Plumbing Rough in for Slab

I don’t see any issues, but wanted to get some feedback. 1/4 inch slope. Toilet is 3", wet vent for toilet and lavatory is 2", shower is 2", shower vent 1 1/2", 2" drain for kitchen sink at the beginning of the run.

For review.


  • P2604.1, P2604.3
  • P2605.1

If you are concerned about the size reduction from the shower/kitchen sink drain line near the WC and Lav drain line it is allowed in that flow direction. However if this is a two story make sure nothing else will be using that line from above as the dfu and pipe size is specifically for that combination.

Is that shale/rock or a shale/rock ledge they cut through?

Thanks for the reply. It’s a single story so it should be good.

That is an old slab that is being covered over by a much bigger slab. This is a metal building out by a pool. It will be a pool house / workshop.

Is the new foundation an engineered design?? Did a Professional Engineer actually approve that??

Kitchen vent? Or is that going in after, above/after drain?

Since it is on an outer wall it is most likely intended to be a wet vent or even an AAV.

Not quite the best bedding material for underground piping.

Is the clothes washer going to be hooked up to that wet vent?

It would still be interesting to note if this was an engineered foundation?

My bad.

No sand no pass;-):wink:

Shouldn’t the drain be plugged, and isn’t the 3 inch pipe supposed to be approx 6-8 foot tall and filled with water for the hydrostatic test to check for leaks?

No it isn’t ! It should be on a sand bed. Thoses rock will someday wear through the pipes and cause it to leak.

Thanks for all the replies. This is out in the country so no city inspector. I agree about the bedding material. The pipes were full of water so I’m assuming that was his version of a hydrostatic test. He is doing the work himself. The slab is not engineered. He said it’s a 5-inch slab with number 3 and number 4 rebar. It’s a single story Mueller steel building. The kitchen sink vent will come off the kitchen drain during install.

Hopefully you are reporting on the significant FU with trying to pour a new slab over that old slab?

Yes they are aware of potential issues with pouring over the old slab. The new slab is longer and wider. With footers all around the old slab. They are willing to take the risk apparently. I was only asked about the plumbing. This was a consultation so no written report. They are long time clients and this was a courtesy. I have referred him to a licensed plumber and structural engineer.

What exactly are the problems with pouring a new slab over an old one? Differential settlement?

Texas has a region of highly expansive soils called The Blackland Prairie which covers a large part of North Texas but also extends down into and around Austin where this Inspector is located. These black soils (black gumbo or black clay) are why foundation repair companies have been so successful here! Also very successful are a lot of licensed Engineers who specialize in residential and commercial foundation design and problem remediation.

These soils are notorious for movement and have literally lifted large commercial buildings causing large foundation problems and failures. This building being built is a relatively lightweight structure compared to even an average sized house. AHJ’s in these areas typically all require a designed foundation for any large structure. Active soils where moisture content can vary significantly can be from 8’ - 15’+ deep. Building in these soils typically does require a clean slate and soil preparation. Even among the black clay there are pockets of sandy loam and heavy rock formations but in this case there is no idea what might be beneath this old slab and around it without proper soil sampling.

Add to the soils problem is what they plan to do with the old slab? What are the chances they would blow it clean and add a bonding agent to try making it a part of the new slab? Even if they claim to be doing that what are the chances of doing it right? Looking at the pictures it appears they may try to cover the old slab with soil and pour on it. These are some of the issues with that.

  • Even if they are lucky enough to compact the soil properly if it becomes wet. which it can easily do, the weight of the structure above can settle pushing those soils out leaving problem voids under the slab. I’ve seen where problems have been caused by mud packing piers causing settlement issues. This is no different.
  • If voids are caused they are collection points for water and that means uneven moisture content of the soils. This is a problem with expansion and movement.
  • These soils can cause the old pad to shift and heave under the new foundation. That can damage anything near it or even above it.

Can nothing at all happen? Absolutely and hopefully it never does. However I suspect they are putting in at least a 30’ X 40’ building. A basic building of that size, without plumbing or utilities, is a healthy $25K - $30K. I know I sure as he11 would not take that chance if it were my building and would ensure my client is well aware of the potentials! All we can do is educate the client and make sure they know what they are getting into.