Last fall, my company was asked to investigate a roof leak originating from a cupola atop an attached carriage-house garage. From the exterior, nothing appeared to be wrong with the cupola or the surrounding roof. But the inside was a different story. According to the homeowners, there had been gradual, periodic leaking around the interior light well since the home’s construction. After a particularly bad storm last summer, the water staining worsened, prompting them to remove the damaged drywall—exposing open-cell-foam-filled stud bays and water damage to the framed enclosure. That’s when they called us.

The house was fairly new, approximately 10 years old. Above the garage was a guest bedroom with a cathedral ceiling that reflected the roof’s four valleys. At the peak, the cupola, with its four awning windows, served as a light well. On our initial walk-through, the clients informed us of the leaky cupola’s history. The original builder had...

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