Roof Decking Calculator: How Many Sheets of OSB Do You Need? (2026)

Roof decking (also called sheathing) is the layer of OSB or plywood your shingles are nailed to. You can't see it until the old roof comes off — which is exactly why it causes so many surprise change orders. On a typical 20-to-30-year-old roof, crews end up replacing somewhere between 5% and 30% of it.

The sheet math

• One 4x8 sheet covers 32 sq ft
• Sheets needed = area to replace ÷ 32, plus about 10% for cuts
• A 2,000 sq ft roof with 25% bad decking needs roughly 18 sheets

What decking costs in 2026

• 7/16" OSB: about $20–$25 per sheet (the most common choice)
• 5/8" OSB: $30–$35
• 1/2" CDX plywood: $35–$42
• 5/8" CDX plywood: $42–$50
• Installed, most roofers charge $70–$120 per sheet including labor and fasteners

How to protect yourself on the quote

Ask for a per-sheet replacement price in writing before work begins, and ask the crew to photograph bad sections before they cover them up. A fair contract says something like "decking replaced as needed at $85 per sheet" rather than leaving it open-ended.

OSB or plywood?

OSB is cheaper and perfectly fine for most asphalt roofs. Plywood handles moisture a bit better and holds nails well, so it's worth the upgrade in humid climates or under heavy tile.

FAQ

How do I know if my roof decking is bad?

Soft spots when you walk the roof, sagging between rafters, dark stains in the attic, or daylight around nails all point to damaged decking.

Can shingles go over old decking?

Only if it's solid and dry. Nailing new shingles into soft decking voids most manufacturer warranties.