Missing Head Flashings and Sill Pans on New Flanged Windows
On a new two-story house in a rainy climate, the crew has installed 12 flanged vinyl windows in 2x6 walls with 7/16 in. OSB sheathing, a spun-bonded housewrap lapped 6 in., and fiber-cement lap siding. The windows were set after the housewrap, flanges were nailed through the housewrap to framing at 12 in. on center, and low-expansion foam was applied from the interior. Sill pans were installed on only four windows; head flashings were omitted because the flanges were sealed with sealant. The building inspector has flagged the installation before siding is complete. At one 60 in. by 72 in. window, the housewrap seam above the window laps over the top flange, and the side flanges are not taped. The crew argues that the sealant and foam make the system watertight. The general contractor wants a repair plan that does not require replacing all 12 windows.
1. Evaluate the water-management errors and explain the correct layering sequence around a flanged window in this wall assembly.
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The installation is treating sealant and foam as the waterproofing, but the system depends on shingled drainage. The omitted head flashings are a primary error: the head flashing should go over the top flange and under the weather-resistive barrier above, so water draining down the wall passes over the window rather than behind it. The missing sill pans on eight windows mean water that reaches the sill has no sloped, end-dammed path to the exterior. Side flanges should be taped or otherwise integrated with the barrier so water on the sides does not enter the framing. The barrier above the window should lap over the head flashing, not directly over the top flange. Interior foam does not substitute for exterior flashing, and foam at the sill can block drainage if it fills the rough opening. Correct sequence is sill pan integrated with the barrier, window set and shimmed, side flanges integrated, head flashing over the top flange, barrier above lapped over the head flashing, then siding.
2. What repair sequence would you require before siding is installed, without replacing all 12 windows?
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The general contractor can keep the windows if the units are undamaged and the flanges are not crushed. For each window, remove the siding and barrier locally to expose the opening. Remove and reset the affected unit only where a sill pan must be installed; install a sloped pan with end dams and a back dam, integrated with the barrier at the sides. Tape or otherwise integrate the side flanges. Install a head flashing over the top flange and extend it beyond the opening, then lap the barrier above over the head flashing. Replace any wet or damaged sheathing before closing. Reinstall the barrier in shingle fashion and reinstall siding. For windows with only head-flashing omissions, the unit may not need removal, but the flashing and barrier still must be corrected. Verify fastener spacing and shim condition during the reset, and water-test a representative window before final close-out.
3. The crew also asks about anchorage and foam. What should you check, and what should they stop doing?
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Check that flange fasteners penetrate framing, not just sheathing or foam, and that spacing matches the manufacturer instructions. The reported 12 in. on center may be acceptable for some units, but corners and any mullion or operating hardware usually need tighter spacing. Fasteners should be corrosion-resistant, driven straight, and not overdriven through the flange. Shim at the jambs and sill as required so the frame is not distorted; do not use shims to bridge a twisted frame. Stop relying on sealant as a substitute for flashing. Low-expansion foam is acceptable for interior air sealing only if it does not distort the frame or block exterior drainage. Keep the sill weep path clear, use backer rod where joint depth requires it, and follow the unit manufacturer instructions for sealant type and placement.