rafters, and the second number indicates maximum spacing for floor joists. A 24/16 panel works on roof rafters up to 24 inches on center but only on floor joists up to 16 inches on center because floors carry heavier loads.

Actual thickness varies slightly from nominal ratings. A panel marked 15/32 inches measures between 0.451 and 0.484 inches depending on manufacturer. This matters when sheathing abuts wall sheathing or other materials—expect minor height differences that require shimming. Plywood tolerances run tighter than OSB, with less variation between sheets in a bundle.

Edge support requirements influence thickness selection. Sheathing panels without tongue-and-groove edges or H-clips require rafters spaced at the shorter span rating. Adding H-clips between rafters or using T&G panels allows you to use the full rated span. A 24/16 panel on 24-inch rafter spacing needs edge support; without it, you must space rafters at 16 inches or use a thicker panel.

Plywood vs. OSB Performance Characteristics

Plywood sheathing consists of thin wood veneers glued with grain directions alternating 90 degrees. This cross-lamination resists splitting and holds fasteners well. Plywood edges exposed to rain swell less than OSB and dry faster, reducing the risk of roof deck telegraphing through shingles. Premium plywood costs 20-40% more than OSB but delivers better dimensional stability.

OSB (oriented strand board) uses wood strands arranged in layers and bonded with resin under heat and pressure. The resulting panel offers consistent strength properties without the weak spots caused by knots and voids in plywood. OSB swells more than plywood when wet, but modern moisture-resistant formulations minimize this issue. Once dried and covered with roofing, OSB performs equivalently to plywood in service.

Installation speed favors OSB because sheets are larger and more uniform. Plywood sheets sometimes contain gaps, knots, or thin spots that create soft areas during nailing. OSB's homogeneous structure means every fastener gets the same holding power. Roofers appreciate OSB's consistent surface for walking and working, though wet OSB becomes dangerously slippery—always cover completed sections before rain arrives.

Layout, Fastening, and Code Requirements

Proper sheathing layout starts at the eave with the long edge perpendicular to rafters. Stagger vertical seams at least 4 feet from row to row, creating a brick-pattern layout that distributes load. Leave 1/8-inch gaps between panel edges and 1/16-inch gaps at ends to allow for expansion—panels swell in humidity and buckle if installed tight.

Fastening schedules vary by panel thickness and edge support. Common practice uses 8d ring-shank nails spaced 6 inches on center along panel edges and 12 inches in the field. High-wind or seismic zones require closer spacing—sometimes 4 inches at edges and 6 inches in the field. Always follow local code and manufacturer recommendations, especially for engineered shear wall or diaphragm designs.

Roof sheathing carries both gravity loads (weight of roofing, snow) and lateral loads (wind, seismic). The nailing pattern and panel layout create a rigid diaphragm that transfers lateral forces to the walls below. Cutting large openings for skylights or chimneys weakens the diaphragm, often requiring doubled rafters or rim blocking around the opening. Engineering input prevents over-weakening the roof structure.