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In a big home improvement projet we are planning to move the bathroom.

The plumber want to run a new waste line to the basement which imply opening a wall, removing laminate flooring, breaking concrete and digging in the home theater room! This new line would be, horizontally, 7 feet long with a 90 degrees bend and then 3 feet long before going down vertically to the old waste line in the basement.

Of course I would like to avoid all this work and would rather use the existing waste line. I believe we could align the toilet to fit between the floor joist and run a straight 14 feet long waste line to the old waste line. I have a dropped ceiling in the basement and there is a minimum of 8" of clearance and almost 12" near the old waste line. IMHO running this new line would be much more easy and there should be enough clearance for a slope of 1/4" per feet.

Here is some ASCII art (side view) :

                Actually              Plumber new waste line        My suggestion
                                 T           T                          T
1st floor  ======================|    =======|==============|    =======|==============|
                                 |    ,------'              |           `--------------+
                                 |    | (+3' on other axis) |                          |
                                 |    |                     |                          |
Basement   ======================|    |=====================|    ======================|
          <----------------------'  <-+---------------------'   <----------------------'

Plumber said this is "patchwork" and he refused the idea. Do you believe he is right about that?

Note :

  • Venting shouldn't be a problem : there is only one floor so running the vent in the attic and inside the new bathroom walls should be easy I guess.
  • The old waste line will still be used by the kitchen sink but there won't by any additional fixtures.
  • It's not clear what the difference in the two plans is other than route. Do they both tie into an old line at some point? What's the pro's concern, exactly? – isherwood Jul 27 '16 at 19:42
  • @isherwood Yes they will both end in the old line. He only said "patchwork". Please have a look at my diagram. – ForguesR Jul 27 '16 at 19:56
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    I voted to close as Primarily Opinion-based, but I'll give you mine. If your existing drain is known to be in good workable condition, and the new arrangement will have adequate slope, hire a plumber who'll make the connections as you describe. I see absolutely no problem doing that in a remodel situation. Worst-case scenario is that you end up doing the messy, expensive version later. – isherwood Jul 27 '16 at 20:00
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    i have no idea what that diagram is trying to illustrate. – Alaska Man Jul 27 '16 at 20:56
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    If the question wording was changed to say can I tie in to the existing drain line because I don't believe the cost of rerouting is reasonable. Should be acceptable. – Ed Beal Jul 28 '16 at 0:17
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Could be related to fixture units meaning the old line has too much stuff dumping into it to accommodate the new bathroom OR could be your proposed 14' run is too far to wet vent back to the stack (unless you're already planning on a separate vent, then it doesn't matter.

If this is a 4" stack you should be fine on fixture units (unless you have 6 bathrooms upstairs) but you could be pushing the limit on wet venting at 14'. Therefore, run your waste line into existing as you suggest and then run a 2" vent out the back of the closet bend. In your illustration the plumber's proposal on the opposite wall would need a vent anyway so I assume it is going vertically? Run the 2" up that chase instead. I may be missing the boat here, hope this helps.

Personally, I'd redesign the bath to avoid dynamiting the home theater and digging a mine shaft down to the main, that is guaranteed to end in tears.

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