Roof Sheathing Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Sheathing sheets are sloped roof area divided by the net sheet area, plus waste. A 2,000 sq ft roof needs about 70 sheets of 4×8 plywood at 32 sq ft per sheet with 5% waste.
Your measurements
Enter the values you know — results update live.
True sloped roof area in square feet.
Standard plywood or OSB is 4 ft wide.
Standard length is 8 ft; 10 ft sheets are also common.
Analysis
Live results from your measurements.
Show the working
- Net sheet area:
4 × 8= 32 sq ft - Area with waste:
2,000 × 1.05= 2,100 sq ft - Sheets:
ceil(2,100 ÷ 32)= 66
What this calculator does
Roof sheathing is the layer of plywood or OSB that spans across the rafters or trusses and provides a nailing base for shingles, underlayment and other finish roofing. It is sold in sheets, and the count depends on the roof area, the sheet dimensions and how much waste is generated by hips, valleys and dormers. Standard 4×8 sheets cover 32 square feet, but the usable area is slightly less because panels must be spaced for expansion and edges must land on framing. The calculator rounds up to whole sheets because suppliers do not sell partial panels.
How to use it
- Enter the true sloped roof area.
- Enter the sheet width and length you plan to use.
- Choose a waste allowance based on roof complexity.
- Divide the roof area plus waste by the net sheet area and round up.
The formula
Net sheet area
net area = sheet width × sheet length
Gross area of one sheet before any expansion gaps or cuts.
Sheets required
sheets = ceil((roof area × (1 + waste)) / net sheet area)
Rounds up to whole sheets.
Worked example
A 2,400 sq ft roof sheathed with 4×8 sheets, 8% waste.
- Net sheet area: 4 × 8 = 32 sq ft
- Area with waste: 2,400 × 1.08 = 2,592 sq ft
- Sheets: 2,592 ÷ 32 = 81
Order 81 sheets of 4×8 roof sheathing.
Reference chart
| Roof area | Sheets | Roof area | Sheets |
|---|---|---|---|
| 1000 | 33 | 2500 | 83 |
| 1500 | 50 | 3000 | 99 |
| 2000 | 66 | 3500 | 115 |
Sheathing panel options
| Panel | Size | Net area | Best for |
|---|---|---|---|
| CDX plywood 4×8 | 4 × 8 ft | 32 sq ft | General residential |
| OSB 4×8 | 4 × 8 ft | 32 sq ft | Most common, cost-effective |
| OSB 4×9 | 4 × 9 ft | 36 sq ft | Longer rafter spacing |
| Plywood 5×5 | 5 × 5 ft | 25 sq ft | Metric regions, tile roofs |
Common mistakes
- Using footprint area instead of sloped roof area.
- Forgetting waste for hips, valleys and dormers.
- Not allowing for panel expansion gaps.
- Ordering exact count with no spare for damaged sheets.
- Using wall sheathing thickness on a roof; roof panels are usually 7/16 or 5/8 inch.
Assumptions and limits
- Sheets are laid with edges supported by framing.
- Expansion gaps and edge support are handled during installation.
- The roof area entered is the true sloped surface area.
- Panels are not mixed in thickness or type across the roof.
Frequently asked questions
How many sheets of plywood do I need for a roof?
Divide the sloped roof area by the net area of one sheet and add 5% to 10% waste. A 2,000 sq ft roof needs about 66 sheets of 4×8 plywood.
What thickness roof sheathing should I use?
7/16-inch OSB or 15/32-inch plywood is standard for rafters at 16 or 24 inches on centre. 5/8-inch panels are used for heavier loads or 24-inch spacing.
Should roof sheathing be OSB or plywood?
Both meet code when rated for roof sheathing. OSB is more common and less expensive. Plywood handles moisture cycles better and is preferred in very wet climates.
How much waste for roof sheathing?
5% for a simple gable, 8% to 12% for hips and valleys, and up to 15% for roofs with many dormers or odd shapes.
Do I need to leave expansion gaps in roof sheathing?
Yes. Leave a 1/8-inch gap between panel edges to allow for expansion and contraction. Panels that are butted too tightly can buckle.
Can roof sheathing be installed over existing shingles?
No. Shingles should be removed before installing new sheathing so the deck can be inspected, fastened properly, and covered with underlayment.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.