Question on skylight installation plus new roof on an old roof

If you are putting in a new roof on top of an old roof, and you also want to put in a skylight, does the flashing of the skylight just rest on the old roof, then put the new roof on top of the flashing, or should the entire flashing go under the old roof/paper and all? The reason why I ask is I have been trying to troubleshoot a chronic leak in in one of my skylights. And today I removed part of the sheetrock on the side and discovered that whoever did this job some years ago just rested the skylight on the old roof, then installed the new roof on top of the flashing. Something tells me this is not right and the water is finding its way through the old roof from another penetration up higher and working its way down to the skylight. The actual flashing when viewed from the top looks fine. I can't find any thing that jumps out at me that indicates a problem.
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The skylight should be flashed for the new roof. I highly recommend a curb-mounted, properly step-flashed type of skylight, not the "self- flashing" type, The self flashing kind relies only upon tar and roof pitch to keep water from penetrating into the structure. Bad juju. Tom
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Then the only true way to fix it is to strip everything down and get a new roof, right? I'm guessing this roof is 15-16 years old. I've been here four years.
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No, not the only true way. Of which type of skylight are we speaking? The way you worded your original question, it's the self-flashing type, right? If you're bound and determined to use one of these, it can be attached _through_ the old roof shingles into the decking after you cut the rough opening. New shingles are laid up to and _under_ the bottommost flashing on the new skylight, then are laid on _top_ of the flashing on each side of the skylight with a _good_ vertical bead of tar between each course of shingles, a couple of inches out from the sides of the cut shingles. Make sure the cut shingles leave enough room for a good downpour to travel along the sides of the skylight, maybe a half-inch plus. When you reach the top of the skylight you'll want something like ice and water shield laid about a couple of feet above, and a foot or so to the outsides (depending on the pitch) and lapped out onto the top of the last course of shingles on the skylight. Again, I don't recommend this type of installation, as it will leak. A curb-mounted type will require tearing off down to the deck, but only in the immediate vicinity of the curb installation, filling in the underlying roof with old or new shingles around the new curb, then roofed, stepflashed and counterflashed as normal. Make sure your roofer also gets new plumbing vent flashings and chimney flashing, as they must work for your_new_ roof, not the old roof. HTH. Tom
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I would replace the entire roof, no doubt he cut other corners
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Mikepier wrote:

The new construction flashing should rest on the roof decking. The tarpaper and shingles (all of them) should be on top of the flashing.
Flashing on top of the old shingles still leaves water a way to get under the flashing. That's likely where your leak comes from.
--
Steve Bell
New Life Home Improvement
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If you're thinking of putting in a skylight, make sure you swing by Wal-Mart first. Pick up a few plastic buckets to keep in the closet in the room with the skylight. That way, you can set them out quickly when it rains so your carpet won't get all wet.
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You better keep an eye out for more of that guy's work. -----
- gpsman
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