How to Properly Configure This Drain

[Cboneill0099]

I’m getting two bathrooms remodeled and now that we’re up to installing fixtures it’s hitting me it doesn’t look quite right. Hoping to get some advice before I talk to the contractor. Tried to do my best with a diagram and some pictures, but unfortunately the walls are closed up.

It’s a double vanity. The pic with the yellow line shows where the drain runs through the wall and ties into the vent. My drawing tries to show the height off the floor and distance from the vent for the sinks drain connections. The vent is a 4” stack that runs through the roof.

There is also a third sink in play in my guest bathroom which is on the other side of the wall pictured.

The diagram with the single line off the vent, the wye coming up to one drain and continuing to the next drain is how it is now. I believe this is wrong. I drew up the way I think it should have been done. That’s the diagram that splits off the stack and runs vertical lines down to each sink for vents.

Would love some advice from anyone willing to share their knowledge. Really want this done correctly.

Thank you.

 

[wwhitney]

Georgia uses the IPC per up.codes. The IPC allows so-called "common venting" at the same level. That would permit two sink trap arms to join and then proceed to a common vent, as long as the trap arm from each trap to the common vent complies with the trap arm rules. One of those rules is that the trap arm may fall at most one trap diameter between the trap outlet and the vent connection.

So looking at your first drawing (which does not show the 3rd sink), if the wye were pointed out (horizontal, or rather at 2% slope), and if the far sink's trap arm from the trap outlet to vent complied with the trap arm rules (which means that if the slope is the perfect minimum of 1/4" per foot, and the trap is 1-1/2", the trap arm including the portion outside the wall could be at most 6' long), that would be an example of allowed common venting.

But with the wye pointed up it's wrong, the fall on that trap arm is shown at 6". Also the fall from the far trap to the vent is shown as 2", still too much for a 1-1/4" or 1-1/2" lav trap. And with a 3rd sink it's wrong, common venting is only allowed for two fixtures.

Your second drawing is fine; the vertical vent you show above each san-tee that ultimately connects back to the vent stack may be called a revent. With a single horizontal branch near the trap elevation like in your first drawing (at least with respect to the far sink), it's possible to vent everything with a single revent to the stack, rather than the 3 revents shown in your second drawing. But that would require at least part of that horizontal branch in the wall to be a 2" drain. If the details of how to do that are going to be useful to your project, ask and I'll explain the options for that.

BTW, all of the above assumes that the stack labeled "vent" in the drawing has no drainage connections above the drainage connection of the lavs.

Cheers, Wayne

 

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[Cboneill0099]

Georgia uses the IPC per up.codes. The IPC allows so-called "common venting" at the same level. That would permit two sink trap arms to join and then proceed to a common vent, as long as the trap arm from each trap to the common vent complies with the trap arm rules. One of those rules is that the trap arm may fall at most one trap diameter between the trap outlet and the vent connection.

So looking at your first drawing (which does not show the 3rd sink), if the wye were pointed out (horizontal, or rather at 2% slope), and if the far sink's trap arm from the trap outlet to vent complied with the trap arm rules (which means that if the slope is the perfect minimum of 1/4" per foot, and the trap is 1-1/2", the trap arm including the portion outside the wall could be at most 6' long), that would be an example of allowed common venting.

But with the wye pointed up it's wrong, the fall on that trap arm is shown at 6". Also the fall from the far trap to the vent is shown as 2", still too much for a 1-1/4" or 1-1/2" lav trap. And with a 3rd sink it's wrong, common venting is only allowed for two fixtures.

Your second drawing is fine; the vertical vent you show above each san-tee that ultimately connects back to the vent stack may be called a revent. With a single horizontal branch near the trap elevation like in your first drawing (at least with respect to the far sink), it's possible to vent everything with a single revent to the stack, rather than the 3 revents shown in your second drawing. But that would require at least part of that horizontal branch in the wall to be a 2" drain. If the details of how to do that are going to be useful to your project, ask and I'll explain the options for that.

BTW, all of the above assumes that the stack labeled "vent" in the drawing has no drainage connections above the drainage connection of the lavs.

Cheers, Wayne

Thanks Wayne. This is extremely helpful.

