DWV Question for Cast Iron San Tee Replacement

[JBurt09]

Hello everyone! Working on a Florida block home built in the 60s that most of the cast iron drains have been replaced in but still a bit lingering around in places we're working to replace.

The toilets in the hall bath and master bath are back to back and drain into a 4" cast iron double sanitary tee that has a 2" inlet on one side that the master shower and hall tub/shower used to connect together and drain into. Bottom of the double sanitary tee goes out to septic and top goes out the root to vent.

The master shower stopped draining and upon inspection found the horizontal from the p trap to the wye was completely collapsed. I've since cut and capped that section and I was looking into swapping with a PVC double sanitary tee with a side inlet (like here), but after reading everything I could find on many forums it appears this is not advisable; however, it seems that a double fixture tee is not advisable either. The double fixture and/or double wye will require some rework / rerouting of the hall tub/shower and the master shower with how to get those in and still vented.

Thanks in advance for the insights and feedback.

 

[breplum]

Back to back WCs need a double fixture tee, though, most people don't really have any major reason to replace an old CI double tee. I still live with my double tee...yeah, sure, when one WC flushes it slightly moves the water in the bowl...no issues for me. Some WCs are more sensitive than others. . below that double fixture tee then would be where to add other fittings.
Old galv. water supply piping is bad to have.

 

[JBurt09]

Back to back WCs need a double fixture tee, though, most people don't really have any major reason to replace an old CI double tee. I still live with my double tee...yeah, sure, when one WC flushes it slightly moves the water in the bowl...no issues for me. Some WCs are more sensitive than others. . below that double fixture tee then would be where to add other fittings.
Old galv. water supply piping is bad to have.

Thank you for taking the time to provide your expertise! Just because I know terminology seems to differ from place to place and person to person, can you post a picture of what you're referring to when you say a double fixture tee? I'm seeing some places where folks use double fixture tee and double combination wye interchangeably...

Also...I can definitely wye in (or whatever the appropriate fitting is) the shower and tub/shower below but the drain slope is going to be far greater than 1/4" per ft - is this a problem?

 

[breplum]

Trap arms for shower/tubs or any fixture must be 1/4" per ft. After the vent for the fixture, then greater than 1/4" per ft slope is fine.
You can get arguments about solids not draining when slope is too steep but that is hogwash to my knowledge.

Depending on the material of the fitting it can be hard for outsiders to grock. Fig. 5 is also a common reference.

 

[JBurt09]

Trap arms for shower/tubs or any fixture must be 1/4" per ft. After the vent for the fixture, then greater than 1/4" per ft slope is fine.
You can get arguments about solids not draining when slope is too steep but that is hogwash to my knowledge.

Depending on the material of the fitting it can be hard for outsiders to grock. Fig. 5 is also a common reference. View attachment 43357View attachment 43356
View attachment 43355

The amount of back and forth I've been able to find online about the double wye vs double fixture vs double san tee is truly impressive. More hotly debated than American politics! From the siphoning in the pipe versus the toilet being self siphoning, the weir lines in relation to the vent and the suction, and the water level drops in the toilets. I can't believe there's so much contentious debate on something that is grounded in science and physics.

One thing I was wondering/considering was getting rid of the t stack completely and dropping both toilets and showers into a horizontal run and then wye the toilets and showers into it. Thoughts there?

Thank you again for all of your help and expertise breplum!