Hello,

I’ve been reading a renovation book that has a couple of pages on Kerdi in the bathroom renovation chapter. I would like to get the opinion of the pros on this site regarding what I’ve read with regards to Kerdi and shower walls.

The book’s author, a nationally known and “trusted” renovator, recommends using concrete board underneath the Kerdi for the shower walls as concrete board is mould resistant, doesn’t rot, and makes a great backing for tiles. However, he does acknowledge that the concrete board is hard to work with.

I have two questions I would like to ask, if I may.

I’m undecided about whether I should use concrete board, regular drywall, mould resistant drywall or something else underneath the Kerdi? Your advice would be greatly appreciated with this dilemma I’m having.

Of the four shower walls, two of the walls would be wood framed as they are interior walls to my house and are not shared with my neighbour, one of the walls is made of cement blocks with my garage on the other side, the remaining wall is a concrete wall that divides my house from my neighbour’s house.

2nd question, do I need to also wood frame the latter two walls as well, or can the Kerdi go directly on top using the appropriate thinset?

Thank you very much for your answers.

To answer question #1, drywall will work just fine. It's easier to work with, and provides a flat, smooth surface to apply the Kerdi. There is nothing "wrong" with using some type of cbu, it's just more expensive and more of a hassle to work with. Whatever you use doesn't need to be mold resistant or waterproof or whatever, the Kerdi does that job by not allowing the moisture through to begin with.

Question #2: The concrete wall would be iffy. It would need to be completely flat, plumb, and smooth to accept Kerdi. I suppose if it was smooth enough, you could fill any spots that were out of plumb with thinset. That sort of answers the question about the concrete block wall. All the ones I've seen are much too rough apply Kerdi to directly. If space is an issue, you can frame the walls with 2x4's laid flat. That would save you a couple of inches. Good luck.

Thank you for the response Kman.

I’ll go with the regular drywall as the underlayment for the Kerdi around all four walls.

The bathroom is 61 1/8” wide. I could go up to 44” deep without running into space problems with the toilet. I would like to use the Kerdi 32”x60” shower tray without having to cut it – I’m trying to avoid creating problems for myself.

Assuming the above, some questions come to mind that I would like to ask for advice on.

1. Would I be able to use 1x2’s laid flat against the concrete and the cement block walls, instead of 2x4’s ?

2. Would screwing 1x2 wood slats into anchors in the concrete/cement walls be acceptable?

3. Is a vapour barrier required?

Thank you very much for your responses.

Hi Buzz,

If you use 1x2 as furring you're not going to have much for the nail or screw to bite in to. I recommend 2x lumber.

Kerdi is the vapor barrier, you do not want two.

Where you stated; "The bathroom is 61 1/8” wide. I could go up to 44” deep without running into space problems with the toilet." Are you saying the remaining 17" is for the toilet?

Jaz

Thank you for the response Jaz. Just to clear up a few things first.

The two walls enclosing the space where I would like to put the shower stall are 61 1/8” inches apart. These walls are fixed and cannot be moved. One wall is made of 2x4’s, while the other, opposite, wall is made of cement blocks. The comment about the ‘44” inches deep’ was with regards to the other two perpendicular walls. One of these wall is in a fixed position as it is made of concrete and is shared with my neighbour, I’m in a semi-detached house. The other wall is non-existent at the moment and is where I would have some wiggle room as to where I would choose to set it. I had figured on a shower with a maximum depth of 44” inches as this would be the maximum depth that would allow for a 24” inch shower door to open and closed without hitting the toilet. In hindsight, I kind of went off the rails here as the dimensions of the shower stall will be bound by the 32”x60” Kerdi shower pan that I’m thinking of using. The actual bathroom itself is roughly 5’x11’.

If I go with the 2x lumber laid flat on the cement block and concrete walls, I believe that will mean I’ll have to cut the Kerdi Pan to fit the space.
61 1/8” [Breadth of shower space] – 1 ½” [for 2x4] - 1" [for drywall] < 60” [58 5/8”]

Is it ok to cut the pan or is this something to be avoided?
The pan will be about 1 3/8” too wide for the space.

I'd like to cut off about 1" on either side of the pan. Will I need a particular tool for this as the product is made of polystyrene?

Hello,

Anyone with any recommendations on how to attach pressure treated 2x4's to concrete?

Thank you in advance for your responses.

There are a number of fastners that you could use. Tapcons, u-drives, Ramset or Hilti shot fired pins.

Buzz, what are you going to use the PT lumber for? Pressure treated lumber is not for interior use.

Jaz

Thanks for the responses.

I had planned to use the PTW in the shower area against the concrete and cement walls; however, I now see the error of my ways. I had read an article about saunas and using PTW against concrete; but the vast majority of the literature says that it shouldn't be used indoors. Thank you for protecting me from myself.

I've attached a diagram of what my plans are for the shower. I'll be going with the kerdi pan, membrane and drain.

Please let me know if my plans are ok or not so far?

Thank you in advance for your much appreciated responses.

Hello,

I'll be tapconning all the 2x4's to the concrete and cement block walls. The base plate of the partition wall will be tapconned to the concrete floor. All lumber used will be non-ptw.

I have a question about the partition wall base plate.

Do I need to use a vapour barrier or some form of adhesive between the concrete floor and the base plate?

Thank you in advance for your responses.