how to move Vanity drain from side wall to the back

Question

I am renovating a bathroom and the drain is coming from the side. This was done because the vent stack is on that side wall. The wall directly behind the vanity would be an outer wall.

I am planning to put an Ikea Godmorgan vanity with drawers. I would like to move the plumbing to the back so there is no interference with the drawer. Since its an outer wall, I prefer not to go behind the wall, instead go down through the subfloor.

1. Would the proposed new drain run be viable? (would it be vented, up to code, etc.)

Answer 1

What you draw won't work, if you want it all to come out of the floor you'll need to do an island vent or use an AAV if one is permitted in that location.

Answer 2

Depending on your code in force, there is a way to do something like this and still vent properly. In my case, this section is struck from locally adopted code, so I could not do it.

"The trick" is that the downward pipe from the trap is 2", ensuring that a 1.25 or 1.5 inch lavatory drain can't make it siphon. There are also limits on distance.

IPC section 917 "single-stack vent system" for details. More importantly, check your local adopted code which may or may not be based on IPC and may or may not include this section or a similar one. It's evidently baed on a Philadelphia code with over 100 years of successful use, and was adopted in IPC in 2012.

Answer 3

Unfortunately, the way it's vented in your drawing won't meet code. Your vent is going to have to branch across and tap into the horizontal run out of the P trap - probably where it goes vertical, using a sanitary tee.

The rule of thumb is that the trap weir in the P trap needs to be able to 'see' the vent.

There are also code regulations with minimum and maximum distances that the vent must be from the P trap based on the drain pipe diameter. For example, for a 1 1/2" diameter drain the vent must not be closer than 3" to the trap nor more than 42" away.

Option 2: You could possibly run the P trap to the right and take it directly into the wet vent in the wall on your right. The issue will be working around any drawers/shelves - maybe behind a drawer?

Option 3: As mentioned in another answer here you might check to see if an AAV (air admittance valve) can be used under a vanity in your locale.