Cast Iron drain/waste pipes under slab

[makethatkerdistick]

I'm trying to think a bit ahead here. My 50 year old slab ranch has 4 in cast iron drain pipe buried under the concrete. So far, aside from one belly on one branch, there are no problems at all, even with low-flow toilets. We also make sure to strain hair at all shower locations and use very little toilet paper. It's working as intended. Whenever I remodeled bathrooms and got to peek into the the CI or had to cut section out, it looked in decent shape.

I have no desire to touch these pipes at this point and hope that they'll keep working for a couple more years at least. But at some point they'll probably corrode at the bottom and need replacement. When that happens I want to have a plan and the funds to replace them. Cast iron can be long-lived (like 100 years) but from what I've heard this is not the case in buried application. I've been told that my pipes statistically are nearing the end of their life.

My preference would be to repipe with PVC, minimizing the piping under the slab and completing the runs in the backyard whenever possible. Two of the three bathrooms have their drains located on an outside wall. The other bathroom, the laundry room and the kitchen are a bit farther in but are close to each other, minimizing the slab cutting that might have to happen in this area.
Since we have expansive clay soils here, I wonder if I get more durability and stability by using Schedule 80 piping? Is this something that plumbers consider? Or is this wasted money?

Another option would be to reline the drain pipes with an epoxy treatment. I would never consider doing this with pressurized piping (like water supply) but would consider it for this application. Ideally, this would only make sense if we got good service life out of this (I'd think at least 20+ years).

I know this all depends on so many more specific variables but what is your professional inclination? PVC Schedule 40 or 80? Epoxy relining? Just trying to gather some impressions here.

Thanks so much!

 

[Reach4]

I am not a plumber. This is just a theoretical discussion. I suspect your cast iron will last fine, so ideally, this will be just a mental exercise. For your plan ahead thinking, I wanted to point out that there is something in between the 4 inch DWV pvc (cellular core) in the local store and schedule 80 pressure-rated pipe. I am thinking of schedule 4o pressure-rated pipe (ASTM D 1785).

https://www.charlottepipe.com/Documents/Brochures/BR-PK.pdf page 14 gets into it.

Also, 3 inch PVC is often plenty. So in your planning, where you are feeding from 3 inch or less, you could also consider 3 inch. PVC is smoother than cast. However you normally can't reduce from 4 to 3. But where you can, you might consider a switch to 3.

Another thing to consider is that it can often be easier to run a drain line outside of the perimeter of the house and take the long way. Just abandon failed cast under the slab. The required slope gets smaller with bigger pipes. If you do that, do put in an outside cleanout.

 

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[Sylvan]

Point of information "But at some point they'll probably corrode at the bottom and need replacement"

Would you believe tat most cast iron failures are from the top of the pipe not the bottom

The 5" lines pictured below had 104 feet was cracked on the top because of the sewer gases ( poor venting) were allowed to destroy the CI soil line

External corrosion is caused by exposure to high levels of acidity in the soil caused by hydrogen sulfide, salt, and other acids. Moisture-rich environments like Florida

Drain cleaners are amazing for killing piping as the ingredients are extremely harmful t ferrous metal system as well as non ferrous metal piping

To much pitch is worse then to little as the velocity will cause erosion at the bottom