stack question for bathroom remodel

wally123October 13, 2011

I am remodeling a small bathroom, 30 sqft. The current sink goes through the slab to the septic. The vent is a feed to the main stack on another wall. My main question is the vent to the sink is along an outside wall and I would like to remove it. Assuming I bust up the floor, can I connect all the drains from the sink and shower over to the main stack, utilizing just one drain and cap off the second drain location that the sink currently feeds?


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It depends upon which code your under.

If you are under the IRC it could be done by making a vertical riser up above the point where the sink trap fixture arm will attach and installing an "AAV" (air admittance valve) on the top of the pipe, with the fixture arm attaching to the riser with a tee.

If you are under the UPC you cannot do it.

    Bookmark   October 13, 2011 at 1:44AM
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what's UPC and IRC? Let's assume I cannot. What's the minimum size vent pipe I would need to connect a sink to the main stack? Total distance would be 6 ft

    Bookmark   October 13, 2011 at 6:42PM
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IRC = International Residential Code
UPC = Uniform Plumbing Code

Suffice it to say that all codes prohibit running a horizontal vent line under the floor. Under no circumstances may a vent line run horizontal until it reaches an elevation that is at least 6" higher than the flood level rim of the tallest fixture served by that vent.

The height of a bathroom lavatory cabinet is generally about 36" and on average the bowl is 8" deep. That means the drain opening is approximately 28" above the floor.

Code allows a maximum 12" drop from the drain opening to the input of the trap, therefore the trap output will be about 16" above the floor.

The horizontal pipe from the trap to the drain or stack is called the "Fixture arm or waste arm" and the bottom of fixture arm at the trap weir may not be higher than the top of the pipe where it connects to the stack or vented drain.

Under the IRC the fixture arm for a lavatory is supposed to be 1-1/4" pipe and it may have a maximum developed length (actual length of the pipe + fitting allowances) 5'.
Code will allow us to increase the size of a pipe by one nominal trade size, which would then be 1-1/2" and an 1-1/2" may have a maximum developed length of 6' and it must be run with a 1/4" per ft pitch. (Remember, this is not a point to point measurement, it is the actual length of the pipe including all turns)

Under the UPC the maximum length of an 1-1/4" fixture arm is 2'6" and an 1-1/2" is limited to 3'6".

The minimum size for a vent line is 1-1/4" but I doubt if you can even find 1-1/4" pipe and fittings because the lavatory is the only fixture that is permitted an 1-1/4" line, and rather than maintain a separate inventory of pipe and fittings just for the lavatory it has now become an industry standard to opt for increaing the line to 1-1/2". Therefore, due to the absolute minimal sales of the pipe and fittings most suppliers no longer carry 1-1/4".

    Bookmark   October 13, 2011 at 8:29PM
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