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Apprentice answer

Why is reverse-lapped louver flashing a serious water problem

3rd YearRoof Curbs, Louvers & Exterior AirStop / get direction

Reverse-lapped flashing sends water behind the wall protection. Stop and correct the shingle-lap sequence so water drains out, not into the wall.

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Plain-English answer

Exterior flashing works like shingles: upper layers must drain onto lower layers. If the head flashing is placed on the wrong side of the weather barrier, water can run behind the flashing into the wall cavity.

Stop covering work, expose the head condition, and coordinate the correct lapping detail with the GC/envelope trade. Usually the top leg must be integrated behind the weather barrier or approved flashing tape system, with side/end detailing per the project envelope detail.

Ask Foreman

The louver head flashing at [location] appears reverse-lapped with the building wrap. I checked the water path and it drains behind the flashing. Do you want this held for envelope correction before cover?

Verify before acting

Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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