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Old 08-04-2011, 12:45 PM   #1
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venting of two bathroom sinks

I have a 4 inch vent from the roof down into the attic space and I need to tie it into two separate drains for both the sinks (they are 2 inch drain pipes).

The wall is 2 x 4 inch framing. Is it ok to reduce the 4 inch attic vent to a smaller size and how should I connect it to the two sink drains?


Last edited by Sandramc; 08-04-2011 at 12:57 PM.
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Old 08-04-2011, 09:50 PM   #2
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It's kinda hard to give you an idea what to do, I don't have a diagram to go off of or anything like that.

I do know that you can't reduce the size of a vent or "choke it down".


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Old 08-05-2011, 07:12 AM   #3
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Without a schematic of what you're dealing with it is very difficult to advise you.
However, if these two sinks are side-by-side bathroom vanity sinks, then they should need only one shared trap and one vent after the trap.
The easiest thing to do is to install an air admittance valve under the sink base after the trap, IF allowed by your local building code.
Check out about AAVs. Studor is a well-known worldwide brand name,
Then call your local building inspection department to see if AAVs are allowed in your jurisdiction. They are easy to install right after the trap under the sink with basic plumbing skills.
The alternative, if AAVs are NOT allowed, is to run a 2" pipe from a T in the wall behind the sink base with the trap draining into the T (and a 2" vent going up to a 2" connection to a 4" fitting installed in the vent pipe going up through the roof). The T will allow the water to drain in and downward and the air to come in from the top to vent it.
Good Luck!
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