I want to add a tub to my master bath that currently has a shower, two sinks, and a toilet. See attached photo for drain layout and proposed addition. It looks to me option A would be the easiest way to do it. This would tie the tub drain into the 3" line after the lavatory drain/vent. A remodeling contractor suggested option B, extending the 3" drain along the wall and tying the tub drain into that. Option B seems like a lot more work. Is there a problem with Option A I'm missing? The 2018 IRC applies here.

[1drv.ms]

Thanks.

You can't do either unless you install a "new vent" for it.

Wouldn't it be wet vented like the rest of the bathroom group?

the only way it would be 'wet vented' would be if you put the "Y" into the 2 inch lav drain.

I thought the existing group is already wet vented. The 2" from the lavs extends above the lavs into the vent system. I thought it's an existing wet vent from where the two drains from the lavs join the vertical 2" pipe to the point where the shower drain joins the 3" pipe. There are no other vents for the toilet or for the shower. Looks like there's something I don't understand. Are you saying the new tub drain would have to join to a double wye directly opposite of where the 2" from the lavs and vent joins the 3"? In any event, isn't this still a wet vent down to where the shower drain joins the 3"?

'groups' are not wet vented

fixtures are wet vented

you can not 'wet vent' one fixture by using another wet vented fixture


(wet venting, while code compliant, is second best and NOT best practice in and of itself)



by your logic ONE fixture vent could wet vent an entire home

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

if the shower drain drops directly into the 3 inch then it is not properly vented.
i can't tell from the photo if that is the case?
a wet vent in laymans terms is when the drain of one fixture is also used as a vent for another.
if you tie the tub into the lav drain then the upstream portion of the lav drain (being properly vented) acts as a vent for the tub as well.
if the shower were to tie into the lav drain then it would be wet vented thru the lav.
so, the toilet is wet vented thru the lav but the shower nor your proposed tub would not be.

bern, we've argued over this subject before.
in my opinion 'wet venting' is better than dry venting.
reason..
in the case of a toilet being wet vented thru a lav drain.
the lav drain by code must be increased one pipe size. 1 1/2 to 2 inch.
if this drain becomes clogged thus eliminating the toilet venting, you will know because the lav will not drain.
if the toilet is individually vented with no other fixture able to 'wash' the vent, if the vent becomes clogged you will never know and in most cases never care.
so for this reason i am a big fan of wet venting.

we have never argued


we have, however, debated


i am almost convinced re: wet venting a toilet

but

remain adament re: non-sanitary fixtures




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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Thank you all for your responses. I understand some may not agree with things allowed by code, or there may be better ways than what code allows. I can see all the drain lines from underneath the floor. The shower drain does in fact pop directly into the 3" drain. The toilet does not have a separate vent. The 2" comes from the lavs. This is a brand new house, and if in fact it isn't vented correctly, that's a pretty major blunder. What led me to believe the wet vent is between the lavs and the shower drain is this portion of the 2018 IRC: "Any combination of fixtures within two bathroom groups located on the same floor level shall be permitted to be vented by a horizontal wet vent. The wet vent shall be considered to be the vent for the fixtures and shall extend from the connection of the dry vent along the direction of the flow in the drain pipe to the most downstream fixture drain connection. Each fixture drain shall connect horizontally to the horizontal branch being wet vented or shall have a dry vent. Each wet-vented fixture drain shall connect independently to the horizontal wet vent. Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain." When I say "bathroom group" I mean the fixtures within a bathroom. I'll consult with the local authority on what is permitted here. Interesting discussion--thanks.



Edited 1 times.

1002.3 - Prohibited Traps - #5 "S" traps

The shower has an S trap.

the section of 3 inch pipe you refer to is NOT a vent. it is a toilet drain that is vented thru the 2 inch lav drain/vent.
as i said earlier if the tub is connected to the 2 inch (which is considered the toilet vent) then by the definition you posted, you are within the code.
look again at the shower drain. are you sure that is a 90 where it drops down into the 3 inch. could it be a sanitary tee standing upright with a vent coming off the top?

Thanks for the additional explanation. I follow what you said. This photo is from a different angle and shows the entire shower drain--no separate vent: [1drv.ms]

wow... you have a generous inspector.
that shower drain is 100% illegal.
also the pipes running between the joists are supported by metal bridging? that too is not correct.
i would at least slide some rubber between the metal and the pipe.
what is that black thing holding the toilet pipe up?
while i'm being critical, no cleanout in the 2 inch lav drain. how you supposed to snake it when/if it clogs?
also, can you see or feel any mortar or plaster under the shower base. look or fish around thru the hole at the shower drain fitting.

I appreciate the information. Yep, pipes between joists are on metal supports. Where the horizontal pipe is against the basement wall it's on J supports attached to the joists. Vertical pipes have steel strapping wrapped around them and attached to the studs on either side. Looks like everything else connected to the 3" pipe between the shower and where it exits the wall to the public sewer has it's own vent. The black thing is actually a strap holding up the duct from the bathroom exhaust fan, so it has nothing to do with the pipe other than being in the same space. If we ever have to snake the drain I guess it's up on the roof and hope we can make all the 90 degree bends (or cut into the 2". I do see mortar under the shower base. I think I'll consult an architect/engineer buddy who has a local mechanical/electrical/plumbing design practice and see if this is normal around here.

the cement under the shower base is a good thing.