I'm working on a layout for plumbing under slab. Currently I'm only planning on venting the system using a single vent stack which will also house a clean-out. Will this pass inspection, or do I need to vent directly from each trap?

plumbing layout


Part of the issue is I'm running into is that my plumbing is in the footing of my monolithic slab. Is there any issue with running some of the plumbing for the toilet in the slab to get it to the single vent?

plumbing layout v2

  • Where are you on the planet?
    – mmathis
    Commented May 5, 2017 at 15:50
  • Virginia, US of A Commented May 5, 2017 at 15:55

2 Answers 2


Updated Awnser:

There are a few issues with your updated layout:

  • In a wet-vent, the toilet needs to be the last fixture connected (or be connected via a symmetrical fitting). Your updated sketch has the shower connecting downstream of the toilet. To fix this, you would need to install a 3" double tee (you don't need the wye and 45° as you can change from horizontal to vertical with a 90° but vertical to horizontal with two 45°s). One side would connect to the toilet and the other side would be bushed down for the shower. This is sometimes called a 'potty on the post'.

  • The vertical distance on the shower may exceed what your code allows, this would be addressed in the above revision as the p-trap would be moved up.

Updated Bathroom Sanitary Layout

As far as installing pipe within the footing, that will depend on your local authority. Here we can do it if required so long as the pipe is wrapped in sill gasket (~1/8 thick foam wrap) to allow for some separation between the pipe and concrete.

Original Answer:

I would make a few changes:

  • Take a more direct route to the sink, eliminating some of the 45° fittings (this eliminates a lot of the tight turns and should be less likely to plug)

  • Combine the vertical sink and vent pipe into one 3" vertical pipe (creating a wet vent for all fixtures)

  • Eliminate the s-trap on the shower and use a p-trap

Here's a sketch of the above changes:

Bathroom Sanitary Layout

This would be to code in Canada but I'm not sure of where you live and any differences between codes. For the most part, codes in the USA are close to Canada with small differences and different terminology.

If this is the only plumbing in your house/building, there may be a code requirement that the vent stay 3" to roof as they typically call for one main vent of that size.

  • Thanks for the help. I added an update to my question with a clarification. Commented May 5, 2017 at 18:21

I personally would have at least two vents. I would have a vent directly behind the toilet and one you have pictured. But since I don't know the distance between the vent and fixtures it's hard to be exact. Depending on the distance you may have to move the vent. And I would suggest keeping the same diameter pipe on the main trunk all the way to the shower, and not have it reduced like in the picture. Also I would either move or maybe add a clean out to the end of my line behind where the shower connects.

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