What is the difference between a roof deck and a floor deck?
Thickness and spans
- Roof decking: commonly 7/16 inch OSB or 1/2 inch plywood over framing spaced about 24 inches apart, sometimes thicker on steep or long spans.
- Floor decking: commonly 23/32 inch (sold as 3/4) tongue and groove panels, glued and screwed or nailed to joists.
Structural panels carry a span rating stamp such as 32/16. The first number is the maximum spacing for roof framing, the second for floor framing, which shows how much stiffer a floor needs to be.
Edge support
Floor panels use tongue and groove edges so one sheet supports the next. Roof panels between rafters often use small metal H clips at the unsupported edges, and they are gapped slightly so they can expand without buckling.
Moisture
Roof decking lives under the shingles in a hot attic and gets wet whenever a leak or condensation hits it. That is why roof sheathing rot is common in poorly ventilated Georgia attics. See OSB vs plywood roof decking.
On commercial buildings
The terms also describe steel: corrugated steel roof deck carries the roofing and insulation, while composite floor deck is steel that gets concrete poured on top. Homeowners rarely deal with these.
Why it matters to you
During a reroof, rotten roof decking is replaced sheet by sheet, and estimates list a per sheet price. Roof anatomy explained shows where the deck sits in the roof system.