How to Modify 3” stack for second bathroom?

[Cfleming53]

Working on remodeling our one bathroom upstairs Wife wants to add master bath on otherside of the pink wall, including shower toilet and vanity. In existing bathroom/in wall going between proposed master and existing bath, a 3” stack was already in place which vents through roof.

Reachable before carving into downstairs walls is the sanitary t currently running to bathtub/shower. Here’s my plan on replumbing the main stack and running the various things for new bathroom. See a rough diagram I put together via online program. Let me know if I’m wrong/advice/etc. very much amateur when it comes to plumbing. Just want to make sure if I flush a toilet I don’t suck out all the p traps:

I would swap existing San t to existing bathtub shower with 3” san t with side inlet that runs into a wye which wye 3” runs to new toilet which sits on the other side of wall and in the stud bay between the stack stud bay and the current vanity stub out bay and 2” of wye runs to new shower located appx 5/6 feet on other side of wall stud bay where the drywall is cut to. side inlet of replaced san t running to the existing bathroom shower/tub.

Then above new san t with side inlet on stack another San t to run a line to new vanity on other side of wall.

Any issues with this setup? Any way to better set it up? Good way to separate the san t fittings that seem to be fitting to fitting?

Photos: diagram. Pink wall and stack is current bathrooms vanity stub out. Picture of butchered joist is the stack looking down below the floor. Toilet and shower is the existing bathroom where the existing plumbing runs to.

Thank you all in advance!

 

[wwhitney]

This is the top story, so nothing drains into the stack above the existing lavatory?

Right now the stack is venting the lav and the WC, and presumably the shower. Is the fall on that lav trap arm (from the trap under the lav, which is temporarily gone, to the stack) less than one trap diameter (1-1/4" or 1-1/2", depending on which you use)? Likewise on the shower trap arm (from the trap outlet to stack) less than one trap diameter? Those are requirements for any vent for a trap.

If your new vanity is going to join the stack so low, does that mean it's going to have it's own vent takeoff that reconnects to the stack higher up?

By having the new shower join the new WC before they join the stack, you will need to dry vent the shower and have it wet vent the WC. Also, I'd have to check, but I think you might need to have both WC join the stack at the same elevation, below all the other fixtures, i.e. with your configuration the stack can no longer vent the existing WC, and you'd need to separately dry vent it. Not 100% sure, will double check when I get a moment.

Cheers, Wayne

 

[Sponsor]

 

[wwhitney]

MI uses the IPC, and IPC 912.1.1 requires that for vertical wet venting, "Water closet drains shall connect at the same elevation. Other fixture drains shall connect above or at the same elevation as the water closet fixture drains."

So yes, in your proposed diagram, the stack could not be the vent for the new shower or the existing WC.

Chapter 9 Vents: Vents, Michigan Plumbing Code 2021 | UpCodes

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Cheers, Wayne

 

[Cfleming53]

Thank you for the insight, Wayne. Modified design to place both water closets on lowest and same elevation. Used one sanitary tee to branch off of main stack and would use a wye to run both toilets into the San tee.

Higher up stack would connect new shower and existing tub shower to one sanitary tee with outlet. Then higher would be another new San tee for the vanity.

This is second floor with nothing else on this line, connects to main sewer.

Thank you.

 

[wwhitney]

A couple comments: it's slightly off to connect the two WCs with a wye before they join in a single san-tee to the stack. You should really use a 3x3x3x3 double wye, which means that the two WC should approach the stack from opposite sides (although you could put street LT90s into the branch inlets on the double wye, letting the two WC approach the stack from the same side but spread apart enough). There does not appear to be a 3x3x3x3 san-tee with side inlet, so that's not an option.

On the 3x3x3x2 san-tee with side inlet, what you really want is a 3x3x2x2 san-tee with side inlet, which doesn't exist. So I'd suggest a 3x2 bushing in the 3" side inlet hub, rather than a short section of 3" pipe with a 3x2 "reducer". You could also stack two san-tees, one on top of the other, if those heights work out for you. The upper san-tee could be street if you want them closer together.

And just a reminder: the connection order (from the top) of lav/lav/tub/shower (any order) - WC/WC (same level) lets the stack vent everything. And the WC fixture drain length from the closet flange to the stack is unregulated. But for the 4 external traps (2 lavs, tub, and shower), the vent connection for the trap must be within one trap diameter of fall, which at the minimum allowable slope of 1/4" per foot means at most 6' of run for a 1-1/2" trap and 8' of run for a 2" trap, along with no downward offsets.

So if a trap's connection to the stack meets those limits, then the stack can vent the trap. If the trap's connection to the stack has to violate those limits (e.g. the shower trap arm looks very long), you need to take a dry vent off that fixture drain (via an upright combo or if space is tight a san-tee on its back) and run the dry vent vertically upwards to 6" above the fixture flood rim before any horizontal segments, and up to at least 6" above the lav flood rim before connecting that vent back to the stack.

Or if it would be easier, you could do that separate vent for one of the lavs instead, and then route the lav drain to join that trap arm to wet vent.

