Window and Roof-Wall Leaks in New Fiber-Cement Siding
A two-story house is being sided with fiber-cement lap siding over 7/16 in. OSB sheathing and a spun-bonded housewrap. The windows are vinyl flanged units, 3 ft 0 in. by 5 ft 0 in., nailed through the nailing fin directly over the housewrap; no sill pan or back dam was installed. The housewrap laps 6 in. at horizontal seams and 12 in. at vertical seams, but the siding crew butted the fiber-cement planks tight and caulked every butt joint. At a lower roof-to-wall intersection, step flashing was woven into the roof shingles, but no kickout flashing was installed at the eave, and the siding was brought down to within ½ in. of the roof surface. Two months after completion, the owner reports dark stains and soft spots on the sheathing at the lower corners of two windows and at the roof-wall corner. The interior drywall shows no staining yet.
1. Identify the most likely water-entry paths at the windows and explain why the absence of a sill pan and back dam matters, even with a flanged window and caulked siding.
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Water on the wall first wets the siding, then runs down the back of the siding to the housewrap. The housewrap is the drainage plane, so every penetration must be shingled so water on the wrap discharges to the exterior, not into the wall. At these windows, the flanged units were nailed directly over the housewrap with no sill pan or back dam. That means water that gets behind the siding and runs down the wrap reaches the rough opening at the sill, where there is nothing to stop it from turning inward onto the sheathing and into the framed opening. The caulked butt joints make it worse: they may keep some water out initially, but they also trap water that gets behind the siding and prevent drying, so the lower corners stay wet. A proper detail would have a sloped sill pan with a back dam, end dams, jamb flashing, and a head flashing lapped under the housewrap above; the housewrap and flashing must be sequenced so water on the wrap goes over the flashing and out. The soft sheathing at the lower corners is consistent with this path.
2. Evaluate the roof-wall detail and specify the correct flashing sequence and clearance for the siding. Include what should happen at the kickout and the bottom edge of siding.
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At the roof-wall intersection, the step flashing is necessary but not sufficient. The missing kickout at the eave is a likely entry point: water running down the roof and wall converges at the roof edge, and without a kickout it can run off the roof edge and behind the siding and housewrap at the wall corner. Bringing the siding down to within ½ in. of the roof surface also violates the principle of clearance: the siding should be held at least 1 in., and commonly 1–2 in., above the roof surface so it does not wick water and so the flashing can drain. The correct sequence is roof underlayment, step flashing over the underlayment and lapped behind the wall’s drainage plane, kickout flashing at the bottom to divert water out past the wall, then the housewrap and siding above with the required clearance. Any caulk at the siding-to-roof joint is a sealant, not a flashing, and should not be relied on. The repair should remove the lower courses of siding, install the kickout and proper step flashing, verify the WRB laps over the flashing vertical leg, and maintain the clearance.
3. Recommend a repair sequence, including moisture verification, before reinstalling siding and trim.
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Repair sequence: (1) protect the area and remove siding and trim at the two windows and the roof-wall corner to expose the sheathing; (2) probe the soft spots and cut back all decayed or delaminated sheathing to sound material-do not cover soft OSB; (3) dry the framing and check moisture with a meter, requiring readings near 15–19% or at least stable and below the threshold for the finish system; (4) replace the sheathing and install a proper window sill pan with back dam and end dams, jamb flashing, and head flashing, integrating the housewrap so upper laps drain over lower flashing; (5) at the roof-wall, install kickout flashing and correct step flashing, and hold the siding 1–2 in. above the roof; (6) reinstate the WRB with correct laps, then re-side, leaving the required clearances and using sealant only at intended joints. Do not simply caulk and paint the stains. Verify the repair with a water test if possible, and check the interior after the next rain.