1) Tiled floor using cement board: If you have a plywood or OSB-type of subfloor that meets all the deflection requirements, and the surface is clean and in good condition, you can install cement board right over it. Bed it in thinset using a 1/4" notched trowel and then nails or screws to fasten the CB to the subfloor. The thinset is not for adhering the cement board to the subfloor. The fasteners do that. The thinset is simply a "gap filler" to ensure that there will be no micro-voids resulting in vertical flexing/movement of the cement board when it is walked upon.

The thinset is not optional. It's mandatory. Unmodified thinset can be used.

If your subfloor is wood planking, you'll need a layer of plywood between the plank subfloor and the cement board.

If your "prewar subfloor" is a concrete slab, then different rules apply.

2) Tub surround walls: Sounds like you're using a topical membrane. So yes, cement board on the studs, then the membrane on the cement board. Some topical membranes require fabric reinforcement in the corners. Then tile on the membrane, then grout.

You mentioned you're using a Kerdi drain in the shower. If that means you're using Kerdi on the shower walls and Kerdi on the tub surround walls, you don't need any detailing at the cement board seams or wall corners. The Kerdi is its own reinforcing fabric.

If using a membrane like RedGard of Hydroban, then you might need reinforcing fabric, or the cement board seams might need mesh tape and thinset, prior to applying the membrane.

3) Shower stall floor: Subfloor, then a slip sheet of some sort over the subfloor. Can be tar paper or plastic sheeting. I'll then tack down expanded diamond mesh. Then the sloped deck mud bed over that, with the Kerdi drain set into the sloped mud bed. Then Kerdi over that. Then tile over the Kerdi. Use unmodified thinset when tiling over Kerdi.

There are some drawings of a couple of ways to use a topical membrane in a tub surround in this thread. In that same thread, there are photos of a shower floor with Kerdi and a Kerdi drain.

There are a few ambiguities in the air due to there being minimal details in your original post. So certainly ask more Q's if you have them.

Best, Mongo

Thanks Mongo

I'm sure the ambiguities have to do with my lack of understanding (sadly) about what to include/describe and what not to. Thanks for the response though.

To clarity: Our subfloor is likely to be wood planking as that is what we found when we remodeled our kitchen a few years ago (see photo). So will then add plywood based on your recommendation above.

Re 2), this is for a bath with a tub. I will suggest Kerdi as the contractor has not said anything yet. Looking at some pics of a project he was completing nearby, it looked like he was tiling right over the CBU. I was a bit worried seeing that and it made me think that I want to insist he use a membrane over the CBU.

Re 3) this is for a small bathroom that will only have a shower stall and no tub. Was thinking of using the Kerdi drain there.

Does this help clarify a bit better? Thanks much

docrck

For the bathroom floor, you'll need a layer of 3/4" plywood, set perpendicular to the joists. (screwed down) and then a second layer of plywood of at least 3/8" over that. Try not to screw into the joists. Finally, a layer of either 1/4" cement board, set in morter and screwed into place OR a layer of something like "Ditra," set in morter as per Schluter's recommendations. NOW, you have a good surface to tile onto.

I like your windows. 2V over 2V are among my favorites.

If the plank floor is 3/4" and in good shape, then I'd recommend 1/2" ply underlayment over the planking and then cement board and tile over that, as previously described.

Your added info is good:

"Re 2), this is for a bath with a tub. I will suggest Kerdi as the contractor has not said anything yet. Looking at some pics of a project he was completing nearby, it looked like he was tiling right over the CBU. I was a bit worried seeing that and it made me think that I want to insist he use a membrane over the CBU. "

You need a membrane somewhere in the wall. If it is behind the cement board, then usually it's 6-mil polyethylene sheeting stapled to the studs and draped over the tub flange. The cement board goes over that, then you tile on the cement board. There's nothing wrong with that type of installation when properly done.

If the membrane goes "in front" or on the cement board, then it's a topical membrane...either a sheet membrane like Kerdi or Nobel, or a "paint on" membrane like RedGard, Hydroban, something like those. Then you tile on the membrane.

The poly would be the least expensive, the topical membranes are more expensive.

"Re 3) this is for a small bathroom that will only have a shower stall and no tub. Was thinking of using the Kerdi drain there. "

To use a Kerdi drain, you generally use it in one of two ways. The first is as part of the Kerdi shower system. Kerdi drain, then Kerdi membrane on the floor and walls. Along the lines of the shower in the link I posted previously.

The other way is to do a deck mud slope with a Kerdi Drain installed, also like the shower in the previous thread. But instead of using Kerdi membrane on the sloped floor and walls, you use Hydroban.

I do prefer topical membranes, they result in less deep wetting of the shower structure and thus they help minimize mildew, etc, especially on a shower floor.

Mongoct-- (sorry OP)
If not using Kerdi shower pan, but just a mudbed/liner, can you use a topical membrane for a shower floor? I've posted previously, but it didn't catch your attention. I'm trying to understand this more as well...
Thanks!

"Mongoct-- (sorry OP)
If not using Kerdi shower pan, but just a mudbed/liner, can you use a topical membrane for a shower floor? I've posted previously, but it didn't catch your attention. I'm trying to understand this more as well... "

Sorry if I missed your previous post. I seem to be hit and miss on the forums these days.

