I'm sure the professionals will be along shortly to reassure you but meanwhile, we used CBU (I think it was Hardi) in our MB shower and it worked great. This time around we're using Wonderboard. Like you, we want the extra protection. Yes, CBU needs to be kept moist while applying the Kerdi (spray with water bottle) but it's not that difficult once you get the hang of it.

And when you mix up your thinset mix it very loose - like pancake batter.

I didn't want to use the schluter recommended drywall b/c if the adjacent bathroom's shower install failed eventually one day b/c of lack of vapor barrier, I didn't want the drywall to be weakened through the back side.

Smart move.

I didn't leave the 1/4" gap b/c I didn't read the instructions. How bad is this?

It's completely immaterial, being that there will be a membrane between the hardi and everything else.

As for using Hardiboard, I'm with Mongo on this. I use Durock almost exclusively. That's not to say you can't use the Hardi, though.

Bill & Mongo - someone over at johnbridge forums seemed to think that the 1/4 gap was going to be necessary to prevent the grout/tile cracking if the hardi ever got wet and expanded and pushed off the subfloor. I tried scoring the 1/4" off as Mongo suggested but after about a 6" length in a half hour I gave up. Are you positive that it doesn't matter? I really don't want to take the boards down, so I like your answer, it's just that I don't want my tile to crack later either.

As a side question - what's your recommended technique for packing the mud under the wide Kerdi drain? Mine comes in today and I'll be pouring the shower deck mud soon. This is upstairs so I do not have access to the plumbing from below. It's a 1 shot deal for me since the ABS glue sets up so quickly.

So you want the 1/4" gap in the shower, at the wall-floor junction, right?

If so, just as Bill wrote, no need to worry. If the hardie is down to the subfloor, when you pack your preslope deck mud perimeter, you're going to have a few inches of floor deck mud packed tight, right up against the hardie.

That will lock the hardie in place.

These photos are may be slightly different than your proposed order of work, but in this shower I hung the cement board on the walls and hung hardie on the ceiling.

I then Kedied the walls and ceiling.

I then tiled the walls, leaving the bottom course of wall tile off, and tiled the ceiling.

Then I packed the deck mud, right up tight against the kerdied walls.

Then I Kerdied the floor, tiled it, then filled in the bottom course of wall tile.

As to the drain installation, I had the same setup here. No access to the plumbing from below. After the P-trap was glued in place, I dry fit the stub and drain so the flange stood the required distance off the floor and so the flange was LEVEL.

Then as you're packing the deck mud, when it's time to set the drain, mix mud a little looser than the dry pack mix, set a generous ring of loose mud around the drain opening in the floor. Glue the drain stub and set the drain in place, working the stub of the drain into the p-trap and the flange of the drain into the loose mud, all in one smooth motion.

Clear as...deck mud?

Mongo

Awesome.....

Now if I can only do the drain as smoothly as your description....I've heard about 1 1/4" of mud under the drain. Is this what you typically do? Then just slope it up 1/4" per foot to the far wall, mark the screed, and off you go?

Some other quick questions, since I got someone answering questions.....

1) Around the mixing valve & showerhead - how close do you cut the cement board? I did the piece around the valve last night and have about 1/8" - 1/4" gap between the board and the plastic valve protector. Had some pretty nice measuring. I'm going to do the piece around the shower head tonight and need to know how close to cut that one. Is this a suitable gap or more/less?

2) When Kerdi'ing, what's the recommended technique around these protrusions....kerdi right up to it and caulk? Kerdi and caulk around trim?

3) Should I put the finished chrome shower head stub into the brass bend BEFORE I put the wall up w/ teflon tape and get the downward angle for the showerhead just right? Right now there's a raw galvonized dummy stub that the plumber put in.

I would seal the backer with a bonding agent such as Link or Plasterweld. This will still allow the thinset to stick without drawing out the moisture, which will allow an amateur such as you or I a lot more working time to set the Kerdi nice and flat and with a good bond. I only wish I had known about this before I started my tub surround. The hardi has an insatiable thirst. Use a 1/2x1/2 notch trowel too.
Casey

This is the first I've heard of sealing CBU when using kerdi. And also that trowel is way bigger than the 1/4" x 3/16" recommended by schluter & in john bridge's kerdi book. Confused.....