The way it’s currently done (first pic) I should have mentioned that horizontal drain is currently 2”. The first pic does not show the 3rd sink. I forgot to include that. That one is probably 12” closer to the vent (to the left of the left sink). It is configured the same way as the left master sink, a wye coming up and 1 1/2” coming out to the p-trap for the sink.

Would love another option with a single revent if having 3 of them is overkill.

 

[wwhitney]

You can do the following in the wall, going downstream from the upstream-most lav (#1), everything horizontal (2% slope), intervening horizontal pipe segments omitted:

Horizontal LT90 (for lav #1) (or a quarter bend for the IPC)
Vent location option 1
Horizontal Combo (for lav #2)
Vent location option 2
Horizontal Combo (for lav #3)
3x3x2 San-tee or Combo at the Stack

The single revent would connect at one of the two locations specified, via an upright combo (horizontal barrel, vertical branch) or for the IPC a san-tee on its back. The revent can be 1-1/2".

With the vent at location 1, you have a horizontal wet vent for lavs 2 and 3. The outlet of the combo for lav #2 needs to be 2"; upstream may be 1-1/2". With the vent at location 2, you have a combination vent for lavs 1 and 2, and a wet vent for lav 3. The outlet of the vent fiting at location 2 must be 2"; upstream may be 1-1/2".

Either way the fall from lav #1 to the vent takeoff is limited to the trap diameter (1-1/4" or 1-1/2"). As long as the trap arms outside the wall are also horizontal (2% slope), that should ensure compliance for all the trap arms.

Cheers, Wayne

 

[Cboneill0099]

You can do the following in the wall, going downstream from the upstream-most lav (#1), everything horizontal (2% slope), intervening horizontal pipe segments omitted:

Horizontal LT90 (for lav #1) (or a quarter bend for the IPC)
Vent location option 1
Horizontal Combo (for lav #2)
Vent location option 2
Horizontal Combo (for lav #3)
3x3x2 San-tee or Combo at the Stack

The single revent would connect at one of the two locations specified, via an upright combo (horizontal barrel, vertical branch) or for the IPC a san-tee on its back. The revent can be 1-1/2".

With the vent at location 1, you have a horizontal wet vent for lavs 2 and 3. The outlet of the combo for lav #2 needs to be 2"; upstream may be 1-1/2". With the vent at location 2, you have a combination vent for lavs 1 and 2, and a wet vent for lav 3. The outlet of the vent fiting at location 2 must be 2"; upstream may be 1-1/2".

Either way the fall from lav #1 to the vent takeoff is limited to the trap diameter (1-1/4" or 1-1/2"). As long as the trap arms outside the wall are also horizontal (2% slope), that should ensure compliance for all the trap arms.

Cheers, Wayne

Thanks Wayne. I’ve had a crash course in plumbing the last week trying to make sure this is done correctly.

After thinking about it, I’d like to do the single revent, but it may be easier to do the 3 revents as long as that’s up to code.

Reason being is the far right drain (labeled master lav 1) is too low for my vanity and needs to be raised up. If we do that, there will be too much drop between that trap and the vent. Right now it’s an 8’ span with 2” pipe for the drain, at 1/4” drop per foot we’re right at the limit, if I understand that correctly.

If we go with the single revent we’d have to raise that tee on the stack the drain connects to. With the 3 revents this doesn’t seem to be an issue because we can leave the drain line where it is?

You are correct, there are no other drainage connections above the drainage connects for the lavs.

 

[wwhitney]

I neglected to mention that in either of the two options I described, the drain downstream of the dry vent connection no longer needs to be strictly horizontal or limited to 2% slope. So you could lose some height downstream of the revent connection, if that's necessary for your desired height of lav 1 compared to the height of your stack connection (which you don't want to raise).

Basically the fixture(s) downstream of the dry vent are being wet vented, and with wet venting the combo itself is considering the vent connection for the trap, so the "one trap diameter" maximum fall only applies from the trap to the combo.

Cheers, Wayne

 

[Cboneill0099]

I neglected to mention that in either of the two options I described, the drain downstream of the dry vent connection no longer needs to be strictly horizontal or limited to 2% slope. So you could lose some height downstream of the revent connection, if that's necessary for your desired height of lav 1 compared to the height of your stack connection (which you don't want to raise).

