I have a PVC furnace exhaust pipe coming out through my roof. Unfortunately, the guy who installed it used the wrong size flange. So instead of the rubber rising upwards against the pipe, it is sunken downwards, and water puddles in the depression and eventually drips inside. I tried using roof sealant caulk, but it didn't do the job. Is there some kind of thick, sticky sealant that I could drop in there to solve the leak?

roof image

  • I have used this stuff called Black Jack in the past. You just slather it on with a trowel. Use tons of it. It comes in a can at Lowes. Have you tried that/ Nov 19, 2012 at 21:20
  • @0A0D - that looks like just what I need, thanks!
    – kurkevan
    Nov 19, 2012 at 22:33
  • 2
    Can you gently run a wire hook (welding rod with a 3/8-1/2" L bent into the end) under the rubber boot and pop it up into the proper position? Looks like the installer pushed the vent up through the boot, found it was too high and pulled it back down, inverting the boot. As long as you have that dish, water will always collect and drain in. -- Sorry, read the note below. On newer installs, this could be done. Nov 19, 2012 at 23:54
  • 100% silicon only! All the rest of caulking is junk. Roof cement is cement, not sealant, 5yrs (or less) it will leak again. Silicon is only one guaranteed 50yrs.
    – Randy
    Aug 18, 2013 at 14:13
  • What is the purpose of the pipe (is incidental water entry into the pipe OK?)
    – Bryce
    Nov 23, 2013 at 23:15

4 Answers 4


Sounds like you should just get the right sized pipe boot rather than trying to glue the wrong sized pieces together.

  • yeah, but that would require an hour or two of work on the roof, which I'm not keen about doing.
    – kurkevan
    Nov 19, 2012 at 20:36
  • And, I would have to cut the pipe in order to switch the hardware... much easier to just put in some good sealing material!
    – kurkevan
    Nov 19, 2012 at 20:37
  • 1
    @kurkevan you could buy the right size boot and cut a funnel off it and side it over the existing dektite to get rid of the hollow or you could put a piece of wire under the edge of the current one pull it up and fold back the edge of the boot and silicon or similar the edge to the pipe which will seal it and prevent it from dropping down again
    – UNECS
    Nov 19, 2012 at 22:08
  • @UNECS - The problem is that the pipe is wider on the top, so I can't slide a new boot over it. The rubber is also quite hard by now, which would prevent it from being pulled upwards.
    – kurkevan
    Nov 19, 2012 at 22:32
  • 1
    @kurkevan you'd be able to pull a new boot over the bend but if your worried just cut of the bend and glue another one on there cheap, by the by have you checked if the bend is even glued on?
    – UNECS
    Nov 20, 2012 at 1:40

You could either

  1. lift the edge of the old dektite(boot) up and silicon it to the pipe
  2. Get a new Dektite (boot) and cut the bottom of it using it as a apron flashing over the old boot, by pulling it over the bend and over the old boot
  3. (Most professional way without totally removing the old boot) Get a weathering apron 100mm weathering apron cut the bend off the top of the pipe side the apron on and glue it in place over the dektite (boot) then glue the bend back on the opposite way to which it was (eg. saves buying a new bend)
  4. Remove the old dektite and replace it with a new one (preferably with a aluminum/poly base instead of just rubber as the old one is)
  • Thanks... the weathering apron looks interesting, but I can't locate anything by that name in the US; what else could it be called?
    – kurkevan
    Nov 20, 2012 at 2:48
  • 1
    not really over there but you could probably just use a 4"x 8" or whatever size reducing bush to do the same thing you might just have to file out the pipe stop on the inside of the bush to push it onto the pipe
    – UNECS
    Nov 20, 2012 at 3:19

Just get a tube of good quality silicon caulking and fill that entire area and create a positive slope for water run off. Tool the caulking around the PVC pipe and you're finished. It really is as simple as that.

  1. Cut the PVC pipe just below the elbow.
  2. Slip a proper sized flashing on. Ensure it drapes "like a woman's dress" over the old flashing.
  3. Flip the elbow over, and cement the elbow back on.

But if you were in a situation were the elbow could not be cut, then you'd want sticky roofing tape. This has much more structure than a tar like substance, and far easier to work with than a caulk. Drill a small hole in the old flashing so any condensation can vent away, then apply the tape:

Roofing tape

After you install the roofing tape, you can use silicone caulk on the top edges just to be sure.

Black Jack will not work for long: it gets hard then starts pulling away from the pipe. Use Black Jack only if you're a Realtor trying to flip the house.

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