I have just finished stripping a leaky old shower stall in my basement down to the studs and cement floor (took the shower pan out too). Am replacing most of the studs due to mold at the bottom.
Intent is to rebuild a tiled shower stall with door in the same place.

The concrete was not level, had large cracks and was not securely holding the drain in place, so I made the (possibly foolish) decision to break it up and remove it - leaving dirt.

Current plan is to have a plumber in to redo the drain/p-trap before I lay down new concrete sub-floor, then concrete board walls & do the full Schluter shower system before laying tile.

Want to ensure the plumber leaves things the right way.
I think *but am not sure) the right way to go is to have him leave the new 2" (p-trap a few inches below the bottom of the concrete subfloor) pipe sticking up a foot or so with a 4" diam spacer pipe around it.

Assume I then pour the concrete sub-floor, let it dry and cut the 4" spacer pipe flush w/ the surface. Next cut the 2" pipe to attach the kerdi drain to it to fit with the pre-sloped kerdi shower pan on top of the concrete.

Other route that I can think of is to have the plumber attach the kerdi drain to the 2" pipe (no other 4" spacer around it) and then pour the concrete subfloor.

Don't want to 'set things in concrete' the wrong way & have to chip it all out.

Any help greatly appreciated.

The Kerdi tray is not that thick where the drain sits so you will need to have the 4" pipe spacer placed in the concrete pour to allow for the wide portion of the Kerdi drain to fit.

The center section is detachable from the tray so you can have the plumber dry fit everything to the right heights.

Randall - thanks very much for the quick and clear reply.

Two follow-up questions:
1. I have the Schluter kit here - What is the best way to mark the target height of the top surface of the kerdi drain for the plumber ?

2. What is the best concrete to use for the subfloor ?

Assume I have to project the top surface of the concrete sub-floor plus some space for the mud bonding the kerdi tray.
The old concrete was max 2" thick - not sure that meets current code. Want the floor as low as possible (I'm a tall guy & in the old shower stall, my hair brushed the ceiling), though assume top of slab in the rest of the bathroom (not broken up) is a good starting point.

Cheers. Patrick.

It sounds like you have removed the existing shower pan plus the entire concrete slab under the pan. Is this correct?
Remove the center section of the Kerdi tray and use it as a guide for the plumbing rough-in height. You first need to determine your final slab height then using the Kerdi tray pc and Kerdi drain you can calculate the stand pipe height.

Use regular pre bagged concrete mix.

Concrete should be a minimum 3" thick for non load bearing slabs.

Randall,
You are correct - have removed all of the concrete so the plumber can get at the p-trap.

Your answer makes sense, though my main anxiety is projecting the final slab height before pouring it and permanently fitting the kerdi drain before doing so.

After reading your answer, and watching the Schluter video, I think I missed something pretty obvious & am now thinking through the installation this way....

When the plumber puts the new p-trap in, he can dry-fit a 2" standpipe (length not important) and put a 4" spacer around it. I then pour the concrete sub-floor around the 4" pipe.

After the concrete dries, I cut the 4" spacer flush to the surface, remove the dry-fit standpipe, position the Kerdi-drain over top (w/ centre section of the Kerdi shower pan underneath it) and measure for the length of standpipe req'd. Then permanently fit the cut-to-length standpipe.

Next, mud the concrete and put down the Kerdi shower pan. Then lay down mud on the centre section and permanently fit the Kerdi drain into the 2" pipe while pushing it into the mud.

This saves me from permanently fitting the Kerdi drain or cutting the stand pipe before laying the concrete.

Does this make sense or have I missed something again ?

You got it exactly.

Randall - thanks very much. The quality of your answers and speed of turnaround (same can be said about quite a few of the senior members of these fora) is truly incredible.

Will likely have a number of additional questions as I go through the project (which now includes putting down underfloor heating).

In a small attempt to give back, will post photos of a few key stages in the build.

Cheers.

Randall - thanks very much. The quality of your answers and speed of turnaround (same can be said about quite a few of the senior members of these fora) is truly incredible.

Will likely have a number of additional questions as I go through the project (which now includes putting down underfloor heating).

In a small attempt to give back, will post photos of a few key stages in the build.

Cheers.

Unless I'm mistaken, you will want to remove the 4" spacer pipe after the concrete sets, but before it completely dries, since the Kerdi drain is 4" at the thick part where it flanges up. That section will be below the top of the slab unless you have built it up higher.
Your project sounds identical to one I'm working on.
Check this out: kerdi drain rough in
BJ

maybe I'm missing something here. Make sure that your kerdi drain is glued in place first at the right height. You need to set the drain in place before you mud around it. Don't try to glue it afterwards, it may screw up. If you have a schluter tray then the blocks that come with the drain tell you what your finished floor height should be.
If you already know this , then sit back and drink beer!

BJ - you are right - thanks. Great photos and illustrations at the link you provided.

Randy - thank you - good advice. Had not thought through the order of ops for mudding yet.
I have dry-fitted the 2" standpipe joining the p-trap & the Kerdi drain (KD). Once the concrete is dry, I will cut the standpipe to seat the KD at the right height off the slab, then glue the standpipe and KD in place before
mudding, etc.

By way of update -
Had the plumber in on Thursday. Poured the sub-floor this afternoon. The trap and 4" ID spacer pipe are in place. As pointed out by BJ, the KD is 4-1/2" OD for the first inch or so, so I cut the spacer off below the surface of the slab and used a bit of tin as a temp outer spacer until the concrete was firm enough to hold its shape.
The tin is now out, so lots of room for the KD.
If I ever do this again, I'll use a 5" ID spacer pipe.

Two issues I can see coming up:

1. How level is the slab and do I need to pour a bit of Self Levelling Cement (SLC in this forum) to make it "just so" for the shower pan ?
Think I will just place the Kerdi shower pan on the slab & measure for level. Assume mud can fill in very minor dips, but anything more will require SLC.

2. When I seat the standpipe and KD, do I need to fill the entire space between the 4" spacer pipe and the 2" standpipe with mortar ?
From reading the Schluter stuff, does not seem to be critical, but likely smarter to do so than not.

Next step - some framing and cement board mounting - then time to get mudding with the drain & shower pan.

Have photos - will send to Harry once I have a few more steps completed....

if you could send pictre then that would be great.

Photos sent to Harry - hope the file doesn't blow up his inbox....