In my attic there is (what I think is) the fan part of a radon mitigation system. There's a lip, and water pools there, then overflows and gets water in the attic.


It seems to be worse when it rains. We got a new roof a few weeks ago, and we just moved in, so I don't know the history of the problem, but if I had to guess, it's that it's just a big open pipe pointing straight up coming out of our roof, collecting rain water.


A bunch of things I read online say that I shouldn't put a rain cap on top of this thing, that it wouldn't need it. So, either that's just wrong, or however the unit is supposed to deal with water isn't working. My best guess is that the fan is too weak? Does anyone have any tips or insights?

  • 1
    Where is the lip that you mentioned? It would be helpful if you would mark up the photo to indicate the lip and/or the place where water is leaking.
    – MTA
    Commented Nov 7, 2021 at 19:14
  • MTA, the mobile site won't let me add a picture when editing. I will tonight. the lip is in the middle of the wide white part. the water pools there then spills over
    – Sam H.
    Commented Nov 7, 2021 at 19:55

3 Answers 3


You say the water is pooling at the lip and spilling out, and your comment says that the "lip" is in the middle of the wide part.

So here's what's happening --

What you're calling the lip is the two flanges that join the upper part to the lower part of the fan housing. Those flanges are joined either by solvent welding or ultrasonic welding. The joint is supposed to be air tight and waterproof, but yours has an imperfect seal. If you put some soapy water on the lip, you'll see bubbles. That's where it's leaking.

When rain falls into the vent pipe and into the fan -- or when moist, relatively warm air from under your house condenses inside the fan during cold weather -- the spinning fan catches the water and flings it radially outward, directly at the "lip", and drives the water through the space between the flanges that is supposed to be perfectly sealed. There's your water leak.

If this fan is still under warranty, the manufacturer should replace it because their airtight waterproof seal is leaking.

If it's no longer under warranty and you want to repair it, make sure the lip is clean and dry, roughen the outside circumference of the lip with some coarse sandpaper, mix up some epoxy and coat the lip with it to plug up the crack between the upper and lower flanges. Best to do this with the fan turned off, or the crack may blow epoxy bubbles and fail to seal. If you use 5-minute epoxy, leave the fan off for an hour till it's good and firm. It won't leak any more.

Leaving the fan off for just an hour won't make any measurable difference in radon infiltration.


Vent pipe generally are built to accept rain water entering the top.

If the leak is coming from around the outside of the pipe where it penetrates the roof then you need a better roof flashing. I had a plumbing vent pipe that didn't get a roof boot until 10 years after the house was built and a really small amount of water would come down and eventually dissolved the drywall below a bathroom ( we thought it was just a bathtub caulking issue ).

If the leak is coming from inside the pipe and getting out at a connection like the furnco (rubber compression fittings) then you need to adjust that connection. If it is from a solvent join then you should fix the solvent join.

Rain water that does get into the pipe should travel down to under neath your slab where the radon gas accumulates. The rain water would then make it's way to your sump or perimeter drain.


There is another possible source of water on the outside of the pipe. a variation of which occurs in our basement. To wit:

Cooler air from the sump will cool the pipe. If the attic air is warmer, and it's humid enough, moisture will condense on the outside of the pipe. This will run down and accumulate where its path is obstructed - the pump flange, for ex.

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