Bill the big difference is that between the tile and the Kerdi you have 1/8" of thinset.

Between the tile and a CPE membrane with a clamping drain, you have not just that 1/8" of thinset but you have a thickness of mud too.

So yes, the advantage of Kerdi is there's less potential wetting of what is under the tile because there is less material to be wet under the tile.

CPE/CPVC membranes and a clamping drain can work fine. The bigger issue with failures of a CPE installation is that installers get lazy and install the membrane flat on the floors, then pile a couple of inches of sloped mud on top of the flat membrane. It's normally not the system, it's the installation that is flawed.

And you're correct, Kerdi does not have any weep holes.

Gotcha. Thanks, Mongo.

Mongo has it right. Just for clarification, I'll add that, with Schluter Kerdi, there is NO pre-slope because there is none needed. The Kerdi on the floor is impermabile (sp) and water cannot penetrate it. Kerdi is adhered to the fleece flange of the drain assembly and the water simply hits the Kerdi and flows into the drain. No "weepholes" are needed as the water never gets into the morter bed.

With a Kerdi shower, you can use it as soon as the Kerdi is adhered. The tile just makes it look good.....

This looks like a mistake. With Kerdi, there IS a preslope.

Any membrane applied on the top is called "topical". They don't need weepholes. There are liquid membranes that you paint on, or trowel on. They dry or cure or set or whatever. Then, you tile over them. Same principle as Kerdi, and been out longer than Kerdi.

Kerdi is a sheet that has to be overlapped or taped. Corners require a lot of attention. A trowel-on membrane has no seams. Many people use these trowel-on liquid membranes.

there IS a preslope under any topical membrane.

The term "pre-slope" implies that there would be a second slope on top of the membrane, which there is NOT with Kerdi.

Yea, I get - this thread has gotten a little misleading. With the kedri/brush-on membreane system, the pan is sloped and it is monolithic - laid down all at once. It's because of the kerdi corner thing that I decided to use the Hydro Ban.

Interesting that companies have developed a "rubber-like" brush on material that will adhere to thinset and support tile. There's some fancy chemistry going on out there.

I wouldn't want a pre-slope solution these days - why build a shower that basically stays wet all the time beneath the tiles. Seems like this welcomes life to grow; the kind of life you don't really want under your bare feet or that keeps you cleaning your shower more often than you'd like. My original question was trying to figure out how much less water the thinset layer would absorb and hold compared to a much thincker layer of deck mud. Maybe thinset is 1/2", tops and the mud + thinset would be, say, 1"+. Also, maybe deck mud absorbs/holds more water than thinset does.

Deck mud does absorb/hold more water than thinset does, per cubic inch. It's mostly made of sand. It has more air spaces in it.

To me, the word "pre slope" could have been eliminated years ago. What we need to say is that the membrane is put onto a sloped surface. Then, that membraned sloped surface receives tile.

My question is, can he use the Kerdi membrane over the mud base?

Yes, that is how I built my Kerdi shower. The base is not necessary for the shower system to work properly. It often isn't the right size for the shower and it is less expensive to use mud. Regardless of whether mud or the Kerdi base is used it is all covered with Kerdi, making it watertight.

Should he use the Kerdi Drain?

Of course he will use the Kerdi drain. It ain't a Kerdi shower without the Kerdi drain.

If you want a lot more information on the Kerdi system I suggest you head over to John Bridge Tile Forums where there are many pros and DIYers who have used the Kerdi system. John Bridge even has an ebook available there on the Kerdi system.

I don't mean to corrupt this thread. I have been searching for info. on the Schluter system and can't find the threads. We are re-doing our two small bathrooms. We are replacing our tub and stall shower with tiled showers. After reading the previous posts regarding the Schluter system, I'm hooked. One of the contractors said he uses the Schluter system, but he will build the shower floor without using the Kerdi Pan. My question is, can he use the Kerdi membrane over the mud base? Should he use the Kerdi Drain? I'm not sure what his hesitation is in not wanting to use the Kerdi pan. I really do not want leaks. He said it would about $300 more for the Kerdi Pan.
Thanks.

The Kerdi pan is not necessary to build a waterproof Kerdi shower, as long as he slopes the shower floor appropriately before applying the Kerdi waterproofing membrane. I prefer the Kerdi drain, but ask him what he intends for the drain and why. The Kerdi pan is not required in order to use the Kerdi drain.

Thank you both for your replies. This forum is a great place for ideas and information!

Of all the Kerdi line, the drain is the best thing. This drain can be membraned with liquid (paint-on) membrane. Slope is always necessary and can be made with anything, not necessarily with a styrofoam pan.

