Window Leak After Wind-Driven Rain
After a wind-driven rainstorm, water stains appear on the interior drywall below the head jamb of a newly installed 1.2 m × 1.5 m vinyl slider window. The wall is 2x6 wood frame with oriented strand board sheathing, a spun-bonded weather-resistive barrier, and fiber-cement lap siding. The window was installed with a nailing flange, but the siding installer caulked the entire perimeter of the window and the flange to the WRB. At the sill, the pan flashing is flat, with no back dam, and the WRB was lapped over the sill flashing at one corner. The head flashing was installed after the siding, face-nailed through its top edge. The homeowner noticed the stain after the first storm, and the builder wants to “caulk the inside and monitor it.” The window manufacturer’s installation instructions are available but have not been followed. The climate is rainy and the wall faces the prevailing wind.
1. Diagnose the likely water-entry paths and explain why interior caulking is not a repair.
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The likely paths are at the head and sill corners, not through the drywall itself. A flat sill pan with no back dam or end dams can let water run inward, and the WRB lapped over the sill flashing at one corner creates a reverse lap that traps water. The head flashing was installed after the siding and face-nailed through its top edge, so it may not be integrated with the WRB or the window flange. Caulking the entire window perimeter to the WRB can also trap water behind the siding and prevent drainage. Interior caulking only hides the symptom; it does not stop water from entering the wall cavity, wetting sheathing and framing, or damaging finishes. The repair must correct the drainage plane and flashing sequence, not seal the interior.
2. What repair sequence would you propose?
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Remove enough siding and trim to expose the window flanges, WRB, sill pan, and head flashing. Remove and replace the flat sill pan with a sloped pan that drains outward and has a back dam and end dams. Re-establish the WRB in shingle-lap order so upper layers drain over lower layers. Install the window and integrate the flanges with flashing tape or the manufacturer’s specified method. Install head flashing in the correct layer so water drains over the window and behind the siding, not through a face-nailed top edge. Replace damaged sheathing or framing if found. Reinstall siding with proper clearances and caulking only where the manufacturer requires it, not as the primary waterproofing. Then verify with a controlled water test before closing the wall.
3. How would you verify the repair and prevent recurrence?
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Verification should include a visual inspection of each layer before it is covered: sloped sill pan, back dam, end dams, WRB laps, flashing tape, head flashing, and siding clearances. A controlled water test at the window, sprayed from the exterior while the interior is observed, can reveal remaining paths, but it should be done before final interior repairs and with a method that does not force water into unrelated assemblies. The manufacturer’s instructions should be on site and followed. The repair should be documented with photos of the flashed layers. To prevent recurrence, the crew should be retrained on the difference between a drainage plane and a caulk seal, and the builder should require flashing inspections at rough-in. The interior drywall should not be patched until the wall has been tested and any wet insulation or sheathing has been removed.