Direct Answer
Roof decking rots from moisture — either a long-standing leak soaking the wood from above, or poor attic ventilation condensing moisture on the underside of the deck. The fix is replacing the rotten wood and eliminating the moisture source, or the new wood rots too. Rot is always a moisture problem.
Wood rots when it stays wet. In a roof, that moisture comes from two directions: from above through leaks, or from below through condensation. Identifying which one is causing the rot is essential — replacing the wood without fixing the source guarantees the rot returns.
Causes of Decking Rot
- Roof leaks — a long-standing leak soaks the decking from above (failed flashing, pipe boot, or shingles)
- Poor ventilation — warm moist attic air condenses on the cold underside of the deck in winter
- Failed flashing — water enters at penetrations and saturates the wood
- Ice dams — backed-up water soaks the deck edge
- Exhaust vented into the attic — bathroom or dryer vents dumping moist air under the deck
Signs & Symptoms
- Soft or spongy spots when walking the roof
- Dark stains or mold on the underside of the deck in the attic
- Sagging between rafters
- Shingles that won't lie flat over soft spots
Repair Options
- Replace rotten sheets of decking
- Fix the leak causing the rot (flashing, pipe boot, shingles)
- Improve attic ventilation to stop condensation
- Re-route any exhaust vents that dump moisture into the attic
When to Call a Roofer
Soft spots, attic stains, or sagging mean the decking is rotting. Get it inspected — and during any replacement we check every sheet.
Local Considerations — Upstate SC & Western NC
Carolina humidity makes attic ventilation critical. A poorly ventilated attic in Upstate SC and Western NC condenses moisture on the deck underside and rots it from below — even without a leak. We check ventilation as part of any rot diagnosis.
How Beyond the Ladder Handles It
We replace rotten decking and fix the moisture source — leak or ventilation — so the new wood stays solid.
Related Questions
Can rotten decking be repaired without replacing the roof?
In small accessible areas, yes. But it's far easier and cheaper to replace decking during a full tear-off when the roof is already open.
Will rotten decking come back after repair?
Only if the moisture source isn't fixed. We always address the leak or ventilation issue causing the rot, or the new wood rots too.