Setting Plumbing Under Slab

[Basketcase]

I am in the process of backfilling to prep for my slab. I have to bring up the grade 2' on average.

I need to set my main drain pipe and toilet before I have my finished grade. What height should I have this pipe set at? Should it be cast in the concrete? Or a certain depth below? What height should it exit the building to the septic or storage tank?

I didn't sleeve the walls when pouring the foundation so I have to core a hole where I need to go. I knew this when we were forming.

Should I also bring my electrical in under the slab? I planned on bringing it up outside the wall along with my gas piping.

 

[rburke65]

No expert her, but I would think you Need to know your finish floor elevation. How would you know how much concrete to order, or the size of the garage door to buy?

 

[Basketcase]

No expert her, but I would think you Need to know your finish floor elevation. How would you know how much concrete to order, or the size of the garage door to buy?

Im not exactly sure what you're asking?

I know my finish floor elevation. The walls are poured and I am currently backfilling to get to the finished elevation to pour the slab.

What I am asking is, how far below the slab should the plumbing be? 6"? 2'? 4'?

I want to drop the piping in as I am backfilling instead of getting it up to grade and then trenching it after.

 

[Radix2]

Im not exactly sure what you're asking?

I know my finish floor elevation. The walls are poured and I am currently backfilling to get to the finished elevation to pour the slab.

What I am asking is, how far below the slab should the plumbing be? 6"? 2'? 4'?

I want to drop the piping in as I am backfilling instead of getting it up to grade and then trenching it after.

With plumbing, you have to start with where your water is going, and work back from there.

you are going to a septic system.
Gravity fed drainfield?


Find out the height of your farthest point, this sets your maximum depth, then work back based on distances and slopes.

Usually for a gravity fed septic, your drainfield depth is the constraint.

Estimate your drainfield height if you can do gravity, Work back to your tank height.

Your inflow pipe is about a foot below the top of the tank, out flow a bit deeper.

Now see what you can do to get back to your farthest floor fixture at the proper slope.

Normally you want your pipe below concrete, the distance below does not matter much. If you can do whatever, I'd shoot for 6-12 below your farthest fixture to allow for easy install of sweep elbows, etc.

 

[Basketcase]

With plumbing, you have to start with where you water is going, and work back from there.

Are you going to a septic system?
Gravity fed drainfeild?
Sewer?

Find out the height of your farthest point, this sets your maximum depth, then work back based on distances and slopes.

Usually for a gravity fed septic, your drainfeild depth is the constraint.

Normally you want your pipe below concrete, the distance below does not matter much. If you can do whatever, I'd shoot for 6-12 below your farthest fixture to allow for easy install of sweep elbows, etc.

Thanks,

I am going with either a septic or buried holding tank. And I can dig that to wherever I is needed to be for the slope. The grade around the shop is getting raised 2-3' everywhere, so this isn't really an issue.

The depth below the slab is what Im really looking for. That will give me my height for exiting the poured wall so I can core that out and stub out the pipe. I will connect that up and figure out where my tank will be later down the road.

The shop is in a 4 acre field, so space isn't really an issue.

You can see what I'm working with

 

[Leaflessshadetree]

From memory I set mine for a 1/8" per ft slope. Which was recommended for 4" PVC. I believe 3" PVC would have a steeper slope.

 

[Basketcase]

From memory I set mine for a 1/8" per ft slope. Which was recommended for 4" PVC. I believe 3" PVC would have a steeper slope.

I will also be running 4" PVC and will be using 1/4"/ft slope.

I am in Canada and didn't know if drainage had to run under the frost line (4') or not. It is an empty pipe, so didn't assume so, but was unsure.

 

[readhead]

It may not seem like it but it is a lot easier to backfill to grade and then dig for the plumbing. Most of the rough in will be right below the bottom of the concrete. Most of the time you can hand dig with a shovel.

 

[Basketcase]

It may not seem like it but it is a lot easier to backfill to grade and then dig for the plumbing. Most of the rough in will be right below the bottom of the concrete. Most of the time you can hand dig with a shovel.

That is exactly what I wanted to know! This means I can keep on truckin to bring up the grade and worry about the plumbing later.

Thank you!

 

[Radix2]

That is exactly what I wanted to know! This means I can keep on truckin to bring up the grade and worry about the plumbing later.

Thank you!

I forgot to mention that directly - it would be pretty hard to properly compact and slope with the pipe on loose fill. You really need to fill and compact in lifts, then install your pipe. If you compact with the pipe in place, there is no way to know if it is staying straight.

 

[barnee]

I just did this and in my area the plumbing code required the pipe to exit the foundation at least 18" below ground, so I needed to install the pipe from the foundation wall back to the bathroom area to keep from having to dig that far down if I did it after I backfilled.

After I backfilled I finished the under slab bathroom plumbing (sink, toilet and shower)

 

[850xpeps]

Yes like advised backfill and pack to slab height then excavate for pipes. Make sure you have no bends in the slab. Backfill and pack with sand. Make sure your pipes can’t sink and you’ll be fine.

 

[maxpat82]

Quebec: my home septic exit the house only 18" below grade.
for my garage I have a 65' run to do to connect to it. I exited the wall at 8" to have the slope needed.
that's not much and I would have liked it to be a bit deeper, but that's what I had to work with.

If you're building from scratch: check the code, but I doubt it gonna be less then 18" below grade

Figure the tank spec(location, etc) and code first and work from there. Septic system have very strict code nowadays....

 

[spudley]

Just did exactly what Radix2 advised last week.
Remember, don't go more than 1/4" per foot. Some think a greater slope is better but you need water to carry the waste away.
You have a nice sized building there.