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I'm adding a secondary bar sink in a kitchen area, which will extend the waste pipe to it's farthest length, so I need to put in a waste drain vent pipe up the wall and through the roof.

The drain pipe is 1-1/2" diameter and I'm wondering if the vent pipe going up the wall can be a smaller diameter, like 1" or 3/4", so I don't need to cut such a wide channel in the drywall and fight with fiberglass insulation.

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I've found this:

Vent piping should never be less than half the diameter of the fixture drain it serves, and usually should be closer in size. For example, waste pipes of 3" and 4" diameters should be serviced by a minimum 2" vent pipe. And regardless of piping size, no vent should be smaller than 1 1/4" pipe. Dual vents which are to service more than one fixture should be slightly larger than required for a single fixture.

source: plumbing-basics.com

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