Plumbing a bathroom on second floor. I worked on 2 plans, which one is best?

[stlows]

Hi !
First post.
We want to add a second bathroom to our house on the second floor. We have 8" floor joist (real 8"). 16" appart.

My rules were:
- Don't pass through a joist with the toilet 3" drain pipe.
- Good venting
- Run the main 3" drain parallel to joist (between 2 of them) to go down to the chute (we already have a box in a wall on the forst floor) to the basement. In the basement, I'll make a box to run the pipes to the main drain stack.

Here are my 2 plans I worked on. Also including a general plan of the bathroom.

Which of them would be best or any other comments would be appreciated !

My plan is to use 2" pipes for shower and the sink. The vent will go from the sink to the roof. I will be using 2" pipe from there for wet venting the shower and the toilet.

 

[wwhitney]

The use of a double wye with the barrel horizontal is marginal. That's because you can't independently adjust the slope of the two branch inlets. Best case you get them equal, and then their slope is 70% of the slope of the barrel. So to make them at least 2% slope the barrel would need to be 3% slope.

Better just to use two individual wyes, one after the other. That raises the question of whether your plumbing code requires the WC to be last on the wet vent. If it does, then your 1st option doesn't work.

On the second option, another question is what your plumbing allows as the maximum distance form the shower trap to its vent, which is the wye where the shower drain joins the lav. The maximum allowable is 8' if you run the 2" shower trap arm at exactly 1/4" per foot slope, but some plumbing codes have a shorter limit. [The fall from trap outlet to vent is always limited to one pipe diameter, as otherwise the trap arm can become a siphon.]

So I would suggest a combination of the two plans. Run the shower like in the second option, but run the lav like in the first option. The lav drain exits the wall into the joist system running perpendicular in plan to the joists. After going through the joist, it hits a 45 (possibly street), then a 2" wye (possibly street) to receive the shower drain (barrel on the diagonal), then a 3x3x2 wye to join the WC drain (barrel parallel to the joist). I think that would all fit in one joist bay, but it bears confirming. Also, this avoids having to drill any joists on a 45, which is harder to do and results in a larger hole.

Cheers, Wayne