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Off center vent stack (see picture). Can I use a 90 degree elbow out of the vent stack, run drain pipe along the wall, and then another 90 degree elbow so that the p track will point directly backwards (towards the wall)? The drain pipe and 90 degree elbows will be 100% level. Will this siphon the trap? Please enter image description herepost any problems with this idea.

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The specifics of this largely depend on your local plumbing code, and so what is acceptable during an inspection depends on that. Your inspector will be able to tell you for sure.

Elbows

My reading of Ontario build code section 7.5.6.3 says this would not be allowed:

(c) the trap arm does not have a cumulative change in direction of more than 135°.

90+90 = 180°, so that's too much.

Trap Arm Length

The other factor is trap arm length, and that depends on pipe size. Since this is a vanity I'll assume 1 1/4". To make that work, you need:

  • 1.5m (5 ft) maximum length
  • 1:50 slope (eg, 1/4" drop for every 1')

Fix

One option, if there's space, is to use a 90+45 (135° total) to route the pipe out:

90 and 45

Or you could also do it the other way around, though that would require opening the wall and cutting into the vent stack:

45 and 90

And if you're going to cut into the wall anyway, another option is re-route the pipe to use a single 90:

rerouted pipe through wall

Which one makes more sense is going to depend on space available and where the studs are.

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  • Could also go directly to the drain location just behind the drawer box, and maintain 2% slope. – Ecnerwal Mar 6 at 17:30
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    If the drawer is short enough you could just point the drap straight at the vent, with no elbows ... or if that won't work perhaps 45 degree elbows will avoid the drawer and give you 90 degree total turn. Or even a single 45 degree elbow off the trap, THEN pointed straight at the vent will avoid the drawer. – jay613 Mar 6 at 18:00
  • @jay613 Excellent point. Added a picture of that. It would still will require cutting a hole in the wall and cutting into the stack pipe, but definitely could be easier if it also avoids having to route the pipe through a stud. – gregmac Mar 6 at 22:54
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The proposed layout would likely work just fine for the sink in a vanity where generally you are just draining water or soapy water. It would be wise however if you could raise the level of the P-trap just a little closer up toward the sink drain so that the horizontal run along the wall has a small bit of slope to it. This will help to ensure that water does not stay lingering in that section of the pipe.

To raise the P-trap some it may be necessary to shorten the tail piece coming straight down from the sink drain.

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