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Old 01-05-2017, 09:46 AM   #1
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can I use/convert a toilet drain to a sink drain


I'm doing a reno to our house, by converting a small bathroom into a mudroom. I already removed the toilet, and want to now add a vanity and sink to where the toilet was. I also now ran a hot water line to the area. BTW - the bathroom did have a sink in a different area, which I removed and capped.

My question is whether I can use the toilet drain (on the ground) as a sink drain? I assume this can be done, since I confirmed that the old sink tied into this same sewer drain (by looking at my crawl space). I guess what I'd like to know is what type of adapter I need to convert the floor toilet 4" drain to a 2" sink drain, and whether this would cause any issues?

FYI - I also had a air vent in the old sink which I capped when I removed the old sink. Being that I was unable to relocate the vent to the new sink area I plan to use the portable ones available at hardware stores. Again, do you see any issues with this?

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Old 01-05-2017, 04:33 PM   #2
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Re: can I use/convert a toilet drain to a sink drain


Just sewer smell and illegal to code other then that none.
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Old 01-05-2017, 04:39 PM   #3
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Re: can I use/convert a toilet drain to a sink drain


There ought to be a nearby vent that was for the toilet. Presumably 2". You can cut a 2x1.5 tee into that for your sink and cap the toilet.

If you use the toilet's pipe as it is in the floor, you'll be creating an S Trap which aren't legal but can still be purchased... But idk, seems like an s trap along with questionable venting is 2 risks of the same type.

As for adapting it, if you are really set on it, is it a 3" pipe or a 4" hub or still the closet flange?
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Old 01-05-2017, 07:56 PM   #4
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Re: can I use/convert a toilet drain to a sink drain


BlackTiger - thanks for the info. I now have a few more questions:

The original bathroom was on the 1st floor of a 3 level house. The original sink (that I removed) had a vent pipe going up (I assume to the roof and tied into the sink in the 2 floors above), and then drained down which tied into the 4" sewer pipe. When I removed the sink, I capped the vent at the ceiling (again on the 1st floor) and then in the crawl space capped the 2" sink drain pipe (which again tied into the 4" sewer pipe). So my questions:

1) by capping the vent pipe in the ceiling of the 1st floor, will that prevent venting for the sinks on the 2nd and 3rd floors? I assume it should be fine since I assume those sinks would also be tied to this same vent pipe which leads to the roof

2) by capping the 2" sink drain in the crawl space (which leads to the 4" sewer pipe, and which was originally connected to the vent), would that mean my sewer pipe is no longer vented? Would other toilets in the house normally be tied into a vent of there own, or would all toilet pipes eventually lead to the crawl space and then use 1 2" pipe as vent? Is there anyway for me to know if my toilets are now not vented? Not that I have not had any issues with flushing, and no odors and I did this capping over 2 months ago.

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Old 01-05-2017, 08:12 PM   #5
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Re: can I use/convert a toilet drain to a sink drain


I assume you'd have noticed by now if your upstairs sinks were no longer draining, so it is apparently a dry vent all the way through the roof. I'm worried that the capped vent pipe is going to slowly collect water from condensation or precipitation. Capping the bottom of it shouldn't affect the venting of other fixtures attached to it higher in elevation.

Normally toilets are required to have a 2" vent within a certain distance. It's not uncommon to wet-vent a toilet by using it as a lav drain. Possibly that is the situation you were in. Your sewer pipe is probably vented through other connections also.

Did you remove the whole wall that the lav drain was coming out of? Or was there a specific reason you couldn't have removed the tee without removing 1 story of the pipe? That would take care of collecting water in the vent pipe as well as any sewer main venting concerns you have.
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Old 01-05-2017, 08:28 PM   #6
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Re: can I use/convert a toilet drain to a sink drain


I just went down into the crawl and took a good look at all my sewer (4") pipes and water/drain pipes (2"). I have about 8 other 2" drain pipes tied into various sections of the sewer pipe. So yes, I assume my toilets throughout the house are vented OK through one of the other 8 drains/vents tied into them.

Yes, the drain/vent that I capped was a dry vent, and not tied into the sinks above. I assume the sinks above are those other 8 drains tied into the sewer pipe. Is it safe to assume those other 8 drains are also vented to the ceiling? How are vents tied together? I would assume there is only 1 pipe out the roof and all sinks then tie into the main vent pipe - is this correct?

Yes, I had a wall in the bathroom which I knocked out. This is why I had to cut and cap it in the cieling and then cap it in the crawl space.

Currently I have removed the toilet and just have a portable plug on it (not sure if that is what it is called, but it is the type that uses a bolt to tighten the seal from within the pipe). I could permanently plug the toilet drain and then drill a hole and bring in the 2" drain that is tied to the sewer below the toilet (I removed), but that is extra work not necessary if I just use the toilet drain for my sink drain. I was thinking of a 4" to 2" adapter and then using a basic P trap (or S trap) to tie into the toilet drain direct. would that work? any issues? I did buy a portable vent attachment, but do I really need it being that the sewer pipe has the other 8 drains/vents already (isn't it already well vented?)