Basically the fixture(s) downstream of the dry vent are being wet vented, and with wet venting the combo itself is considering the vent connection for the trap, so the "one trap diameter" maximum fall only applies from the trap to the combo.

Cheers, Wayne

Makes sense. I gave it a shot to sketch out what this may look like. Am I remotely close?

 

[wwhitney]

No.

Makes sense. I gave it a shot to sketch out what this may look like. Am I remotely close?

No, I meant that the unspecified horizontal pipe segments connecting two of the horizontal fittings in my list could have an vertical downward offset within them via a pair of 45s, say.

It is true that for Option 1, you can additionally raise the san-tee for lav 2 like you do. But in your last drawing, (a) it makes no sense to have a san-tee for lav 3, as you show nothing coming out the top and (b) lav 3 would violate the 1-1/2" max fall from the trap. But your latest diagram Option 1 would work if you add a second revent out of the lav 3 san-tee.

I thought you said you wanted lav 1 higher than the other lavs. Did I mix that up, and you want it lower than the other lavs? In that case you need at least 2 revents if everything is always draining down or to the left (*). Because the drain from lav 1, being lower than the traps for lavs 2 and 3, can't be used to wet vent lavs 2 and 3.

Cheers, Wayne

(*) If lav 2 and lav 3 traps were sufficiently higher than lav 1 trap, then you could drain lav 3 to the right towards lav 2, at which point or shortly to the right of it, the combined drain turns down and then hits an upright combo to pick up and wet vent lav 1. Lavs 2 and 3 would either be common vents, or lav 3 would be dry vented and wet vent lav 2. The minimum height difference would be determined by the height difference of the horizontal connections to an upright combo with a street quarter bend or san-tee in it.

 

[Cboneill0099]

No.
No, I meant that the unspecified horizontal pipe segments connecting two of the horizontal fittings in my list could have an vertical downward offset within them via a pair of 45s, say.

It is true that for Option 1, you can additionally raise the san-tee for lav 2 like you do. But in your last drawing, (a) it makes no sense to have a san-tee for lav 3, as you show nothing coming out the top and (b) lav 3 would violate the 1-1/2" max fall from the trap. But your latest diagram Option 1 would work if you add a second revent out of the lav 3 san-tee.

I thought you said you wanted lav 1 higher than the other lavs. Did I mix that up, and you want it lower than the other lavs? In that case you need at least 2 revents if everything is always draining down or to the left (*). Because the drain from lav 1, being lower than the traps for lavs 2 and 3, can't be used to wet vent lavs 2 and 3.

Cheers, Wayne

(*) If lav 2 and lav 3 traps were sufficiently higher than lav 1 trap, then you could drain lav 3 to the right towards lav 2, at which point or shortly to the right of it, the combined drain turns down and then hits an upright combo to pick up and wet vent lav 1. Lavs 2 and 3 would either be common vents, or lav 3 would be dry vented and wet vent lav 2. The minimum height difference would be determined by the height difference of the horizontal connections to an upright combo with a street quarter bend or san-tee in it.

You did not misunderstand. I just did a poor job of putting it all together/explaining. But I understand what you’re saying now. Ideally they should all be at the same height.

Right now, the way it was installed originally, lav 1 was lower than the other 2. When I said it needed to be raised up, that was to bring it in line with at least lav 2. The reason why lav 1 is lower than the other 2 is because they came out of the wall with a 90 for that one.

For lavs 2 and 3 they have a wye and came out the top of that for the drain connections. When re-working this id like them all the same height.

 

[wwhitney]

For lavs 2 and 3 they have a wye and came out the top of that for the drain connections. When re-working this id like them all the same height.

Great, and if that height is higher than your existing stack san-tee which you would like to reuse, downstream of the lav 3 connection to the common horizontal drain, you could use a pair of 45s to jog the drain downward. Or if horizontal space is really tight between lav 3 and the stack, a quarter bend (to turn down) and a LT90 (to turn back to horizontal). But that would have a much higher minimum drop achievable (what you get with a street quarter bend going directly into the LT90.

Cheers, Wayne