How thick should roof decking be?
Framing spacing sets the minimum
Decking spans from one rafter or truss to the next, so thickness depends on that gap. Most modern Georgia houses use trusses at 24 inches on center. Older stick-framed roofs are often 16 inches, which allows thinner panels but rarely gets them today.
Read the stamp on the panel
Every structural panel has a grade stamp with a span rating such as 24/16. The first number is the maximum roof span in inches, the second the maximum floor span. A 24/16 panel, usually 7/16 inch, is rated for roof framing at 24 inches. A 24/0 panel, usually 3/8 inch, can span 24 inches only with edge support such as clips, which is why old 3/8 inch roofs often sag between trusses.
When to go thicker
- Heavy coverings like tile or slate.
- Framing wider than 24 inches.
- Homeowners who want a flatter looking roof plane with less waviness.
- Replacing a sagging 3/8 inch deck during a reroof.
Match thickness on repairs
When only some sheets are replaced, they should match the thickness of the surrounding deck. A thinner sheet leaves a dip and a thicker one a hump, and both show through the shingles. Older homes with 1x board decking are usually about 3/4 inch thick, and repairs there are often made with plywood of similar thickness.
For the material choice itself, see OSB vs plywood roof decking, and check roof anatomy explained for how the layers fit.