Are pre-made rain caps made for plumbing waste vents?

My house was built 1938 with galvanized DWV pipes which have a series of elbows before reaching the roof. All such elbows retain a certain amount of rain or dew water. I quite incidentally discovered one elbow is paper thin at the bottom:

Rusty drain waste pipe galvanized

The run is more or less sloped. See also: Covering / Cap over DWV vent pipe This is 1.5 inch pipe.

  • These type of caps are normally used for gas combustion vents (like on a gas water heater, because they are metal pipes), but there's no reason you couldn't use it on a drain waste vent. Does that work? – JPhi1618 Dec 10 '15 at 15:02
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    Three-quarters of a century in there's probably not much point to protecting the pipes now. They'll always be moist simply due to condensation from vent activity. I assume that you'll replace the run in the photo. How many does that leave? – isherwood Dec 11 '15 at 19:19
  • @isherwood, that leaves four more, all buried in the attic. The run in the photo I will abandon in place and cap the top. The rest are inaccessible without major work. – Bryce Dec 14 '15 at 6:25
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    Unless you live in monsoon territory, the amount of rain entering through the stack is negligible, and you can't do much about the condensation from below. I have never seen a cap on a plumbing vent in my life, and I live in a rainy area. If the pipes are rotting, replace them but a cap is likely not the answer to anything. – MickeyfAgain_BeforeExitOfSO Apr 12 '17 at 21:38

I used a quick and very cheap solution, taking a standard plastic elbow and jamming it on to form a vent cap:

DIY rain cap for 1.5" plumbing vent

This will not affect vent operation, but will keep a large fraction of the rainwater out of the pipe, potentially extending the pipe lifetime by a few decades. This also has a presumably minor benefit of keeping rainwater out of the sewer.

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