Kitchen sink drain through floor, p trap arm length

[Rick1526]

Hello,

I am remodeling my kitchen and moving the kitchen sink about 6' to an outside wall. I would like to drop the sink drain through the floor underneath the base cabinet rather than plumbing into the wall. I would like to do this to avoid cutting into a number of studs/cripples that are supporting a large window. Also, I would worry a little bit about freezing if the sink was plumbed along the outside wall.

I understand that I must vent the waste line after the p-trap arm before the line turns to go down.

My questions are, how long must the p-trap arm be to avoid being an S trap? And, is there a requirement on where I vent the waste line? e.g. could I use a 45 bend-nipple- to a 45 wye and then run my vent line off the top?

In this scenario the waste line ultimately goes through the floor into a crawl space, turns and goes straight across into the main soils line. The vent would T under the base cabinet, enter the wall, rise 45 degrees across the wall, turn up and tie into an existing vent line that goes to the top of the soils stack.

See pictures below. Is this a good plan?

 

[Terry]

The vent would be "above" the trap arm that goes to the p-trap in the cabinet.
No p-trap below the floor.
A 1.5" trap arm can go 42" to the vent, but if this in below the kitchen sink, it's close anyway.

 

[Sponsor]

 

[Reach4]

My questions are, how long must the p-trap arm be to avoid being an S trap?

minimum 2 pipe diameters, so 3 inches for 1.5 trap arm.

 

[Rick1526]

The vent would be "above" the trap arm that goes to the p-trap in the cabinet.
No p-trap below the floor.
A 1.5" trap arm can go 42" to the vent, but if this in below the kitchen sink, it's close anyway.

Thanks for the info. Could you elaborate a little on the vent being "above" the trap arm. If the P-trap arm terminated in a T and the vent line was connected to the top of the T would this be considered above the P-trap arm?

 

[Terry]

The vent or AAV is above the trap.

 

[Jadnashua]

I think that the plumbing code in Canada may allow a p-trap to be beneath the floor, but none of the USA codes allow it. There is a limit on the height before the p-trap, too...too long, and gravity can make things go faster than you want and make the trap malfunction.

 

[Bryan in Toronto]

Hi Terry,
Firstly thanks for taking the time to help all of us homedepot you-can-do-it-types.
I’m renovating my kitchen. The previous kitchen sink was on the wall (properly vented). I kept the vent connection but shortened the drain line about ten feet to allow for a new island sink drain. There’s really no way of pin pointing exactly where the drain pipe will come up out of the bottom of the sink’s cabinet during rough in, so getting it “into” the cabinet was the best I could do.
The conundrum I’m facing now is that the drain comes up out of the bottom of the sink cabinet almost directly below the sinks tailpiece. In Canada (and I believe in all 50 states as well) s-traps are illegal. I dry fitted what I thought isn’t technically an s-trap. Something still bugged me about my setup though. While not technically an s-trap.... it still kind of is. Look at the picture and I have a feeling you might agree. The potential for siphoning the water fence in the bottom of the trap still nags at me. Am I wrong? The drawn grey line is what I think I should do to definitively negate any potential siphoning issues. By running a piece of abs off the p-trap to go to the back of the cabinet, then a 90 and a short section of abs to go downwards, another 90 to come back to the front of the cabinet with short piece of abs, and one final 90 to connect to the drain in the cabinets floor I think might be a better setup.
Am I overthinking this or can I stay with the dry fit you see in the photo? For reference, the picture’s viewing angle is from slightly left of centre.

Thanks for any advice and help.

Bryan

 

[Jadnashua]

If they allow AAVs where you live, take the trap arm straight (horizontally) into a sanitary T, up to the AAV as high as you can get it in the cabinet, and the other side goes down to the drain.

While S-traps have been used for ages, they are subject to being siphoned dry.

 

[Mliu]

I dry fitted what I thought isn’t technically an s-trap. Something still bugged me about my setup though. While not technically an s-trap.... it still kind of is. Look at the picture and I have a feeling you might agree. The potential for siphoning the water fence in the bottom of the trap still nags at me. Am I wrong? The drawn grey line is what I think I should do to definitively negate any potential siphoning issues. By running a piece of abs off the p-trap to go to the back of the cabinet, then a 90 and a short section of abs to go downwards, another 90 to come back to the front of the cabinet with short piece of abs, and one final 90 to connect to the drain in the cabinets floor I think might be a better setup.

Bryan:

Technically, what you have shown absolutely is an S-trap. And your proposed re-design is also an S-trap. You must have a vent above the trap arm before it drops vertically.

