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Just bought the house and I don't really know anything about roof work. After some stormy days we noticed a ceiling water spot. A vent flashing is the most likely culprit far as I can tell; there are other vents but they don't appear to have any issues. The problem one was not nailed or caulked to anything; it can wiggle freely with hand force. The collar part was slightly inverted around the pipe, allowing water to pool right at the seam, whereas things I've read indicate that area should be caulked too. Is there a circumstance that would make it right to leave the whole thing unsecured, or did someone screw up?

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That was installed wrong. The metal part of the boot should definitely be nailed down. Since it is not nailed, the wind is free to blow it up. When it moves up, the rubber seal can become inverted or otherwise deformed. Repeated movement will end up prematurely cracking the rubber, and of course when the wind blows it up, water will get blown underneath it.

The rubber boot shouldn't need to be caulked if everything is secure. Normally you'd see a few nails in the metal section, and those nail holes would be caulked. There should also be a few nails covered up by the course of overlapping shingles.

Also, it's unclear from your photo, but the lower edge of the metal should not extend past the shingle that it is on top of. if it does, then that row of shingles should have overlapped the metal. When the metal overhangs the shingle, it gives wind and water more of a chance to lift and penetrate the boot.

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    I'd nail it once on each side, under the overlapping shingles, and call it good. The layering of the bottom half doesn't concern me much.
    – isherwood
    Commented Nov 27, 2017 at 21:24
  • I agree with @isherwood. You could put a strip of roofing tar under the lower edge of the metal, so it will adhere to the shingle below, much like shingles do to each other. You do want to fix the inverted boot. If it were me, then I'd put a "permaboot" over the whole thing. (search for that brand. they sell them at some big box stores.) Those rubber boots will all eventually fail and crack from solar/uv, and the permaboot over it will prevent that as well act as a backup.
    – Tim B
    Commented Nov 27, 2017 at 22:08
  • Thanks all. I wound up just replacing the whole flashing because the collar seemed like it was distorted or looser than it should be, and the stores around here didn't have the replacement/supplemental collars on hand as a replacement. Plus a lot of what I was reading advised a caulk layer under the flashing so I wanted to be able to put that in place.
    – Nathan Knutson
    Commented Nov 28, 2017 at 1:36

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