Side-by-side bathroom remodel

[KDB]

I purchased a small house with side-by-side bathrooms that I plan to remodel. The existing DWV is a mix of a ABS, and copper. I need to do the work as the budget does not allow for a plumber.

I want to move the 3” main stack and a toilet.
I can do the work but am confused as to the correct fitting choices for double fittings.

Currently the main stack has two main fittings. The top one is a 3” double fixture servicing one WC per side. The bottom one is a 3x3x2x2 double wye servicing sinks on one side and shower/washer on the other side.

A lot of the existing plumbing is newer but incorrect. For example shower drain comes out p trap to horizontal then drops vertically to horizontal then meets the vent. The washing machine p trap is under the floor. All drains are 1.5” etc. washing machine is not vented at all. Since I know so much is wrong I am questioning it all. I believe a double wye is incorrect. That makes me worry about the double fixture.

Does the vent need to go straight up through the roof or can it jog over to the existing opening?

Is the double fixture the correct fitting for the WC’s? If not, what?

Is the double wye the correct fitting for the drains? If not, what?

Does the order of fitting stacking matter?

I have already cleaned up the rats nest of copper and pex supply. Converted it to trunk and home run branch.

Thanks

 

[wwhitney]

A double fixture fitting is generally the correct fitting when a stack is venting two fixtures connecting at the same level. However, in the case of (2) WCs, the correct fitting is a double wye. Because WCs don't have a fall limitation on their trap arms, and the double wye will give less cross-communication (water from one side jumping across to the other side).

Any drains tying into the stack below the WCs would need to be separately vented, not vented by the drain connection to the stack. [The separate vent could reconnection to the stack above all the drainage connections, and 6" above the flood rims of the fixtures.] And for connecting vented drains to a stack, a double wye is a good choice.

The stack can offset to connect up to the existing opening. If the offset is at least 6" above the flood rim level of all the fixtures it is venting, then the offset could be done with a horizontal segment (using two 90s). Otherwise you'd need to use (2) 45s, as the diagonal offset that provides is still considered vertical.

Cheers, Wayne