Where skylights leak, and why.
A skylight is an opening in the roof with glass in it, and nearly everything about keeping it dry happens at the edges. Step flashing is woven into the shingle courses along each side. Head flashing at the top turns water around the frame. Sill flashing at the bottom sheds it back onto the roof. When any one of those pieces fails, or was never installed right, water gets in, most often at the upper corners where snow and ice sit the longest.
We handle skylights three ways. When the unit is sound and the flashing is the problem, we reflash it with bent metal, not a bead of caulk. When the glass seal has failed, the frame is cracked or hail has damaged it, we replace the unit. And on a re-roof, we recommend replacing an aging skylight, or at least its flashing, at the same time, because a new roof around an old opening is only as watertight as its oldest part.
Not every wet skylight is a leak. In a Minnesota winter, warm, humid indoor air hits cold glass and condenses, and the water runs down the shaft exactly as if the roof were leaking. We check for that before quoting a repair you do not need.