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Old 01-05-2017, 08:44 PM   #7
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Re: can I use/convert a toilet drain to a sink drain


The right thing to do would be to put a pipe in the wall where you want your sink to be now, and connect that into the vent and drain that you capped, and cap the toilet permanently. Or else create a new drain connection into your main, but still connect to that capped vent.

I feel pretty strongly that leaving that vent capped from underneath will not be a good idea, in the same way that having a random low point on your drain system would be a bad idea.

But if you're determined, you'll have to get down to 1.5", unless you're planning on gluing up the trap yourself. The slipnut ones at the store are 1.5".

I feel like mentioning now that drain pipes are measured inside diameter so maybe the 2" pipes you're mentioning might actually be 1 1/2" and the toilet pipe is probably a 3" pipe. Just a guess.

Anyway yeah if you want to hook it up that way just rig something up. You should probably still use the air admittance valve because the next nearest vent is too far away. But you'd probably get away with not because it'd be draining into such a large pipe so nearby.

There won't ever be a better time than now to do it right, though, so that's what I'd recommend. The rules can be finicky but especially when it comes to venting they're there to prevent big problems down the road.
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Old 01-05-2017, 09:46 PM   #8
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Re: can I use/convert a toilet drain to a sink drain


BlackTiger - thanks for all the input.

My main concern is the capped vent. How are vents tied together and feed to the roof? Is there 1 main vent in the roof and the various (8 in my case) sinks/drains all tie into them? What diamter pipe is the main vent going to the roof? My concern is whether the capped vent is perhaps the main vent pipe to the roof and the other drains/vents tie to it? My concern if this were the case is what happens if this main vent pipe (which would be capped) would get full of rain water - would that basically plug my vents? Also, what prevents rain water from entering the main vent on the roof, or is it exposed since it basically drains to the main sewer pipe anyway?

Lastly, what is the "right" way to cap the vent permanently? People do reno's all the time and knock out walls and take out sinks. There must be a way to disconnect a vent properly?

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Old 01-05-2017, 10:20 PM   #9
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Re: can I use/convert a toilet drain to a sink drain


There are numerous ways to tie vents together. It depends on the wall layout and location of other fixtures. It could be that the capped vent connects to another vent in the joist space between floors, connects to another vent above a second floor fixture, or goes independently out the roof. They are all approved methods of venting.

Since this is on the main floor, you should bring a new line to your proposed lav location and properly vent it- this includes not using an auto air vent. Cap the toilet lins off under the floor.

Do you know which code model is used in your area? It will tell you the proper drain and vent sizing as well as trap arm lengths, also if auto vents are approved. Hate to see the inspector tell you to rip it out.....
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Old 01-06-2017, 10:35 AM   #10
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Re: can I use/convert a toilet drain to a sink drain


I've been doing alot of reading on venting and plumbing in general.

Being that the vent I capped is on the bottom floor (just before it connects to the main sewer pipe), I suspect it should be OK since even if this dry vent is shared in the 2nd or 3rd floors, it will still connect to the roof vent to get air. And since vent connections to the main vent stack need a wye, water enterring the dry vent is not likely.

As for connecting the new sink, I do have a question. I was originally planning to simply put a 3" to 1.5" adapter into the toilet flange and then use a p or s trap to connect to the new sink using an auto vent. I'm now considering a second option but need your advice: instead I would drill a hole in the floor near the toilet flange and run the 1.5" pipe in the crawl space (which I capped) up this hole, and then I would connect the sink to this 1.5" pipe again using an auto vent. This method would have the 1.5" pipe coming up vertically from the floor. Basically it is the same setup as my original plan since the 1.5" pipe in the crawl (that is now capped that would feed the sink) is tied to this same toilet flange, but this new option would 'look' more proper? Your thoughts? Is it against code to have a vertical sink drain pipe, as most I've seen always have a horizontal extension off the vertical - is this mandatory?

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Old 01-06-2017, 01:04 PM   #11
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Re: can I use/convert a toilet drain to a sink drain


What you're describing is the difference between an s trap and a p trap. An s trap is illegal because the drain turns down before it is vented. It is illegal for a drain to turn down before it reaches the vent because of increased potential of your trap getting sucked dry. That's why S traps are illegal.

Your new idea is moving toward the right idea, but suffers from the same problems. It doesn't really matter if you use the old lav drain or the old toilet drain. To me it would be a lot simpler to cap the pipe that's way too big and use the one that's already sized right. But that part doesn't really matter. The problem is that if you come up through the floor, you're probably proposing an S trap unless you are planning on having a visible pipe or using an AAV.

As for the capped vent, maybe you'd never get precipitation in it (I wouldn't count on that, though), but consider condensation. During winter, you have pipes in a warm house carrying hot water, then air as cold as the outside temperature gets sucked down through them. You will have condensation at least in the capped vent pipe.

If you're determined not to use it, you might be able to eliminate it at its next tee, but that would need to be evaluated on a case-by-case. Right now you've created what amounts to a giant drip tee.

I've made a graphic to describe what I've been trying to say because pictures are worth more.


Last edited by BlackTiger; 01-06-2017 at 01:26 PM.
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Old 01-06-2017, 11:07 PM   #12
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Re: can I use/convert a toilet drain to a sink drain


The third picture is the best way.
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