I'm a little confused by the "mudbed/liner" comment and the desire to use it in conjunction with a topical membrane. So let me ramble a bit:

I consider "liners" to be the CPE or CPVC thick shower membrane liners, the ones where you use a deck mud preslope, then install the liner over that with a clamping drain, then top the liner with another layer of deck mud, and you tile upon that. If that's what you're asking about, no I wouldn't use a topical membrane on top of all that.

Here's what I consider to be a liner installation.

Now back-tracking a bit: There are folk that use a topical membrane with a clamping drain, but they have to "dish out" the sloped mud bed as it gets close to the drain. It's sometimes referred to as the "divot method". That way the topical membrane will drain to the drain's weep holes. If you don't use a divot, then the raised part of the clamping drain can sort of act like a dam, causing water under the tile to pool around the drain. The area around the drain might appear perpetually wet.

The divot method is not a technique I embrace. Nothing really wrong with it, it's just not my cup of tea. Here is a photo showing the "divot" carved out of the mud base:

Virtually every shower I do is a one-off size or shape, so I'm almost always doing a sloped deck mud base. As to which topical membrane to use, when using Kerdi, I'll do a sloped mud pan and then cover that with Kerdi.

If using Hydroban, then I'll still use a Kerdi drain in the sloped mud floor but then use Hydroban on the floor and walls.

So if you want to use a topical membrane, then no , do not also use a liner within the floor. One membrane is all you want.

Does that help?

Yes. Thank you! (I am assuming I can substitute "redgard" for "hydroban" in the above comment).

2 more questions on my post that you found... (subject of which was walls with doorless showers.

"I am assuming I can substitute "redgard" for "hydroban" in the above comment"

Not really. With the Divot method you can use either RedGard or Hydroban. With the Kerdi Drain? Either use Kerdi membrane or Hydroban.

The reason? Laticrete (they make HydroBan) approves of and offers a warranty on the Hydroban and Kerdi Drain hybrid installation.

Custom (they make RedGard), to the best of my knowledge, only recommends RedGard with a clamping drain, that's the "divot method".

If your guy uses RedGard, he'll feel perfectly comfortable using Hydroban.

Best, Mongo

And I do have to add this:

Laticrete is now selling their own drains. Trench drains, and flanged drains. The flanged drains are the same idea as the Kerdi drains.

So if you want to use Hydroban, you can use either the Kerdi Drain or Laticrete's "Hydroban" Drain.

Video link for Hydroban with Hydroban drain.

Thank you! Very helpful and generous of you to share your expertise, Mongoct!

Okay, one more, after watching that video...
Do you not have to worry about nicking/tearing/marring the hydroban surface when troweling the thinset on? (Obviously, I am not in a DIY position, and I know it, but I do like to have a good understanding of how things work).
TIA.

"Do you not have to worry about nicking/tearing/marring the hydroban surface when troweling the thinset on? "

Not if you use the same computer generated, remotely operated auto-trowel that they used in the video. ; )

It's really not a problem, the membranes cure to a durable surface. Just the usual care is required, especially underfoot, in terms of not dropping a tool, or not keeping a clean workspace and allowing debris to collect underfoot.

If you did ding up a surface, just spot-apply a repair.

It's tough to ding up Hydroban, especially with a trowel. It's a pretty tough membrane, once it dries. The only place I worry about it is on the shower pan, and what I'll usually do is waterproof it with the two coats same as everything else initially, and then once the walls are tiled, I'll apply one more coat on the floor just for insurance, to make sure I didn't grind a tile chip with my shoes while tiling the walls.

As for the OP, the plank flooring you have right now will give you all the stiffness you need. You DO, however, still need two layers over the top of it, and one of those layers needs to be plywood, and being that you have the "beef" with the planking, you should be able to go with 1/2" plywood, and then for the second layer, you can use either 1/4" cement board or Ditra, which is only 1/8" thick.

Now, as for the shower, you're kind of in a pickle. In NYC, lead pans ARE code, and legal, even though the rest of the country (except Mass.) has figured it out, NYC is a little slow. :-) But because of that, it's tough to get the plumber to switch it out for something else. If you can/ though, you're all that much ebtter off for it. As for the tub, it IS code that they either put up a vapor barrier behind the cement board, or a topical waterproofing OVER the cement board, and that one you CAN (and should) push.

thanks Mongoct and Bill. Just met with the contractor again today.

When we asked about water-proofing, he indicated that he uses polyethylene sheeting behind the CBU. I asked about a "paint on" membrane like hydroban on top of the CBU, for example, and he said he would be willing to do this as well (in addition to the poly sheet behind the CBU). I didn't get the sense that he had worked with it before but he said he'd be happy to do so. Of course what worries me is that if he hasn't used hydroban before, will it result in an application that is of poor quality? Does it require precision and expertise to apply and could he do it in such a way that it negatively impacts the thinset and tiling above?

Regarding the shower, you're right Bill. He said he "has to use" a lead pan being that we are NYC. He suggested using waterproof membrane as well on top of the lead pan? does this make sense?

So much of my concern comes from being in an apartment building built in 1921 and having neighbors below. I don't want any leaks and/or issues once we've done the bathrooms!

Thanks in advance for your advice, MongoCT and Bill V

I will say that you use either a sheeting at the stud wall then CBU OR a paint on membrane like Hydro Ban or Redguard, but not both. The issue is that you don't want to trap H2O, and having both layers apparently cause this to happen. - at least that is what I recall reading.