Roof sheathing (roof decking) is the layer of plywood or OSB boards fastened to the rafters that the roof is installed on. Plywood holds up a little better to moisture; OSB is cheaper and very consistent. Soft, sagging or water-stained sheathing should be replaced during a roof replacement, because installing over damaged decking can void manufacturer warranties.
Key takeaways
- Sheathing is the structural base every roof is nailed to.
- Plywood handles moisture a bit better; OSB is cheaper and uniform.
- Rotted decking is usually only found after the old roof is off.
- Installing over damaged sheathing can void your roof warranty.
In this guide
What roof sheathing does
- Gives the roof a solid, flat surface for underlayment and shingles or membrane
- Holds the nails and fasteners that keep your roof on in high wind
- Stiffens the roof structure, tying rafters together
- Spreads loads like snow and foot traffic
Common types of roof sheathing
Plywood
Thin wood layers glued in alternating directions. Strong, holds nails well and dries out better if it gets wet.
OSB (oriented strand board)
Wood strands pressed with resin. Uniform, strong and less expensive. The most common decking on newer homes.
Plank sheathing
Solid boards with gaps, found on many older NYC houses and row homes. Often needs to be overlaid with plywood for modern roofing.
Sheathing thickness
Roof sheathing is commonly 7/16" to 5/8" thick, depending on rafter spacing, roof load and local code. Thicker decking is stiffer and holds fasteners better. Your roofer should match or upgrade what's there and follow manufacturer requirements.
Signs your roof sheathing is damaged
- Sagging between rafters or a wavy roof line
- A soft, spongy feel when walking the roof
- Water stains, dark spots or mold on the underside, visible from the attic
- Daylight through the boards
- Nails backing out or shingles that won't stay put
Why sheathing matters when you replace your roof
A new roof is only as good as the deck under it. Installing over rotted or delaminated sheathing leads to sagging, nail pops and leaks, and manufacturers can deny warranty claims when their products are installed over an unsound deck.
That's why a proper roof replacement includes a full tear-off and a board-by-board inspection of the deck. Damaged sections are replaced before any new material goes down. Because you can't see the deck until the old roof is off, a good estimate should explain how deck repairs are priced if they're needed.
Learn what goes on top of the sheathing in types of asphalt shingles, or read the full NYC roof replacement guide.
Frequently asked questions
What is roof sheathing?
The flat panels, usually plywood or OSB, nailed to the roof rafters. It's the base for underlayment and shingles or membrane, and it ties the roof structure together.
Is plywood or OSB better for roof sheathing?
Both are code-approved. Plywood tends to recover better after getting wet and holds nails well; OSB is more uniform and less expensive. Proper installation and keeping the deck dry matter more than the choice.
How do I know if my roof sheathing is bad?
Sagging between rafters, a spongy feel underfoot, water stains, dark spots or mold on the underside in the attic, and nails that won't hold are all signs.
Do you always replace sheathing during a roof replacement?
No, only the boards that are damaged. After tear-off we inspect the whole deck and replace soft, rotted or delaminated sections before the new roof goes on.
