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Is this leaking mitered cap corner a seam problem or hidden flashing?
I’m trying to diagnose a leak at the lower corner of an aluminum-capped window before I start removing siding. Indoors, the water shows up on the wall just below the window’s lower corner, but only during fairly strong, wind-driven rain. I do not see water along the rest of the sill or at the opposite corner.
Outside, the aluminum cap has a mitered corner. The seam looks slightly open at the outer edge, and the two pieces do not appear to overlap by much. There is also a narrow gap between the cap and the adjacent vinyl siding. I’m unsure whether this points to an exposed miter seam, failed sealant, an incorrect corner overlap, or damage to the flashing behind the cap.
For those who have diagnosed this type of leak, what measurements or photos would be most useful? I can photograph the full window, a close-up of both cap seams, the corner overlap from below and straight on, and the nearby siding joints. I can also measure the seam opening and note whether the water appears at the interior drywall, stool, or jamb.
Would you treat a localized miter repair as reasonable if the flashing and sill area appear intact, or is water at the interior lower corner enough reason to remove several siding courses and inspect behind the cap? I’m trying to avoid simply adding sealant over a symptom, while also avoiding unnecessary siding removal.
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2 replies
Before removing siding, do a controlled hose test. Start at the bottom of the window and wet only a small area for several minutes, then work upward one section at a time. Keep the hose on a gentle flow, not a pressure-jet, and have someone watch the interior corner.
A slightly open miter seam is certainly suspect, but water appearing below one lower corner doesn’t prove that’s the entry point. Water can get in at a higher siding joint, the window’s upper or side flashing, or behind the cap, then travel down until it finds the first opening. If the leak appears only after wetting the miter itself, a localized repair may be reasonable. If it starts when testing above or beside the window, sealing the visible seam could just hide the symptom.
Photograph or mark the first test area that produces water, and allow the wall to dry between sections. Also avoid sealing the narrow gap at the siding blindly, since that may be part of the drainage path.
Record the timing as well as the test area. Note when each section starts getting wet and when interior moisture or staining first appears, with photos of both. Mark the first wet location inside and outside the wall.
If moisture shows up only when the miter is wetted, that supports treating the seam as a localized repair target. If the interior corner gets wet while testing above or beside the window, or only after a delay, water may be traveling behind the cap or through the wall assembly. Let each section dry before moving upward so the results stay distinguishable.