Material take-off
Roof Sheathing Calculator
Enter roof area to get the number of 4×8 sheets required, plus fasteners and cost. Includes guidance on the right thickness for your rafter spacing.
A 4×8 sheet of roof sheathing covers 32 square feet. Divide roof area by 32 and add about 10% for waste, so a 2,000 sq ft roof needs about 69 sheets. Use 7/16" OSB at 16" rafter spacing and 5/8" at 24".
Calculate sheathing sheets
Based on standard 4 × 8 ft panels covering 32 sq ft each.
Sheets needed
69
4 × 8 panels, waste included
Choosing sheathing thickness
Thickness follows rafter spacing. At 16 inches on centre, 7/16" OSB or 1/2" plywood is standard and code-compliant in most jurisdictions. At 24 inches on centre you need 5/8" to avoid deflection between rafters. A springy roof deck telegraphs through shingles and shortens their life.
Heavy roofing changes the calculation. Tile and slate weigh three to ten times what asphalt does, and both the deck and the framing beneath usually need upgrading before you can install them. That structural review is not optional and is not something a roofing calculator can answer for you.
When decking needs replacing
Tear-off is the only time anyone sees the deck. Budget for replacing 10 to 15% of it on a roof over 20 years old, and expect more around chimneys, valleys and anywhere the old roof leaked. Most contracts price deck replacement as a per-sheet extra rather than including it, precisely because nobody can know the number until the shingles come off.
Ask for that per-sheet rate in writing before work starts. It is the most common source of change-order disputes on a roofing job.