Help with venting and basement bathroom layout

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New Member
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Maine, USA
I am wondering how far "downstream" a toilet and sink and shower can be from a vent? The pipe leading to the vent are 4" cast iron waste pipe then' and connecting back to a 4" pipe which eventually goes out the roof.

I hope to cut open the cast iron pipe and connect somewhere along this 10 foot run in the picture. The vent is about 18 feet away. It is a wet vent (or waste vent, no sure the terms) as it has existing shower, toilet, lav, washer and slop sink on it all ahead of the vent to the roof

I have headspace to run a 3" pipe back to the roof stack right ahead of the upstairs toilet (so I think that would make it a "dry vent"), but would rather not if I don't have to. That whole run would be about 20'. I also could run a 3" pipe into the existing wet vent using a Y, but this would almost certainly make the toilet the first stop on the cast iron and I think that's a no no and would severely limit my ability to add a shower and sink.

Would love to hear suggestions about how folks might lay this all out.

My toilet is wall mount so I have options there. The shower is something I might want to add, so want to plan for it.

There is no possible way to run a new vent stack to the roof from here, so very open to creative (safe/legal) ideas, even if the feedback is "this is impossible"




In the Trades
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Berkeley, CA
The lav trap must be dry vented. Wet venting from a floor above is not allowed. The easiest way to dry vent the lav would be with an AAV, but Maine uses the UPC, which does not recognize AAVs. If there's a Maine or local amendment to allow AAVs, then you could do that.

Otherwise, the only way to dry vent the lav is to provide a vent that connects to pipes that go through the roof. You can pull off your dry vent and the vent can run horizontal once it's 6" above the lav flood rim. For example it can run just under the joists (although it needs to be sloped 1/4" per foot back to the lav.) Then you can run it across to a point where you can run it up through the floor to be alongside an existing dry vent. It has to rise alongside that dry vent until it reaches a height 6" above that fixture's flood rim, and then you can combine the vents. Or you could just run a separate lav dry vent through the roof.

Once the lav is dry vented, it can wet vent the shower and the WC.

Cheers, Wayne
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