As Jadnashua stated, an AAV will solve your problem if it's permitted in your jurisdiction. See Terry's photo for how to plumb the AAV.

If you cannot install an AAV, then look at the plumbing designs for the correct ways to install island sink venting.

 

[Mliu]

See pictures below. Is this a good plan?

Rick:

The detail in the lower right corner of your sketch, you have a wye plus 1/8 bend as your connection between the P-trap tailpiece and the vertical waste drain. That is an incorrect connection! You must use a sanitary tee instead.

 

[Reach4]

The conundrum I’m facing now is that the drain comes up out of the bottom of the sink cabinet almost directly below the sinks tailpiece.

You can do like this, but you would use ABS.

 

[Reach4]

The conundrum I’m facing now is that the drain comes up out of the bottom of the sink cabinet almost directly below the sinks tailpiece.

You can do like this, but you would use ABS.

 

[Kreemoweet]

Am I wrong?

It would seem so. The length of the trap arm has nothing to do with a setup being an S-trap,
it's the lack of a vent connection to the upper side of the arm before it turns down that makes it such.

 

[Bryan in Toronto]

Bryan:

Technically, what you have shown absolutely is an S-trap. And your proposed re-design is also an S-trap. You must have a vent above the trap arm before it drops vertically.



As Jadnashua stated, an AAV will solve your problem if it's permitted in your jurisdiction. See Terry's photo for how to plumb the AAV.

If you cannot install an AAV, then look at the plumbing designs for the correct ways to install island sink venting.

Thanks for the replies guys,

I don’t need a cheater. The line is already vented as I mentioned

and IMHO the redesign is not an s-trap. See attached...

 

[Reach4]

Another think to consider is that in some cases, Canadian code allows traps below the floor, where US codes do not. Maybe you could put a properly vented trap below the floor. You would have to check the rules to be sure what is allowed.

I am not a pro.

 

[Mliu]

View attachment 57402 and IMHO the redesign is not an s-trap. See attached...

I've seen this image and explanation before (don't remember where) and it is WRONG. The piece of straight pipe does NOT magically change that from an S-trap into a legal P-trap. If you pour enough water down that drain, it WILL siphon the trap.

 

[Bryan in Toronto]

I've seen this image and explanation before (don't remember where) and it is WRONG. The piece of straight pipe does NOT magically change that from an S-trap into a legal P-trap. If you pour enough water down that drain, it WILL siphon the trap.

Milu,
Wouldn’t every single bathroom vanity be subject to siphoning then? Look at the picture again. Imagine a wall halfway in the middle of that horizontal pipe. Then the water turns downward and goes bye bye. Every single sink that has a drain through the wall is basically the exact same configuration.... Why/how then is my idea flawed?

 

[Bryan in Toronto]

Another think to consider is that in some cases, Canadian code allows traps below the floor, where US codes do not. Maybe you could put a properly vented trap below the floor. You would have to check the rules to be sure what is allowed.

I am not a pro.

Island is already in. Impossible now. Thanks for the input. Milu is saying the fix is still an s-trap. Technically then every powder room is an s trap too.

 

[Jadnashua]

A properly vented sink would have the trap arm go horizontally into the wall (with the minimum slope), and in the wall, have a vertical vent, with the waste turning down. That is not an S-trap. If an AAV is allowed, that is probably the easiest way to vent the sink when your waste is straight down with no chance to run a vent vertically from the trap arm. FWIW, that also, usually, ends up taking less space in the vanity, at least on the bottom.

While there are probably a lot of S-traps out there, by far, the majority installed are proper P-traps. We've learned a lot over the last 120-years or so once indoor plumbing became the 'thing'...at least in industrialized countries. Lots of really weird stuff in some parts of the world.

 

[Mliu]

Milu,
Wouldn’t every single bathroom vanity be subject to siphoning then? Look at the picture again. Imagine a wall halfway in the middle of that horizontal pipe. Then the water turns downward and goes bye bye. Every single sink that has a drain through the wall is basically the exact same configuration.... Why/how then is my idea flawed?

No, they are not the same configuration. Every single lavatory or sink should have a vent (either a real vent pipe or an AAV) coming off the top of a sanitary-tee connected at the junction between the horizontal pipe (trap tailpiece) and the vertical waste pipe. The vent is what breaks the siphon as "the water turns downward and goes bye-bye." Just because you don't normally see the vent (because vent pipes are usually hidden inside the wall; AAVs are not) doesn't mean the vent doesn't exist.

This is what a normal drain looks like:

This is what an AAV-vented drain looks like: