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Discussion Starter · #1 ·
With a new vanity 'installed', there is only ~2" from the shelf to the bottom of the drainpipe coming out of the wall. Not enough room to fit a standard trap installation.

It seems like I need to glue in some pieces that slope downwards from the trap to the wall pipe. Is that feasible?

Hand Plumbing fixture Plumbing
 

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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
 

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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. :(
 

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Discussion Starter · #5 ·
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. :(
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?
 

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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?
 

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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?
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.
 

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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.
 

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

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Discussion Starter · #14 ·
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.
Yes, so for a 1' length with 1.5" pipe, I can have 1.5" of drop. Right?
 

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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.
refer to Alan's link in post #9
 

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Discussion Starter · #18 ·
So I cut a hole in the shelf in order to the trap low enough.

For the drain pipe connection to the sink, do I use tape and dope on the plastic threads? Do I also tape and dope the threads at the plastic nut that forces the rubber washer tight?
 

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You are going to have problems with that plastic drain.


Here's what I do :

Slide the rubber washer as far down as it will go without disengaging the nut.

Run a generous bead of clear caulking around the top edge of that rubber washer.

This way when you tighten it against the sink, the washer spreads the caulking through the threads.

If you don't do this, water will weep down through the threads. The plastic is not strong enough or the washer is not squishy enough, i dont know which, but they are junk.

The slip joint connections will not require teflon or pipe dope.
 

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Discussion Starter · #20 ·
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.
 
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