Basement bathroom DWV design question

[not-a-plumber-ma]

I'm not a plumber and am trying to understand why things were done a certain way. The plumbers who did the work were not able to give me an explanation that I felt made sense, so I'm asking here to see if someone can explain it better.

I am in the process of building a house. Before I can pour the concrete floor I needed to have the DWV lines installed as they will be in/under the concrete. I recently hired two plumbers who did the work. Some of the work is pictured below.

I asked them why they did not plumb the lav sink from a wye on the same 3" line as the laundry sink (they are on opposite sides of a common wall) and the shower drain from a wye in the 4" stack line nearest to the shower.

My reason for asking them was because the required slope of the pipes made these lines be well above the top of the 4" of concrete. When I hired the plumbers I gave them measured plans including section views showing how much height they had from the sewer line to the top of the concrete. I assumed they would carefully route the lines so they stayed in and below the concrete.

When I originally hired them for the work and gave them the plans they told me there would be no issue staying below the 4" of concrete. After building these lines (which appeared to me somewhat of an on-the-fly process), they told me there was no other way to plumb this bathroom than they way they did and I would just need to pour 6" or 7" of concrete instead. That would be an option, but it reduces the ceiling height below 7'6" and makes this no longer conform to code for habitable space.

If this was the only way to plumb things I can accept that, I just would like to know why it could not have been done a different way.

This is in Massachusetts in case our specific plumbing code may make a difference.

Thanks!

(Second photo is from the other direction, you don't see the 4" stack and line in it, but it would be just at the bottom and left of the photo)

 

[Tuttles Revenge]

The answer why they didn't plumb the laundry and bathroom sinks together is because the bathroom is plumbed using horizontal wet venting and ONLY bathroom fixtures are allowed to connect into that portion of the system, the laundry ties in downstream of the horizontal wet vent system.

Could it have been plumbed differently. Yes. Could it have been plumbed better for your specific needs.. possibly.

Is the point of connection as Low as it possibly could be made? 4" pipe can be graded to 1/8" per foot slope. I'd check that to see if you an Eeek out some inches. Are the runs as efficient as possible... ie as short as possible. A lot of installers avoid using 45's because they're more complicated to figure out angles.. but they can shave precious inches in a very long run.

 

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[not-a-plumber-ma]

If I understand you correctly then it is plumbed this way because:

a) the toilet needs a vent
b) if the shower and sink lines ran to other points then the toilet looses its vent
c) the toilet cannot use the laundry sink vent because it is not part of the bathroom group
d) trying to add a dedicated vent just for the toilet would make things even more complicated/problematic

I did consider asking about changing the 3" to a 4". However by my math -- 10' @ 1/4 = 2-1/2" rise. Going with 4" -> 10' @ 1/8 = 1-1/4" and add 1" more pipe diameter = 2-1/4" rise and you haven't really saved any meaningful height.

In the end, I think it is what it is - especially now that I know the layout does make sense. I'm going to be an optimist and be happy the basement isn't ending up habitable space because now they can't tax me on it.

Thank you very much for your input and explanation.