If you haven't seen this already, here's a thread showing Kerdi over a mud base. The first part of the thread is Kerdi technique. The Kerdi-over-mud base is towards the middle. Skip the end which is distracting drivel.

Mud slopes under Kerdi are done all the time, especially with custom-sized showers. A mud slope is faster and less expensive than trying to fit a square peg into a round hole.

Kerdi Trays are easiest to use when the shower is sized to take the tray from the start. The best use of them I've ever had was when I was doing about a dozen showers in an apartment building. I built the bathrooms around the trays. A dozen prefab trays goes a lot faster than a dozen mud bases. Plus they're a lot easier on the body.

But for one-offs or for custom sized showers, in my opinion mud is preferred.

A tray might also be easier for a novice who is concerned about getting a proper slope.

"A tray might also be easier for a novice who is concerned about getting a proper slope. "

Well, that is me I guess. I just put in a Kerdi shower kit and the thing is almost idiot proof. I understand why a professional might not want to use it due to cost, but I can't imagine an easier/better way to DIY a watertight shower.

For me, it has nothing to do with cost. Rather it's about something called "point load". meaning if you drop something, or even push in on the foam with a toe, you'll see it makes a dent. Now obviously, that stiffens greatly when you apply tile and thinset. But I'm still nervous about it. Additionally, I'd much rather mud the pan, anyway, because even if your shower's the right size for a pan to fit, if your drain's even slightly off center, you have two choices-- either toss the foam pan, or cut it up and fill in the holes with mud. I'd just rather do the whole thing in mud and call it good. Then there's no problems. In fact, tomorrow, I'm doing a kerdi locker room shower. I wonder if they might make a foam pan for a 10'x6' shower, with a drain offset by 18"?

Thank you again for your replies! The Kerdi over a Mud Base thread is great!! I have looked at the work of both contractors. The shower base and the grout lines done by the contractor who uses the Kerdi tray were excellent!! The only complaint I have is that he had very small tiles at the end of every other row. I think it could have been planned better. The tile work and grout lines done by the contractor who did not use the Schluter base were also good. The problem I have with his work is that the shower base was sloped toward the drain, but within inches of the drain, the slope increased dramatically. It looked like a hole, rather than the gradual slope of the Kerdi tray. In another install by this contractor, there was a noticeable bulge at the bottom of an outside corner. the other tile work looked great. This contractor is about $1000-$1500 less expensive than the other contractor. Can I mention my concerns to him and see what he says to address them? The following are shower floor dimensions excluding the curb: One shower will be roughly 36"x36". The other shower will be a Neo-angle about 34"x48". However, the 24" door will be on the angle with one side being about 19" and the other side being about 33". Can they or should they use the Kerdi tray as per Mongot's reply? Am I better off with a properly constructed mud base? I hope you can follow my explanation. Thanks again!!

IMO, you'd be better off with the mud pan, PROVIDED you find someone who knows how to install it. From the sound of it, the one guy you've dealt with is using a secondary slope (the dramatic increase next to the drain). I know several installers who do this (I'm not one of them-- I prefer a gradual slope to spec, like you're talking about, as it's much easier to tile! :-) ), But it's usually not SO dramatic that it's that noticeable, and it's not uncomfortable. If it's either of those, it's been done improperly. If they do a pitch that meets the 1/4" per running foot from the longest wall to the drain, even in a larger shower, that's enough to make the water easily run to the drain. I'm doing a 10'x6' shower right now. I'll try to remember to get pics tomorrow and post them tomorrow evening, so you can see. I just mudded the pan this afternoon-- drain is close to center, 5 feet from the furthest wall, so the pitch from all walls to the drain is 1 1/4". I'll guarantee that it'll drain with no problem. In fact I AM literally guaranteeing it!! LOL

Thanks, Bill. I'm looking forward to seeing the pictures. My wife pointed out that the two contractors are about $2300 apart.

10' by 6', that's a lot of mud! Hope your shoulder is holding up okay. lol

Here are the pics. That's a 5 foot straightedge in the pic going across the shower. it's 10 foot long from end to end.

Some day you'll learn to hang your Kerdi vertically. lol

Nice work!

I just went thru that vertically vs horizontally with a half dozen other pros from John's site on facebook!! Of all that posted, one guy hangs it vertically. As I said there-- so long as it provides for a watertight envelope, whatever works best for you. :-)

Stay safe, mister!

Now that's the gradual slope that I like!! It looks great! Thanks for the pictures.