What size roof sheathing should be used, especially when replacing a few sheets?
Short answer. Roof sheathing comes in 4 by 8 foot panels, and on most Georgia homes with rafters or trusses 24 inches apart it should be at least 7/16 inch OSB or 15/32 inch plywood, with edge clips between supports. When only some sheets are replaced during a reroof, match the thickness of the existing deck so the new roof lies flat.
Picking thickness
- 24 inch spacing: 7/16 inch OSB or 15/32 inch plywood, sold as 1/2 inch, with a span rating such as 24/16, is the common minimum, with H-clips at unsupported edges.
- 16 inch spacing: thinner rated panels may be allowed, but many builders still use 7/16 or 1/2 inch.
- Heavier roofing or a stiffer deck: 19/32 inch, sold as 5/8, or thicker.
- Span ratings are stamped on each panel; 24/16 means 24 inches between roof supports or 16 for floors.
Replacing sheets during a reroof
- Match the existing thickness. A thinner patch leaves a dip and a thicker one leaves a hump, and both show through new shingles.
- Older homes. Some houses have 3/8 inch decking or plank boards. Replacement sheets may need shimming, or the deck may need an overlay, which your roofer should explain.
- Cut to the framing. New pieces should land on rafters or trusses at both ends, not float between them.
- Gap the panels. Leave about 1/8 inch at edges and ends, as panel makers recommend, to prevent buckling.
- Mixing materials. OSB and plywood are often mixed on the same roof, which is fine when the thickness matches.
What to ask
Ask which thickness the contract's per-sheet price covers and how mismatched decking will be handled. See OSB vs plywood roof decking and roofing over existing shingles.