Connecting Kerdi drain to cast iron pipe [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-21-2006, 12:09 AM
After removing my old basement shower and drain hardware, I noticed that the cast iron pipe stub left in the subfloor is pretty much the perfect height for the Kerdi drain and shower pan.

So is it possible to connect an ABS Kerdi drain directly to the cast iron drain pipe?

I was hoping to save the time and expense and jackhammering down to replace the entire cast iron pipe and trap assembly.


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05-21-2006, 12:16 AM
THe only way to make the connection that I have seen posted on this site involves breaking out the floor around the drain, cutting the cast iron, and joining either ABS or PVC using a Fernco-type rubber connector.

How old is your cast-iron plumbing? The trap tends to be the weakest link, and as much as you don't want to replace it now, you'll want to do it even less when a new shower has to be demolished.

05-21-2006, 11:13 AM
Thanks for the reply.

I was hoping on making a direct connection from the Kerdi drain to the cast iron since the cast iron seems to be the perfect height already. The Ferno-connector method would require: cast iron --> fernco --> short ABS section --> Kerdi drain, would it not?

The house is about 60 years old, although the basement plumbing appears to be a little younger - maybe about 30 - 40 years old.

I know that the best way is to replace the whole things from the trap upwards, I just thought it may have been serendipity or something when the cast iron stub ended up being just the right height for the Kerdi drain. :)

05-21-2006, 11:43 AM
What should we call you around here shiu? :)

The minimum you can get by with (as posted) is the Fernco type adapter to a short plastic stub for connection into the Kerdi drain. For two reasons: The CI pipe is not the same outside diameter as the drain connector and there is no way to properly glue CI to plastic.

The reason we recommend replacing the CI trap is because over time it more than likely has become very constricted down there with scale and deposits. Your's might be fine though.

05-21-2006, 11:45 AM
If you really want to find out, try posting at Terry Love's plumbing site (

Just tell them that the Kerdi-Drain has a 2" ABS solvent-weld fitting and see what they say. They're almost as friendly as the guys here.

05-21-2006, 09:12 PM
I took a peek down into the trap. Can't see very much rust or crud down there with my untrained eye.

But I'm probably going to have to rip it out and go with plastic from the trap on upwards at the end of the day after all.

Oh well, thanks for the help guys.

05-22-2006, 06:16 AM
Fernco also makes rubber donuts. These would seal on the inside of the cast iron and the outside of your ABS stub. Naturally, the CI has to be bigger than the ABS, so this probably won't work.

06-11-2006, 04:08 PM
So I've cut down that cast iron drain pipe and test fitted the Fernco fitting and the ABS section in preparation for the Kerdi drain installation.

My question is whether it is necessary (or even advisable) to run a bead of silicone on the inside of the Fernco connector where the ABS and the CI meets "just to be sure". I'm thinking of using GE silicone II.



06-11-2006, 04:11 PM
Fernco couplings are designed to be used without sealants. I've used them numerous times and never had a leak. Just clean any dirt off of the outside of the pipe and tighten the clamp up until it's snug. No sealant needed.

06-11-2006, 04:15 PM
Welcome, Franklin. :)

What Jeff said.

Fernco doesn't recommend the use of any sealants as I recall. I'm guilty of pewtin' a little pipe dope on there when going over old cast iron just to make things slide together a little easier when tightening. That don't mean it's right, though.

My opinion; worth price charged.

And please keep all your questions on the same thread to prevent confusion and duplication. Your thread will rise to the top of the queue with each new post.