Choose your roof geometry
Use measured surface, horizontal run plus pitch, symmetrical gable, or direct known roof area.
Roofing
Roof Sheathing Calculator
Estimate roof-deck area and whole plywood or OSB panel quantities. Combine multiple roof sections, subtract actual surface-area openings, apply a user-selected allowance, enter the panel dimensions you plan to buy, and optionally estimate material cost.
Quantity estimator only.This calculator estimates theoretical sheathing coverage from dimensions you provide. It does not select panel thickness, grade, span rating, orientation, fastening, clips, blocking, structural capacity, snow or wind design, code compliance, or whether existing roof decking is suitable.
How to use it
Estimate roof sheathing in four steps
Combine roof sections
Add repeated groups and subtract actual roof-surface openings only when appropriate.
Set your allowance
Choose the extra percentage that matches your own cutting and project conditions.
Enter the panel size
Use the dimensions of the plywood or OSB panels you actually plan to purchase.
Roof sheathing formulas
Measured surfaceArea = roof length × measured slope length
Horizontal run + pitchArea = length × horizontal run × √(1 + (rise ÷ 12)²)
Symmetrical gableArea = length × full horizontal span × pitch factor
Direct known areaArea = entered roof-surface area
Repeated sectionNet group = (gross area − excluded area) × quantity
AllowanceRequired area = base area × (1 + allowance ÷ 100)
Panel coveragePanel area = panel length × panel width
Whole panelsPanels = ceiling(total required area ÷ panel area)
Roof sheathing example
A symmetrical gable that is 40 ft long, 24 ft across horizontally, and pitched 6:12 has about 1,073.313 ft² of roof-deck surface. With a 10% allowance, the requirement becomes about 1,180.644 ft². If the selected panels measure 4 ft × 8 ft, each theoretically covers 32 ft². The project requires about 36.895 panels and therefore rounds to 37 whole panels, providing 1,184 ft² of theoretical coverage.
Roof sheathing calculator FAQs
Does symmetrical gable mode include both roof sides?
Yes. Enter the full horizontal span from eave to eave. The formula already represents both equal roof planes.
Can I enter a roof area I already know?
Yes. Direct-area mode accepts an actual roof-surface area, which is useful when the area is already known from plans, field measurements, or another calculation.
Does the calculator assume 4 × 8 panels?
No. Enter the actual panel dimensions for the product you plan to buy. A 4 × 8 panel is used only in the example.
Does the panel count optimize the cutting layout?
No. Panel count is based on total theoretical coverage. Real installations depend on panel orientation, support, seams, cuts, stagger, roof shape, and jobsite conditions.
Should I subtract skylights or chimney openings?
You may enter an actual roof-surface exclusion per repeated section. The exclusion must be smaller than that section's gross area. Small openings may not reduce real purchasing by their full theoretical area.
Does this calculator choose plywood or OSB thickness?
No. Thickness, grade, span rating, orientation, fastening, support, structural design, and code requirements are outside this quantity estimator.
Can I use Metric measurements?
Yes. Roof lengths switch between feet and meters, areas switch between square feet and square meters, and panel dimensions can be entered in millimeters, centimeters, or meters.
Why are panels rounded only once?
The calculator first combines the complete required roof-deck area, then divides by panel coverage and rounds the final project quantity upward. It does not separately round every roof group.
Can I save or share an estimate?
Yes. After a successful calculation, the current project can be represented in the page URL when the resulting URL remains within a conservative size limit.
Reference methodology
Roof-pitch adjustment is calculated directly from right-triangle geometry instead of a rounded lookup table. Unit conversion uses the international foot relationship of exactly 0.3048 meter. Panel dimensions are always user supplied, and whole-panel rounding occurs only after the complete project requirement has been combined.
Assumptions and limitations
- Measured-surface dimensions are actual dimensions along the roof plane.
- Pitched-run mode uses horizontal run, not sloped rafter length.
- Symmetrical-gable mode assumes two equal roof planes with the same pitch and uses the full horizontal span.
- Direct-area mode expects actual roof-surface area, not horizontal building footprint.
- Excluded area is actual roof-surface area per repeated section and must be smaller than that section's gross area.
- The allowance percentage is user supplied and is applied after exact net section areas are combined.
- Panel dimensions are user supplied; no panel size is treated as a universal default.
- Whole panels are rounded only once at the final project level.
- Purchased coverage and surplus are theoretical area values, not a panel-layout optimization.
- Material cost includes only the entered whole-panel price and excludes tax, delivery, labor, tear-off, disposal, fasteners, clips, blocking, and other materials.
- This calculator does not determine panel thickness, grade, span rating, orientation, fastening schedule, rafter or truss spacing, loads, or code compliance.
- This tool estimates quantity only and does not determine roof safety, structural adequacy, or product suitability.