Roof flashing sealant question

Stan in Oly, WA

Well-known Member
My natural gas furnace exhausts and gets its intake air through 2" PVC pipes. Recently I discovered dripping water where one of the pipes connects to the furnace. When I got onto the roof, the problem was very obvious. The sealant had failed where the pipes come through a metal boot. The sealant was clear silicone. I peeled it off the pipes and the metal boot, and cleaned everything well. Now I'm wondering what to use as a sealant. We bought this house in 1998, and a new furnace had been installed years earlier. That means that the silicone caulking lasted well over twenty years. Silicone would be easy for me to use to re-caulk. It's readily available, I've used it a lot, and I've got proof that it lasts more than 20 years. At my age, 20 years is likely to be longer than I've got, almost certainly longer than I can continue to live in this house. Is there something else that is proven to last longer and better? I'd much rather use something that I'll get a sure 20 to 25 years out of than something which should last much longer, except that it might not.

Stan
 
There are 2 options. One is to go to Home Depot or Lowes and buy a "sleeve" that slips over the existing pipe, then a "union/cap" seals the top edge from rain water. The other is to use a product called "through the roof" which is clear and can be bought to use with a caulking gun or can be bought to apply with a trowel/putty knife. Through the Roof is fantastic stuff.
 
If I were going to reseal it, I would use Polyurethane roof sealer in a caulk tube.

But anytime you are relying on exposed sealer to prevent a leak, it will eventually fail.

There are many types of vent boots that slip over the pipe and fit tightly around the pipe, depends on what type roof you have as to which one to use.
Vent Boots
 
Steve; The trouble with any new vent boot is that the two pipes are less than an inch apart. The boot that's there appears to have been fabbed by a skilled sheet metal worker, not factory made. I think I'm going to have to work with what I have. About half the responses here so far recommend Through the Roof sealant---enthusiastically---so I guess I'll give that a try. Thanks.

Stan
 

I had 20 year old Oatey brand vent flashing that had a rubber donut that the pipe went up thru, the sun had baked it to pieces, but no leaks for some reason. I got a 2" donut at the hardware store and just slid that over the pipe, still no leaks 8 years later.....just looking at it, should leak, but don't....
 

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