I want to do a pottery studio in my attic and for that I will need to install a utility sink. Because it would be located just above a bathroom and next to the main vent pipe, I was under the impression that it would be a relatively simple job. I was told, however, that if clogged, the sink would create a big problem in my entire plumbing system and such project should not be done. Is this true?

Hi Atticstudio...

Most likely the bathroom below this attic space has used the stack pipe as a VENT PIPE and therefore adding that sink could cause the traps in some of the fixtures below to siphon out and or bubble and allow sewer gasses into the home when draining that new attic sink... so probably NOT a good idea to add a sink to upstairs by just connecting into the VENT STACK in the attic.

You could cut open the wall in the bathroom below and connect into the WASTE pipe down there... BELOW the vents (usually located about 42-48" off finish floor). Then you would run that pipe upstairs to the sink and then could tie the vent into the vent stack up there or even use a mechanical vent (AAV... see picture) so no need to disturb the stack up there...

Using an AAV is not plumbing "legal" everywhere, so it would really depend on whether you would be having this sink installation inspected by a local plumbing inspector or not as to whether you could use one in place of tying back into the vent stack.

Let me know what you are thinking here... glad to hash this out a bit if you want.. ;)

MARK