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I am remodeling my bathroom and have ordered this in-wall tank toilet:

https://www.homedepot.com/p/Swiss-Madison-St-Tropez-Back-to-Wall-Concealed-Tank-Toilet-Bowl-Bundle-in-Glossy-White-SM-WK514-01C/320473696

The wastewater pipe hookup for this toilet is on the wall (not the floor). I need some advice on the best way to run the wastewater pipe to the existing sewer hookup.

Here are the details:

Floor joists: 12 inches high (indicated in blue in the attached screenshot)

Existing sewer hookup: Located in the southwest corner (indicated in purple in the screenshot)

Option 1: Run the pipe from the toilet inside the wall until it reaches the shower, then transition below the floor. This would require cutting 1 or 2 floor joists to route the pipe to the main hookup.

Option 2: Run the pipe from the toilet straight down through the floor, drilling 3 1/4-inch holes through each joist along the way.

My concerns:

Option 1: This route would involve one 90-degree and eight 45-degree turns. Is this a valid concern for wastewater flow and potential clogs?

Option 2: Will drilling 3 1/4-inch holes through the 12-inch joists compromise the floor's structural integrity?

Is there a better, more efficient way to handle this installation? I would greatly appreciate any advice or feedback.

Thank you!

enter image description here

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  • Please search this site for "holes through joists". You'll find dozens of Q&A which will tell you all about what size hole you can drill in relation to the size of the joist. All those bends do carry exactly the concern you've brought up. My preference would be to avoid them.
    – FreeMan
    Commented May 20 at 18:12
  • Got it thank you for that, however by first option is to run it through the wall until the shower. With that many turns, not sure if this is ok. Also not sure if there is another way to run the line.
    – jox
    Commented May 20 at 18:15

2 Answers 2

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I would run the pipe below the joists and deal with a drop ceiling. However, to answer your questions...

Option 1 - May not be physically possible:

The outside diameter of 3" PVC DWV or ABS pipe is 3.5". If your walls are 2x4 construction, the depth of the stud is 3.5", therefore there will be no stud left once you drill out for the pipe. If your walls are 2x6 construction, the studs would be 5.5" deep and once drilling out a 3.75" hole to fit the 3.5" pipe, you would be left with 0.875" of the stud on either side of the pipe.

Even if it is possible to fit the pipe within the walls, the length of the run and the number of changes in direction would be a concern. Low-flow toilets do not do well on long runs without other connected fixtures that add flow to the line. They are prone to buildup as there is insufficient water flow to scour the line. The additional changes of direction will increase this issue.

Option 2 - Possible, but I would not:

Hole Size

If your joists are TJI or manufactured, you must refer to the manufacturer's drilling guidelines. If they are nominal 2x12's, the rule of thumb is that the maximum size of a hole is 1/3 the depth of the floor joist. So a nominal 2x12 would have a depth of 11.25" and result in a maximum hole size of 3.75". A 3.75" hole is the minimum hole size that I would drill for a 3" pipe.

In addition, the hole cannot be in the top or bottom 2" of the joist. (I do not know if there is a limit on the number of sequential joists that you can drill through).

Grade

Next, you need to account for the grade on the pipe. The requirement for 3" pipe is 1/4" per foot. Given that you cannot have your hole within the top or bottom 2" of the joist, the maximum elevation change you have for grade would be 3.75" and at 1/4" per foot, it would allow a maximum run of 15 feet. So as long as the distance between the first joist and the last joist is 15 feet or less you can made the required grade.

Installation

Installing 3" pipe through 3.75" holes in joists is not fun. You may be required to cut the pipe into pieces the width of your joist space and install a coupling in every joist space.

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  • Appreciated! Given this now I am debating swapping the shower with the toilet. However I am also curious about one aspect of option 1 and this comment "Low-flow toilets do not do well on long runs without other connected fixtures that add flow to the line." If the sink is connected to the toilet wastewater? ... or will that be taking a risk and will be much safer to just move the two?
    – jox
    Commented May 21 at 0:55
  • Yes, in option 1, having the sinks drain into the same line as the toilet would be beneficial. However, if you can't add a drop ceiling below but you can swap the shower and toilet, that would be the better solution.
    – pdd
    Commented May 21 at 2:19
  • Yeah, I would think about that swap. If you put the shower where the toilet is now, you only need to deal with running a 1.5" drain pipe through joists, which is likely more possible than running a 3" drain through joists.
    – Huesmann
    Commented May 21 at 12:22
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I would build a new wall behind the toilet to move it out to be away from the rim joist. (Just 2x6 framing, see my attached pic. Please excuse my shaky hand.) Then run the drain straight to the existing sewer line under all the floor joists.

enter image description here

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