Plywood Sheathing Calculator

Structural sheathing is the backbone of any wood-framed wall or roof system. Choosing the right material and calculating accurately prevents costly mid-project shortages.

Plywood vs. OSB

Plywood (cross-laminated wood veneer) and OSB (oriented strand board, compressed wood strands) are both APA-rated structural panels sold in 4×8-foot sheets (32 sq ft each). Plywood is dimensionally more stable and resists edge swelling when wet. OSB is 15–20% cheaper and is the dominant product in new residential construction. For wall sheathing, either works well. For roof sheathing in wet climates or at exposed eaves, plywood is preferred because OSB edge-swells when repeatedly wetted and dried.

Sheathing Thickness by Application

• Wall sheathing: typically ⅜" or ½" (½" preferred for braced wall panels per IRC)
• Roof sheathing: ½" for rafters at 16" o.c. spacing; ⅝" for 24" o.c. spacing
• Subfloor: ¾" tongue-and-groove (a separate product category)

APA Span Ratings

Structural panels are stamped with span ratings like “32/16” — meaning the panel can span up to 32 inches between roof supports and up to 16 inches between floor supports. Always match the span rating to your framing spacing.

H-Clips for Roof Panels

When rafter spacing exceeds 16 inches with thinner sheathing, building codes require H-clips — metal clips installed between panel edges at mid-span to transfer loads without edge blocking. Typically one H-clip per rafter space on the unsupported edges.

Nailing Schedule

Per code (IRC R803.2.3 and R602.3), structural sheathing must be nailed:

• 6" on center at all panel edges
• 12" on center in the field (interior of panel)
• Use 8d common nails (or 10d for ¾" panels)

Roof Slope Factor

The actual roof area is always larger than the horizontal (plan) footprint, by a factor that depends on pitch:

• 4/12 pitch: × 1.054
• 6/12 pitch: × 1.118
• 8/12 pitch: × 1.202
• 10/12 pitch: × 1.302
• 12/12 pitch: × 1.414

Formula: slope factor = √(1 + (rise/run)²)

Wall Sheathing Formula

Net area = (Width × Height) − (Windows × avg. window area) − (Doors × 21 sq ft)

Sheets = ceil(Net area / 32) × 1.10 waste factor

Roof Sheathing Formula (Gable Roof)

Roof area = Horizontal Width × Horizontal Length × slope_factor × 2 (both sides)

Sheets = ceil(Roof area / 32) × 1.10

Worked Example — Roof

A 40×28 foot house (plan dimensions), 6/12 pitch gable roof:

• Each side: 40 × 14 = 560 sq ft (horizontal). 14 = half of 28-foot span.
• Slope factor: 1.118
• Total roof area: 560 × 1.118 × 2 = 1,252 sq ft
• Sheets: ceil(1,252 / 32) = 40 sheets × 1.10 = 44 sheets
• At $35/sheet = $1,540

Staggering Panel Joints

Always stagger panel joints — offset every other row by half a sheet (24 inches). This distributes loads across multiple framing members and is required by building codes for structural sheathing applications.