Can I use horizontal 90 degree elbows to run a p trap along a wall?

Question

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 post any problems with this idea.

Answer 1

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:

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

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

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

Answer 2

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.