Water behind siding means rain or snowmelt has reached the sheathing and framing and is not draining back out. It usually comes from a gap in the flashing at a window, roof-to-wall corner, deck or trim piece, or from a weather barrier that is missing, torn or lapped backward. The damage hides under the siding, so the first signs are often swollen boards, peeling paint or an interior stain. The fix is to find and flash the entry point, then repair the barrier and any rotted sheathing. This page is general information.
On this page: Signs · Causes · Ground check · Fixes · Code basis · When to call
Signs of a wet wall
- Siding that is swollen, buckled, wavy or soft, often near the bottom of a wall.
- Paint blistering or peeling on siding and trim in one area rather than evenly across the house.
- Dark streaks or stains running down from a window corner or roof line.
- Interior signs on an exterior wall: drywall stains, bubbling paint, musty smell, or loose baseboard.
- Rot found under the siding when another repair opens the wall.
Where the water gets in
| Source | Where the damage shows |
|---|---|
| Missing kickout at a roof-to-wall corner | A stripe of damage below the bottom of a roof edge that meets a wall |
| Window or door with no sill pan or a reverse-lapped head | Below the window corners and along the sill |
| Deck ledger with no flashing | Rim joist and sill under the deck door |
| Missing cap flashing over trim | Head trim, window tops and band boards |
| Weather barrier missing, torn or lapped backward | Scattered, often worst on the stormy side |
| Siding run down onto a roof, deck or soil | Bottom courses wicking water |
Interior moisture can also wet walls from the inside. Purdue and Forest Products Lab guidance points to paint failure over bathrooms, kitchens and laundry rooms as a clue to water vapor from inside rather than rain.
Tracing the source from the ground
- Walk the house and mark every area of swelling, staining or peeling paint.
- Look directly above each spot. Is there a window, a roof edge meeting the wall, a deck, a light fixture or a horizontal trim band?
- Check for a sloped metal cap over horizontal trim and window heads, and a kickout at the bottom of each roof-to-wall line.
- Note whether the siding touches the roof, deck boards, concrete or soil.
- Inside, check the wall under each suspect spot for stains.
A contractor confirms with a moisture meter and, if needed, a small test opening.
The correct fix, and the wrong ones
| Repair | When it fits |
|---|---|
| Fix the point source: add a kickout, cap flashing, window flashing or ledger flashing | Damage is local to one detail. |
| Remove siding locally, replace wet sheathing, rebuild the weather barrier with correct laps | The barrier is torn or reverse-lapped; sheathing is soft. |
| Re-side the wall with a drained or ventilated cladding | Damage spreads across a wall, or the wall has no working barrier. |
Wrong fixes: caulking every siding seam and the bottom edges of lap siding, painting over swollen boards, or adding new siding over wet sheathing.
Code and building science
- Code. The 2021 IRC, section R703.1.1, requires a water-resistive barrier behind exterior cladding and a means of draining water that gets past the cladding. Section R703.2 requires the upper layer to lap the lower by at least 2 inches horizontally and 6 inches at vertical joints. Section R703.4 lists where flashing is required, including window and door openings, continuously above projecting wood trim, where decks attach and where walls meet roofs.
- Building science. Straube’s digest BSD-013 calls a drained-screen wall the practical approach for lap siding, says flashing must be waterproof material (not building paper or housewrap), and recommends keeping siding at least 8 inches above grade.
The single most common source is covered in our guide to missing kickout flashing. Related guides: window leaks at the install flashing and deck ledger leaks.
When to call
Call when you see swelling, rot or a stain you cannot explain, especially below a roof-to-wall corner or a window. Integrity installs siding and roofing, so we can trace a wall leak from the roof edge down, open the wall, flash it and re-side the section. The exterior problems guide covers related issues.
Have a wet wall checked
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We photograph what we find and explain the source before we quote a repair. Colorado Springs area: call (719) 488-2800 or book a free inspection. Denver metro: call (303) 209-0152 or book a free Denver inspection. Austin: call (512) 610-5897 or book a free Austin inspection. We reply the same business day.
Sources: 2021 IRC Chapter 7, sections R703.1.1, R703.2 and R703.4 (UpCodes); John Straube, BSD-013 Rain Control in Buildings (Building Science Corporation); Cassens and Feist, Paint Failure Problems and Their Cure, NCR-133 (Purdue Extension and USDA FPL).
