Under Slab Waste Pipe

Under Slab Waste Pipe - Capacity Questions

First, this is a new thread to ask a couple of questions that resulted from an older thread. I don't want everyone to have to read thru the original thread to answer.

Second, thanks to everyone who responded to the first thread. You have provided lots of food for thought. Some final questions have come from that earlier discussion.

1. It is roughly a 24 foot horizontal run of waste pipe, under the slab, from one side of the basement to the other. Originally, I wanted to use that pipe to carry 2 full baths (2nd floor and basement), a 1/2 bath, and washing machine, and a kitchen. I am under the impression that those waste sources would exceed the capacity of a 3" horizontal waste pipe, so I was looking at a 4" pipe. But, I have been told that with low flow fixtures, it is not a good idea to run 4" waste pipe for an extended length. I quote just one source, "In new construction, the 4-in. pipe is now too wide for long, horizontal runs (especially between the home's main vertical drain, or stack, and the street)."

Do you agree? Disagree? Is a 24 foot run a "long" run?

2. Would I be better off staying with the 3" horizontal under-slab run, even if that means rerouting pipes from some of the sources to get under the fixture limit for 3" pipe? (I am using the attached chart, "Fixture Unit Values.pdf," to give me a rough idea of fixture unit values and the capacity of 3" and 4" pipe at various pitches. Does anyone have a serious disagreement with the values in the chart? - I am using them only for this exercise.)

If we say the under-slab pipe runs left to right, with the street exit on the right, then I want to empty into the left side with pipes from one full bath (basement level) and the kitchen (first floor level). I would empty into the right side with pipes from the washing machine (first floor level), the 1/2 bath (first floor level) and 2 full baths (first floor level and second floor level). Entering on the right side would be very close to the exit to the street.

Anything you see wrong with this idea? (Note: this job will be done by a qualified plumber, working for my GC. Note 2: I am ignoring venting and the like, on the assumption that venting would be designed by the plumber and would not be a problem, but if you see something that would prevent proper venting, please let me know.)


There are 2 diagrams appended. One shows a 3" under-slab pipe; the other shows a 4". Also, again, these are not construction drawings - just done to show the general routing of the main pipes. I apologize in advance for the drawings, especially since some of the other posters have done excellent 3-D views.

Finally, many thanks in advance for your thoughts.

Steve

Most homes are plumbed with 3" pipes as long as there are only three bathrooms (toilets)
The fourth toilet requires 4" at that point.

Building sewers (outside the foundation) all seem to be at least 4"

I've seen the report that David mentions, and 3" does have better "carry".
The reason that 4" is required at some point, is traffic logistics.
When you get too many cars on the highway, you need to add some lanes.

pipe

Here, 3" waste pipes are run all the way to the city's sewer connection, as long as there are only 3 toilets or less.

Thank you.

Thank you. Thank you. Thank you.

I have to tell you that it was your efforts in helping me understand what could and could not be done that allowed me to push enough to get a second effort from the design team.

Thanks to your input, the plumber hired by my contractor has revised all the piping to meet my requirements. I now have no pipes on the basement walls that would prevent the placement of cabinetry. In addition, I got a floor drain, eliminating the need for a sump pump.

Thanks to all for your great and helpful input.

Steve20A

Glad you got what you wanted Steve.