I'm adding a new toilet in my basement. There is a cast iron toilet flange coming out of floor approx 3.5”.

There isn’t a way to build up the floor that high, since it's a basement cement slab.

Any ideas how to shorten this?

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  • 3
    That's not a toilet flange. It's just a coupling hub from what may have been a primary stack. Be sure you can add a toilet there without causing problems.
    – isherwood
    Mar 18 at 13:18
  • Can't you raise the floor use wood?
    – r13
    Mar 18 at 17:29
  • That's for a sink. The other hole that's larger (so that you can fit a trap in there) is for a shower. Again, that's not a toilet flange. Amongst other things, you need a sledge hammer, a month of free time, know how to do cast iron, pour concrete, and properly set a flange so that you don't ultimately still have this question.
    – Mazura
    Mar 19 at 0:35
  • @mazura good thing we’ve been isolating at home for the past year to learn to do this 😂 Sledge Hammer- check, Pour concrete-check, cast iron? Nope, would definitely upgrade if I did your way. But I think I’ll go the poop chute route. Thanks! Mar 19 at 1:40
  • @Mazura - for washer drain. Sometimes people added sinks onto this line right there though.
    – DMoore
    Mar 19 at 16:28

There is almost no way in the world that your piece of cast iron here was designated for a toilet.

Edit based on additional pictures: That is almost surely a washer drain. Good news is it is probably vented. Bad news is it isn't big enough for after truckstop buffet. So everything below is still true but you may only need to bust up until you get to the wye that has the vent line. Also if you are doing sink or shower you need to think about tying these in.

So you cannot just cut it off and add a coupling.

Step 1

Have someone come out and put a camera down there and see where it goes. I am guessing your main stack location changed or you had multiple stacks. This one is no longer in service... but are the pipes still connected and in good condition below that concrete? You don't know and doing a ton of finishing work and then figuring out that this pipe is a bust... Well you would be out thousands plus tons of work time.

Bare minimum alternative: Flush your whole house. Means put a crapload of water from everywhere and flush out the sewer lines.
Find the cleanout on the street. Now... and not kidding... dump some human crap and toilet paper down this line - not a kiddie dump, a truckstop buffet dump. Then dump about 10 gallons of water (or run a hose). You should be able to visually see the crap and toilet paper running through the cleanout. If you don't or it takes a while... you need to go back and have a plumber throw a camera down there.

With this "stack" being open it could be clogged with debris, it could be fully functional, it could have roots in it, who knows. This is anywhere from ready-to-go, to needs a cleanout, to can't ever be used.

Step 2

Let's assume you prove this line works. You still have to vent it. You do not want to deal with a toilet venting problem after finishing parts of a basement. So really you need to design how this will vent. And this is why you can't just cut it out and add a flange. So you need to design how you will vent this thing.


You will have to bust up a little bit of concrete. Based on the picture below (this is how I see your toilet functioning) you need to:

  • bust up some concrete.
  • go over 2-3 feet so you can install in order
  • coupling
  • 45 wye
  • straight PVC
  • 90 degree
  • straight
  • flange

This is the bare minimum and will take ~1 hour to do once you have room to install coupling.

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Step 4 The very hardest step is figuring out where your vent is going to go. It is its own question and depends on where the plumbing is in this house and your other options. A toilet must be vented, so take that 2" piece route it to something that can act as a vent. If you are adding more than a toilet here, you can plan your venting out one location.

Step 5

The slope and height of the pvc needs to be reinforced (I am guess ground under) so you need to fill with pea gravel, drainage rock, then concrete.

Don't let these steps scare you. Everything is very DIY friendly. Do be scared about adding a toilet and finishing this space if you miss one of these steps.

  • Thank you! Crazy part is that my parents have the same exact home (built in 1990, mine in 1991) and this is the same location as their toilet stud. I also questioned main stacks but have nothing from the 1 previous owner about it. Also, there is another smaller pvc pipe, which makes me think it’s for a sink? I’ll add picture above. Mar 18 at 23:47
  • Add more pictures of the whole area and down below the pipe... It could be for a toilet. Given the connector I would say there is very little chance of this but you never know. If you are going with "this is for sure for a toilet" you have to have a camera down there because you need to see that it is vented visually.
    – DMoore
    Mar 18 at 23:50
  • +1 for the truck stop buffet descriptor
    – Kris
    Mar 19 at 0:05
  • Great to know! Thank you! Yes to sink, no to shower. Other side of basement is finished and I want just a small half bath for when guests spend the night. Thanks again! Mar 19 at 16:59

Get a fernco rubber coupling to transition from cast to plastic. Assemble your dry fit plastic piece and fernco to determine the proper height to cut the cast iron. Use an angle grinder to cut the metal to the correct height.

As isherwood notes, this was likely not a toilet flange originally you should make sure that it is attached to your sewer line and has proper slope to the street. You'd probably have to rent a scope (homedepot will rent them). Probably best to ask how to determine if this is safe for a toilet install in a different question.


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