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My house has a flat roof and leaks, mostly directly underneath the drains.

I've had problems with leaking around the drains since I've owned the house, mostly dripping, but over time it's turned into gallons of water.

Last year I installed a foam membrane which is basically indestructible and totally leak-proof, but I'm still leaking. (the new roof stopped an unrelated leak in a different part of the house).

I think I've narrowed the leak down to the drain in the upper left of the picture. I think the leak is below the roof membrane, but above the roof decking (it's very hard to tell). The drain has a small bit of chicken wire rolled up in it to keep out debris.

enter image description here

My question is, why do I have two drains so close to each other (they're about 18 inches apart), and is there a reason that I couldn't just seal off the one that I think is leaking? Both drains appear to drain into different pipes, one of which (upper left) has a toilet pipe connected to it in the basement.

The house was built in 1928. Based on the technology level of the drain on the lower-right, I presume it was installed sometime after the house was built (?), or at least someone upgraded it, without upgrading the other one.

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  • Both drains are at about the level. When it rains hard, water pools around them both. If it's supposed to be a vent, it's awfully low - the other vents are the normal sort, 10 inches or so above the roof.
    – Seth
    Commented Aug 30, 2015 at 0:40
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    the one in the lower right is sitting in a well is the over flow drain. Code states that a second drain is to be installed 2 inches above the roofs lowest point.
    – ojait
    Commented Aug 30, 2015 at 0:50
  • It turns out that the "drain" with the chicken wire in it is actually my main plumbing vent, which either got broken off at some point or was supposed to be a secondary roof drain. Both of them leaked at the roof line, and both leaks were easily fixed with a lot of silicone caulking.
    – Seth
    Commented Oct 16, 2020 at 23:52

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You have a redundant roof drain. If one of them were to fail (clog) the other would (hopefully) continue to allow water to drain from the roof. If you were to cap one of them And the only drain were to become clogged water would accumulate on your roof to possibly it failed (or found a new route to the inside).

as for it leaking: it would be easiest to open an access to the drain piping inside the roof. Start were you said the water is wetting the ceiling and cut out a opening (14x14) to allow you to inspect and/or repair the pipe. The pipe, if it is original from the 1930's (geez, hmm) I believe would be 2-4" threaded galvanized? Maybe 2" cast? It may even be large diameter copper pipe. What ever the pipe material replace any rotted or rusted sections with ABS of comparable diameter. Join the new to old with rubber repair sleeves that tighten with hose clamps.

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    The pipe is cast iron, about 4". Water seems to be coming in directly between the two drains. Plugging the extra drain sounds a lot easier, but the drains do clog often from bits of gunk that blow in from a neighbor's tree. Thanks.
    – Seth
    Commented Aug 30, 2015 at 17:42
  • So it isn't the pipe that is leaking , but around the pipe / roof joint? Probably were the pipe exits to the roof. See how well the pipe is secured. If you can try to shake the section around the roof drain. It should be fairly rigid and affixed to the roof joists. If not install a length of perforated plumbers tape under the pipe for stability. It seems you have a roof leak. From outside on the roof you need to look critically around each drain and in the immediate area. Hopefully it is a small gap that can be filled with 100% silicone. Before closing the opening inside test the repair
    – ojait
    Commented Aug 30, 2015 at 18:11

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