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 structural panels rated by APA (the Engineered Wood Association) and 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", with ½" preferred for braced wall panels under the International Residential Code (IRC)
• Roof sheathing: ½" for rafters at 16" on center; ⅝" for 24" on center
• 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.

Unsupported Panel Edges on Roofs

Where roof panel edges land between rafters, the IRC (section R803.2.3) requires that edge to be supported. You have three ways to do it: solid blocking, tongue-and-groove panel edges, or metal H-clips set at mid-span between panels. H-clips are the cheapest and fastest of the three, which is why they dominate on site, but they are an option and not a mandate. Figure one clip per rafter space along each unsupported edge.

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 × 1.10 / 32)

Roof Sheathing Formula (Gable Roof)

Roof area = Plan Length × Plan Width × slope factor

Sheets = ceil(Roof area × 1.10 / 32)

The slope factor already accounts for both roof planes, because the plan footprint under a gable roof is covered exactly once by the two sloping surfaces. Multiplying by 2 on top of that is a common estimating error and doubles your material order.

Note the waste factor goes on the area, before rounding. Round to whole sheets first and then add waste and you round twice, which quietly buys an extra sheet on most jobs.

Worked Example: Roof

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

• Plan footprint: 40 × 28 = 1,120 sq ft
• Slope factor for 6/12: √(1 + 0.5²) = 1.118
• Roof surface: 1,120 × 1.118 = 1,252 sq ft
• Sheets with waste: ceil(1,252 × 1.10 / 32) = 44 sheets
• At $35 a sheet that is $1,540. Panel prices move a lot year to year and by region, so price your own supplier before budgeting.

Staggering Panel Joints

Stagger the panel joints so the end joints in one row land mid-panel in the next. On a 4×8 sheet run horizontally that means offsetting every other row by a full 4 feet, not by a couple of feet. Spreading the joints across more framing members is what makes the assembly act as one diaphragm rather than a set of independent strips, and braced-wall and shear-wall assemblies in the IRC specify the pattern directly.