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Leaky Skylights

Sometimes a drip is not really a leak at all

Ask This Old House Crew
Photo by Matt Kalinowski

We live in an old, crooked house on a little lake in south New Jersey. It has a sunporch with skylights that chronically leak. We've nickel-and-dimed the problem to death with umpteen handymen (we are two old ladies, only one of whom can make it up to the roof), but nothing seems to work. Can you help?

— Laine, Clayton, NJ


Tom Silva replies: Skylights often leak because their flashing is damaged, corroded, or improperly installed, but sometimes those "leaks" are the result of other, less obvious problems. For example, water can get through the roofing upslope of a skylight and then leak out around the skylight's framing. Or condensation may be collecting and then dripping off the underside of a skylight that hasn't been properly insulated against the cold.

Finding out exactly what's wrong is a process of elimination. First have a reputable roofing contractor check the condition of the roofing above your skylight and make any necessary repairs.

But if the next rainstorm brings more leaking, hire a remodeling contractor to strip away the gobs of dried, cracked roofing tar — the typical temporary, handyman fix — and peel away the roof shingles from the sides and top of the skylight. In their place, the contractor should install new metal flashing. I prefer step flashing on the sides because it interlocks with the roof shingles for a good seal. The top of the skylight should be protected with a continuous piece of head flashing that overlaps the uppermost row of the step flashing and is overlapped itself by shingles. That should stop any leaking when it rains.

But if you still have "leaks" on icy winter days, cut back the drywall in the area of the leak and you'll probably find the space between the framing and the skylight isn't adequately insulated. In this case, fill the gap with minimally expanding foam from a spray can. Fiberglass won't do the trick here.


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