Roof decking (or sheathing) is the structural wood layer — typically plywood or OSB — fastened to the rafters that all other roofing layers are installed on top of.
Decking is the structural foundation of the entire roof system — underlayment, flashing, and the visible material (shingles, metal, tile) are all installed on top of it. Decking can deteriorate from long-term leaks or moisture exposure, and rotted sections need to be cut out and replaced (typically billed separately, per sheet) during a reroof if discovered after tear-off.
It's usually only visible once the old roofing material is torn off — soft spots, discoloration, or a spongy feel when walked on are signs of decking damage, most often found near chronic leak points.
Typically $2-$5 per square foot of replacement plywood/OSB, billed as an add-on once the extent of damage is visible after tear-off.
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