What thickness of plywood is used for roof decking?
Nominal versus actual thickness
Plywood is sold by a rounded name, but the stamp shows the real thickness:
- Sold as 1/2 inch: actually 15/32 inch.
- Sold as 5/8 inch: actually 19/32 inch.
- Sold as 3/4 inch: actually 23/32 inch.
That small difference matters when patching, since a new sheet should sit level with its neighbors.
Reading the stamp
Each sheet carries a span rating, such as 32/16 on many 15/32 inch panels or 40/20 on many 19/32 inch panels. The first number is the maximum roof support spacing in inches. As long as it meets or exceeds your rafter or truss spacing, the panel satisfies the IRC table for that span. The stamp also shows the exposure rating, typically Exposure 1, meaning it can handle weather during construction.
Grade
CDX is the standard roof sheathing grade: C and D face veneers with exterior-type glue. Its faces are rough, which does not matter under roofing.
When to go thicker
- Heavy roofing such as tile or slate.
- Framing spaced wider than 24 inches.
- A flatter, stiffer roof plane that shows fewer waves.
- Better hold for metal roof screws and clips.
Plywood next to OSB
Many Georgia roofs mix plywood and 7/16 inch OSB after past repairs. The 1/32 inch difference between 15/32 plywood and 7/16 OSB is minor and does not show through shingles.