· by Michael Blair, Nashville's Roofers
Every roof replacement estimate worth having includes a line for decking, usually priced per sheet with a note that the final count depends on what is found. Homeowners read that as an open door. It is the opposite: it is the contractor telling you in advance that the one part of the roof they cannot inspect until tear-off may need work, and what each piece of it will cost if it does. Understanding what decking is and why it fails here makes that line easy to check.
What is under the shingles
Decking, also called sheathing, is the layer the shingles are nailed to. On houses built since roughly the 1980s it is plywood or OSB panels. On older Nashville homes it is often solid boards, sometimes with gaps between them, and some of the oldest still have the spaced skip sheathing that was laid for wood shingles a century ago.
Shingles hold the water off; decking holds the shingles on. A nail driven into rotten or delaminated decking does not hold, and a roof with poor nail holding is the one that loses shingles in the next straight-line wind event. That is why a roofer will not nail a new roof over decking they know is bad, and why insurance adjusters expect to see it replaced where it is.
Why it fails in Middle Tennessee
Slow leaks are the obvious cause: a failed pipe boot or a valley that has been weeping for years soaks the decking under it long before a stain shows on a ceiling. The less obvious cause is the attic itself. A poorly ventilated attic in our humidity runs damp all summer, and OSB in particular swells, softens and delaminates at the edges when it stays wet from below.
Older boards fail differently. Solid decking dries out, splits and loses its grip on nails, and gaps between boards leave nothing for a nail to bite. Skip sheathing cannot take asphalt shingles at all and has to be covered with new panels regardless of its condition.
How the number should be presented
A fair estimate states a per-sheet price for replacement and either includes a small allowance or says clearly that decking is billed as found. What it should not do is hide decking inside a lump sum, or promise there will be none; nobody knows that from the ground or from the attic.
Ask to be shown the bad decking before it is replaced. A good crew photographs each sheet with the location, and the photos go on the invoice. On an insurance claim those photos are what gets the decking paid for, since most policies cover decking damaged by the covered event but treat rot from long-term leaks or ventilation as maintenance.
- Per-sheet price stated in writing before work starts
- Photos of every replaced sheet, with its location
- Same thickness and type as the existing decking, or better
- Panel clips or blocking where the code and the span call for them
When most of it is bad
On a house with widespread rot, a bad attic ventilation history or skip sheathing, replacing every sheet is sometimes the cheaper and better job than patching, because the crew works once across a clean deck instead of stopping at every soft spot. The estimate should be rewritten as a full re-deck at that point rather than run up sheet by sheet.
Whatever the count, fix what caused it. New decking over an attic that still cannot breathe will be soft again in a decade. Ventilation, the failed flashing, the bathroom fan venting into the attic: these belong on the same job, and a roofer who does not mention them has not looked.