We are planning on making our attic into a master suite with a full bathroom (tub, separate shower, toilet, sink) to a house we are buying next month. Describing what we want to do to our plumber (he has yet to see it as we have not closed on the house) he advised that adding a bathroom to the third floor is not a good idea if we tie the drains into the existing in stack.

He said he usually advises clients to add a seprate drain that connects to the main drain that exits the house. In our case this would be in the basement. His reason being that when water from the third floor travels down the pipe it can create a vacuum that could pull water out of the traps of the other fixtures in the house causing a sewer gas smell and noise in the pipes.

My questions are:

1) Is this a valid reason to add drain? Or is there a suitable workaround as we do not want to open the walls and ceiling below to add a second drain?


2) What is the white PVC pipe that is directly in front of the black cast iron stack? Directly below these two pipes is the second floor bathroom (the bath tub drain is in line with both pipes). Is the white pipe a secondary vent (it does not exit the house)?

Live in Indianapolis, IN, Marion County. City water, city sewer.

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Not sure what it is. Is it capped off or is that an AAV on the top? Is that a dryer vent stretched across the floor?

Hi MpSullivan

1) There is no work around solution here... you absolutely MUST connect the drain for the 3rd floor, at a minimum, below the 2nd floor plumbing. This means that you do not have to necessarily go all the way back to the main drain, but you must connect below the drains for the 2nd floor (unless the first floor also uses this cast iron stack and in that case, you are into the basement, for sure).

2) It looks like a vent from the 2nd floor bathroom and that is usually cut into the cast iron stack just before it exits the building, but in this case, someone opted to install a mechanical vent (studor vent or an AAV) instead of connecting it into the main stack there. When you do the new 3rd floor bathroom, I'd fix that and get rid of the mechanical vent as they can/will fail at some point in time!

Any more questions just let us know, OK?

Mark