Cheers, Wayne

 

[Cfleming53]

A couple comments: it's slightly off to connect the two WCs with a wye before they join in a single san-tee to the stack. You should really use a 3x3x3x3 double wye, which means that the two WC should approach the stack from opposite sides (although you could put street LT90s into the branch inlets on the double wye, letting the two WC approach the stack from the same side but spread apart enough). There does not appear to be a 3x3x3x3 san-tee with side inlet, so that's not an option.

On the 3x3x3x2 san-tee with side inlet, what you really want is a 3x3x2x2 san-tee with side inlet, which doesn't exist. So I'd suggest a 3x2 bushing in the 3" side inlet hub, rather than a short section of 3" pipe with a 3x2 "reducer". You could also stack two san-tees, one on top of the other, if those heights work out for you. The upper san-tee could be street if you want them closer together.

And just a reminder: the connection order (from the top) of lav/lav/tub/shower (any order) - WC/WC (same level) lets the stack vent everything. And the WC fixture drain length from the closet flange to the stack is unregulated. But for the 4 external traps (2 lavs, tub, and shower), the vent connection for the trap must be within one trap diameter of fall, which at the minimum allowable slope of 1/4" per foot means at most 6' of run for a 1-1/2" trap and 8' of run for a 2" trap, along with no downward offsets.

So if a trap's connection to the stack meets those limits, then the stack can vent the trap. If the trap's connection to the stack has to violate those limits (e.g. the shower trap arm looks very long), you need to take a dry vent off that fixture drain (via an upright combo or if space is tight a san-tee on its back) and run the dry vent vertically upwards to 6" above the fixture flood rim before any horizontal segments, and up to at least 6" above the lav flood rim before connecting that vent back to the stack.

Or if it would be easier, you could do that separate vent for one of the lavs instead, and then route the lav drain to join that trap arm to wet vent.

Cheers, Wayne

Thank you Wayne! Now I grabbed a double wye for the toilet. It’s not going to fit in the bay and since it’s vertical in reading rules, I can’t offset it there to make it fit to disrupt the vertical fall.

However I over analyze everything, and per my understanding a horizontal wet vent can drain into the stack. So what I’m thinking about is making a horizontal wet vent since I have enough space under the existing bathroom, and running everything to it. Threw together a new diagram. New lav will have an aav up over the flood rim. It’s further my understanding with horizontal wet vent and Michigan ipc, the water closets don’t need to be the lowest or closest to the vertical but can be anywhere on the horizontal line. And I swear that horizontal pipe running to vertical above existing lav will connect, just not too savvy with this inline pipe planner.

Thanks again for dealing with my over engineering madness. Just trying to get it right before I start.

 

[wwhitney]

My comments on the latest rendering:

Double wye should not be used horizontally (that's the short version; there's a longer answer that would allow its use if the barrel is sloped at least 140% of normal)

Your tub trap arm clearly does not satisfy the requirement I stated as "the vent connection for the trap must be within one trap diameter of fall, which at the minimum allowable slope of 1/4" per foot means at most 6' of run for a 1-1/2" trap and 8' of run for a 2" trap, along with no downward offsets." With wet venting, the vent connection is the wye where the fixture drain joins the wet vent (the drain carrying the dry vented fixture). [The "crotch" of the wye, so any rolling up of the wye beyond the minimum eats into the allowable fall.]

Your shower trap arm looks long enough that it could be a problem as well, not sure. Bear in mind that the "one trap diameter of fall" controls, so a 2" shower trap arm can run 8' if you get the fall at a perfect 1/4" per foot; if for whatever reason you end up with twice the slope at 1/2" per foot, now your shower trap arm can only be 4'. Given your rendering, I can't tell if you are relying on steeper than normal slopes to get around your framing, etc.

I see that a double wye in the vertical position is blocked by that stud next to the stack, as seen in the 3rd photo of your OP. So how about two stacked san-tee? The top san-tee could be street if you want the height difference to be minimized. If the orientation of the existing 3" san-tee's inlet will work for your new configuration, it is possible to drill out the pipe segment entering the top of it, so you don't have to replace that san-tee. See "fitting saver" type tools; there are probably some threads here on which brand/style works best.

With two stacked-santees, you could either make them both 3", which would allow one WC to enter the top one if desired, or make the top one 3x3x2, if it is only going to receive some combination of lavs/tub/shower. And with 4 lavs, you could have up to 4 different horizontal wet vents, although 2 is more plausible. E.g. one set of lavs wet vents both WCs which enter the bottom 3" san-tee; the other set of lavs wet vents the tub and shower and enters an upper (street, perhaps) 3x3x2 san-tee. Lots of other possibilities

Instead of two stacked santees, you could use a 3x3x3x2 san-tee with side inlet, bringing both WCs into the 3" entry, and the tub/shower into the 2" entry. Although that would definitely require cutting out the lower san-tee in your existing configuration. And given the depth that a trap hangs below the trap arm, it's possible that the 2" entry would be too low. Note that a tub/shower tailpiece (between fixture outlet and trap inlet) is limited to 24" in height.

Cheers, Wayne