No sealing. Not even with a bonding agent. As for the 1/2x1/2" notch, don't even try it. Especially as loose as you need to mix the thinset, you'll never get the first trowel full combed out before you're heaving the trowel out the window. and then after, forget about trying to flatten it out ans squeezing out the excess thinset!

1) Around the mixing valve & showerhead - how close do you cut the cement board?

You did fine around the mixing valve. As for the showerhead, cut a hole about 1 1/2" diameter, and you should be fine.

2) When Kerdi'ing, what's the recommended technique around these protrusions....kerdi right up to it and caulk?

Just cover the cement board. The face plate for the mixing valve has a gasket on the back of it, and at the showerhead, unless this is a steam shower, it won't ever see any moisture past the escucheon.

3) Should I put the finished chrome shower head stub into the brass bend BEFORE I put the wall up w/ teflon tape and get the downward angle for the showerhead just right? Right now there's a raw galvonized dummy stub that the plumber put in.

Leave the "dummy stub" there for now.

Thanks Bill! Just wondering - why leave the dummy stub there now? Why not go ahead and put in the finish piece ....is the only reason I might scuff it up? Will hand tightening suffice on the finish piece? The dummy stub the plumber left is pretty dang tight.

"I've heard about 1 1/4" of mud under the drain. Is this what you typically do? Then just slope it up 1/4" per foot to the far wall, mark the screed, and off you go?"

That'll work fine. I think the recommended is 3/4" to 1-1/4", but yeah, I tend towards thicker.

Yup, take the longest wall measurement from the drain, figure 1/4" vertical for each foot of run, and add that to the 1-1/4" flange thickness. Let's use 3', so add an additional 3/4" for a total of 2" thickness at the floor perimeter.

Two other quick cross-checks are that code requires the slope to be a minimum of 1/4" per foot and a max of 1/2" per foot, and the height of the curb is required to be a minimum of 2" above the height of the drain.

So let's figure that the 2" thickness is good. You can attack the layout in a couple of ways:

1) Strike a line around the perimeter 2" of the floor, and pack mud to that height.

2) Use a screed board. If I go that route I rip a 2x4 at an angle, then rip it to the required height, in this case 2". The screed board ripped from the 2x4 will look like an upside-down right triangle. Squint and really use your imagination, it sort of looks like this:

____
I/////
I///
-/

The flat top is about 3/4" wide, the straight left side is 2" tall and goes against the wall, the bottom is about 1/4" wide and the "/" that make up the right side, that's the angled cut.

Place those screed boards around the perimeter of the floor and pack deck mud up against them, the deck mud will hold them to the wall.

Oh, and "pack" means PACK. Your deck mud is not concrete that easily pours or mortar that contains water-absorbing lime, it's a lean mixture, about 4 or 5 parts sand to one part portland cement. Mix it dry, then add just enough water so it looks like beach sand that's just moist enough to hold its shape so you can build a sand castle. Grab a fistful of deck mud and squeeze it, it should be just wet enough to hold its ball shape when you open your hand. No water should squeeze out.

Dump the buckets of deck mud, then pack it into place around the perimeter of the floor. A wood trowel works well, I'll also use a piece of scrap 2-by lumber and hit that with a mallet. You pack it so it's tight and there are no voids.

Anyhow, I pack the perimeter first, then work, sloping down, to the drain. When I get about a foot from the drain I set the drain flange, then fill in between the drain and the floor mud.

Somewhere in there I'll pull out the wedge-shaped screed boards and pack additional mud into the wedge-shaped void.

Take a step back and give it a good look-see, use a straight edge to make sure the slope is consistent and there are no humps or divots.

Then have a beer, although if this is your first mud pan your shoulder might be too sore to raise the bottle to your mouth. It's a good time to be ambidextrous.

If you come back the next day and find a high spot, the beauty of deck mud is that it can be shaved down a bit with the edge of a trowel. Remember, there's enough portland cement in the mix to bind it together, but no so much that it cures hard like concrete.

One other thought about people having trouble getting the Kerdi to bond to hardie. Realize that when you trowel and comb the thinset on the hardie, then set the Kerdi, you can come back even a few hours later and take the kerdi right off the wall with your pinky finger. It (thinset) really does need to fully cure before you get a good bond.

Did you see this Kerdi Shower thread? It's the thread from which I linked the above photos. Please don't post on it, as it'll be deleted if it hits 150 posts.

Mongo

I did see it - it was great....it will likely be my main source of info as I do this... thanks for the replies!