Direct Answer
Roof decking (also called sheathing) is the flat wood panels — OSB or plywood — nailed across the rafters that form the solid base your shingles are attached to. It's the structural layer between the framing and the roofing material. When decking rots from leaks or poor ventilation, it goes soft and has to be replaced so the roof has a solid base.
Decking is the foundation of the roof system. Shingles, underlayment, and flashing all attach to it. If the decking is soft or rotten, nothing above it is properly supported.
What Decking Does
- Provides the solid surface shingles and underlayment fasten to
- Ties the roof structure together across the rafters
- Carries the load of the roofing materials and any snow or wind
Common Decking Problems
- Rot — from long-standing leaks soaking the wood from above
- Condensation rot — poor ventilation condenses moisture on the underside
- Sagging — rotten or undersized decking sags between rafters
- Delamination — plywood layers separate from moisture
Signs & Symptoms
- Soft or spongy spots when walking the roof
- Sagging between rafters visible from inside the attic
- Dark stains or mold on the underside of the deck
- Shingles that won't lie flat over soft spots
Repair Options
- Replace rotten sheets of decking (per sheet, as found during tear-off)
- Fix the leak or ventilation issue causing the rot
- Full replacement if rot is widespread
When to Call a Roofer
Soft spots, attic stains, or sagging mean the decking is compromised. Get an inspection — and during any roof replacement, every sheet should be checked.
How Beyond the Ladder Handles It
We inspect every sheet of decking during tear-off and replace only what's actually bad — and we fix the moisture source so the new wood stays solid.
Related Questions
Is decking the same as sheathing?
Yes. “Decking” and “sheathing” refer to the same thing — the OSB or plywood panels under your shingles. The terms are used interchangeably.
How do I know if my decking is bad?
Soft spots when walking the roof, sagging between rafters, and dark stains or mold on the underside in the attic are the signs. A full inspection during tear-off is the definitive check.