How is roof decking built, and what does the building code require?
Short answer. Roof decking is built from wood structural panels, usually OSB or plywood, laid across the rafters or trusses with the long side perpendicular to the framing, joints staggered, small gaps between panels, and nails at code spacing. The residential code ties panel thickness and span rating to your framing spacing and sets the nailing schedule.
Choosing the panel
- Span rating: panels are stamped with a rating like 24/16, meaning they can span 24 inch roof framing. The rating must match or exceed your rafter or truss spacing.
- Thickness: 7/16 inch OSB and 1/2 inch plywood are common on 24 inch framing. Thicker panels feel stiffer underfoot and hold nails better.
- OSB or plywood: both are allowed when properly rated; compare them in OSB vs plywood roof decking.
Laying it out
- Start at the eave with full panels, long dimension running across the rafters.
- Stagger end joints between rows so seams do not line up.
- Land panel ends on framing.
- Leave about 1/8 inch between panel edges and ends so they can expand in Georgia humidity without buckling.
- Use panel edge clips between rafters where the panel's rating calls for edge support.
Nailing
Code tables commonly call for 8d nails spaced about 6 inches along panel edges and 12 inches along intermediate framing, with tighter spacing near roof edges in higher wind zones. Ring shank nails hold better against uplift. Your local building department and the panel manufacturer set the exact schedule.
On a reroof
Replacement sheets should match the existing thickness so the roof stays flat. Old plank decking with wide gaps may need new sheathing over it before shingles go on. A permit and inspection may apply; see roofing permits in Georgia.