What is roof decking made of?
Short answer. On most Georgia homes, roof decking is OSB (oriented strand board) or plywood, usually 7/16 to 5/8 inch thick, nailed across the rafters or trusses. Older homes may have 1x6 or 1x8 board decking, sometimes spaced with gaps from an original wood shake roof, which needs a solid overlay before shingles.
The materials
- OSB. Wood strands pressed with resin into large sheets. The most common deck on homes built in recent decades. Uniform and economical, but its edges swell if they stay wet.
- Plywood. Thin wood veneers glued in cross layers. Somewhat more forgiving of getting wet and drying out, and it holds nails well. Common on older homes and in higher-end work.
- Board decking. Solid lumber boards, typical before panels took over. Often in good shape if kept dry, but gaps between boards, knots and splits can be problems for nails.
- Radiant barrier sheathing. OSB with a foil face that points into the attic to reflect heat.
How to tell what you have
Look at the underside from the attic. OSB shows a flaky, chip pattern and often a printed grade stamp. Plywood shows wood grain and layered edges. Boards are long strips running across the rafters.
Signs the deck needs work
- Sagging between rafters visible from the street or felt when walking the roof.
- Dark stains, soft spots or delaminating layers seen from the attic.
- Rust on nail points, which signals moisture.
- Spaced boards under an old roof that once held wood shakes.
On a replacement, the crew can only judge the deck fully once the old shingles are off. Contracts usually spell out how replacement sheets are priced, so ask how damaged decking will be shown to you.
Learn more in OSB vs plywood roof decking. Robust Roofing covers rotted decking in its guide on when the plywood under your shingles is the real problem.