Vanity shelf interfering with trap install

I've had to cut a rectanguler shape hole in the shelve.

Your trap outlet needs to be at the level as the pipe in the wall- so as Joe said, you need to cut a hole. Also, you get about 3" of swing in a trap, so you need to move the sink over a tad or put a 45 on the drain arm

Looks like you might as well open the wall up and adjust the elevation and the horizontal arrangement of the drain. That's what I would do. I hate cutting into cabinets.

Alan said:

Looks like you might as well open the wall up and adjust the elevation and the horizontal arrangement of the drain. That's what I would do. I hate cutting into cabinets.

Click to expand...

Well there's another sink on the other side of the wall, so changing the drain in the wall is considerable work. What is wrong with using 45s to slope down from the trap to wall?

It puts your vent below the trap arm which causes siphoning of the trap.

I'm confused about this. Can you have a vent above the trap?

I mean, what is the difference between coming off the trap, dropping down a bit, then going into the wall (where it surely drops immediately), and going directly into the wall (where it surely drops immediately)?

How does siphoning occur unless the vent is before the trap?

I've always been told you want the vent as close to the trap as possible, and there are some permitted maximums. Obviously, a vent 30' from a trap isn't as good as one 3" away.

So if the vent is 6" below the trap, and the water wants to pull on the trap water, the vent won't stop this suction?

cleveman said:

So if the vent is 6" below the trap, and the water wants to pull on the trap water, the vent won't stop this suction?

Click to expand...

No, because water seeks it's own level. Picture a piece of pipe on grade.


Now picture water in the pipe. The water is level in the pipe because again, water seeks it's own level.

As the pipe gets longer, soon you see that on the downhill end of the pipe, the water level is reaching the top of the inside of the pipe. Once this happens, the air is not allowed into the horizontal pipe anymore. It pulls air through by siphoning the trap.

Same thing happens when the trap arm is offset vertically. Someone here has posted a diagram here before of this scenario, and I have a book that shows it. Hopefully you understand what i'm saying.

Here's a diagram

http://www.billbblog.com/Content/images/diy/bathroom/Day8-20/fixtureArm.jpg

OK, I get it.

You must be worried about situations where the vent is out of code. In my experience, a lav sink drains into the wall, where it meets a tee. The tee is the vent, so the trap is about a foot from the vent.

I was also taught to drop a toilet into a tee or elbow with a 2" coming off it for the vent.

And typically the bathtub is about 30" from the toilet.

Anyway, thanks for the clarification.

cleveman said:

You must be worried about situations where the vent is out of code.

Click to expand...

No, like I said if the vent is below the trap arm for instance how the OP is talking about offsetting his trap arm UPWARDS to clear a shelf, the trap can siphon. It really has nothing to do with the vent.

Alan said:

Here's a diagram

http://www.billbblog.com/Content/images/diy/bathroom/Day8-20/fixtureArm.jpg

Click to expand...

Per the diagram, it is allowed to have some minimal pitch. Is that correct?

jkk said:

Per the diagram, it is allowed to have some minimal pitch. Is that correct?

Click to expand...

The diagram is sketched as 1/4" per foot slope. That is why your trap arm maximum length increases with pipe size. If you slope your pipe steeper, the theory is that your maximum length should decrease. Most of the time under vanities we don't have this issue with length however.

Alan said:

The diagram is sketched as 1/4" per foot slope. That is why your trap arm maximum length increases with pipe size. If you slope your pipe steeper, the theory is that your maximum length should decrease. Most of the time under vanities we don't have this issue with length however.

Click to expand...

Yes, so for a 1' length with 1.5" pipe, I can have 1.5" of drop. Right?

In theory, yes, but that is too steep to achieve over a 1 foot section without using fittings.

Why couldn't the blue pipe in the picture be cut so the trap is closer to the bottom of the sink? You should then be able to connect up to the drain with some slip fittings and a 90.

rustyedge said:

Why couldn't the blue pipe in the picture be cut so the trap is closer to the bottom of the sink? You should then be able to connect up to the drain with some slip fittings and a 90.

Click to expand...

refer to Alan's link in post #9

The plastic drain pipe came with the sink. I agree it's junk. But, won't anything else leak since it has to butt against the sink with rubber washer? Is there a better way (short if buying better sink)?

Haven't used tape/dope in compression fittings.