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Using roof gutter as a plumbing vent

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Cole says:
December 30, 2010 at 11:50 am
I need to tie into a vent for a new powder room on my lower level, but there is no vent stack on that level – just the soil stack, and of course we are downstream of everything else in the house.
However, I have a flat roof with a 3-inch drain pipe running right down the middel of the house, very close to the new powder room. Is it ok to tie into that roof drain as my vent? I have a feeling it’s not ok, but I’ve read through the code and I could not find anything specific about this kind of sutuation.
asked in Plumbing by darekrudy (21,730 points)
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1 Answer

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Hi Cole,
    You have a good feeling, sanitary and storm sewers are two separate systems and should be maintained that way, however, I’ve seen them accidently or purposely mixed up and spliced together many times. Also, older system may be designed that way (as one / combined) and include check valves (Palmer valves) that swing in-out preventing sewer water from contamination of the storm sewer section.
    Another thing against such installation is a possibility of that internal roof drain contamination with debris, which would prevent proper venting of… is it just a powder room sink?
    If it is just a sink, you can ask you local building department if they would allow “island venting” installation, also referred to as a loop vent.
    Another option (if possible of course) is to extend that powder room plumbing fixture(s) vent above the ceiling and above the plumbing fixtures on the upper floor, and tie it to the vent stack at least 6in. above the flood rim of the highest fixture.
    There’s also an Air Admittance Valve option (if it’s only the sink that requires venting), which is not approved by some jurisdictions, so you’d have to check with your local building department. The valve needs to be placed where it can be accessed for service / not inside the wall or ceiling cavity, etc.
    There might be some other options to consider but I don’t know how is your plumbing system designed and distributed around that new powder room area… and how much or little damage to the walls and ceiling you’d allow.
    Let me know
answered by darekrudy (21,730 points)


Submitted on 2010/12/31 at 11:28 am | In reply to Dariusz Rudnicki.
The dwelling was built in 1970 – I think the roof and sanitary systems are separate, as they should be. The powder room is on the ground floor, and there is no rough-in for a toilet.
To avoid the mess and expense of breaking up the slab, I am installing a macerating toilet that will pump the waste up and over the ceiling into the garage where it will tie into the main soil stack. However, the unit requires a vent, and it needs to accommodate both positive and negative pressures, so the AAV will not work.
The roof drain is just a few feet away from the soil stack. There is a sanitary vent pipe exposed running up from the slop sink – but it is about 25 feet away; 40 feet of pipe run to tie into that…that’s why I was hoping to avoid going there. But, I guess that’s what I have to do. Thanks for your help.