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.

No vapor barrier needed. You can use liquid nails if you want, but if you're already using tapcons, and you secure them properly through the plate and into the floor, it's not really necessary. You may want to consider using some type of washer to keep the tapcon heads from sinking into the wood.

You need a moisture barrier behind any wood that is in contact with masonry or concrete which is located below grade.

The barrier can be strips of poly or type S roll roofing.

I checked all the posts before my last reply to make sure there wasn't any reference to this room being located below grade. I checked again and still can't find it. Did I miss it somewhere?

Thank you for the responses.

The half of the basement to contain the shower is wholly above grade. The other half starts off at about 2 feet above grade and slopes down to 0. The house has a built-in garage with a walkout basement. (The link to the diagram that I posted July 31 does indicate that the shower will be above grade. I had wanted to embed the diagram as part of the post, but I haven’t figured out how to do that yet.)

From what I see now upon further reflection, that hadn’t dawned on me before, is that the situation may not be so totally black and white due to half of the basement being above grade and the other half being partially below grade by 2 feet and going to 0.

Please correct me if I’m wrong, but there doesn’t seem to be a need for a barrier in this case?

I’m curious about “ … consider using some type of washer to keep the tapcon heads from sinking into the wood”. I had planned to countersink the wall tapcons as I’ll be using drywall overtop.

My question is, would there be a problem with countersinking the base plate tapcons?

Thank you in advance for your responses.

There's no problem with countersinking them. The studs will be pretty sturdy since they'll be supported by concrete, block, or whatever they're stuck to. A base plate for a wall would just be a little more sturdy with a washer on the tapcon, and there's no need to counter sink it since it's inside the wall. I learned this from standing up some walls in my office that they're just a little more sturdy with the washers. It's not a necessity, just a little extra insurance. Keeps the wall from swaying back and forth so much while you attach it to the ceiling, intersecting wall, etc. In my case, some walls ran parallel to and between ceiling joists, so I had to brace it up, and the tapcons with washers held everything a little better.

Thank you for the responses. I'm still making plans and taking notes before I start anything. Thank you to everyone for the advice.

I’ll be using Kerdi with regular drywall in the shower. I would like to ask two questions about this.

• 1. Should the drywall seams be done with drywall compound and tape, some other product such as sheetrock 90, or should the seams just be left alone as the Kerdi will go over top?

• 2. Should I be using rigid foam insulation against the cement block wall. The bathroom is above grade, but there is a drive in garage on the other side of the wall? (There’s an attached diagram to this thread)

Thank you in advance for your responses.

Hello,

I’ve been doing some further research into question #1 above. I would like to report on what I’ve found with regards to the possible available options that I had identified.

1st option. drywall compound. An article I read says that these products are prone to mould and take a long time to dry. The author of the article suggests to conduct an experiment with pre-mixed drywall compound and water to see mould form in a few days.


2nd option. sheetrock 90. Mould is not as attracted to this product as it is to the drywall compound; however, the product dries very quickly. It is not as quick and convenient to use as the mud, but to me this is a moot point for someone interested in doing the job right.

3rd option. do nothing. A David Taylor, who I believe is affiliated with an internet tile supplies retailer, published an article on Feb 14, 2004 in which he states:
"The drywall does not need any special preparation before the application of the Kerdi membrane. Joints do not have to be taped or mudded and there is no need for extra bracing or notching of the wall studs to accept a vinyl pan liner."

Based on the analysis presented, I believe the best way to proceed would be to leave the drywall seams alone which boils down to option 3.

Any comments from the pros?
(question 1 or 2)

Thank you.

You didn't ask for comments from the amateurs, but I'll throw this in anyway. Some of the drywall I use has the long sides tapered off in thickness for about the last two inches. This is so the seams can be taped and mudded without any buildup. If your drywall has the same feature, you may want to fill it in with something (one or two layers of thinset) to make it flat for the Kerdi. Otherwise, the indentation where the two boards meet will telegraph to the Kerdi, and you'll be trying to fill it in when you put the tile up. It'll be easier to fill in before the Kerdi goes up.
RE question #2, what would be your reasoning behind the use of the foam insulation?

Buzz, the exterior walls marked as Neighbour and Built-in Garage need to have insulation and a vapour barrier by code.
If you use styrofoam insulation then a vapour barrier is not required.

Here's a photo of a job similar to yours.


The exterior walls are cinder block and brick. I insulated with foam board and placed the strapping over the foam panels. It's a complete thermal break when done that way.

Hello,

Kman, sorry if I may have offended. I had meant anyone. To me and compared to me, anyone is a pro. But, you are absolutely right, mea maxima culpa for the choice of words.

#1 Thanks Kman. Excellent observation and point you make about the tapered ends and the use of thinset.

#2. Kman & RC. I was having problems deciding if I had an external wall on my hands and thus possible potential air infiltration, ambient vapour and moisture wicking issues. The block wall is at the back of the garage, but the garage does open up directly to the outside. It, the garage, does get pretty cold during the winter and there is moisture in the air, especially on hot humid days. An interesting experiment I’ve read is to tape a piece of clear plastic to a wall and check for condensation in a few days. RC thank you very much for the picture and for setting me straight. The wall marked neighbour isn’t really and exterior wall as I’m in a semi-detached house and that wall is shared between my neighbour and myself. It is not exposed to the outside.

The foam will prevent air movement in the wall cavity and thus prevent cold air from meeting warm air. Condensation forms when cold meets warm. Condendation will in turn lead to the creation of mould.

Thank you.

No offense taken, Buzz. Just kidding around.

I sometimes forget about the different climate you fellows have up there. Nobody uses the foam board around here since we don't have the extreme cold for extended periods like you fellows do. It's in the mid 90's here right now, and we've had a pretty mild summer compared to the last few years. When is someone going to invent a hat with a built-in cooling system?

Hello,

This is a question about the shower curb.

As I’ve mentioned, I’ll be using the Kerdi pan, membrane and drain. I’m now debating on what to do with the curb. The floor substrate is above grade concrete slab. Also, I’ll be using a swinging door as the entrance to the shower.

The two option I see are to either use 3 stacked 2x4’s or the Kerdi curb. I sees pros and cons to both approaches – some of it has to do with me being a newbie.

1st method. wood - I’m concerned about wicking into the wood curb from the concrete. But, I’m also worried about not doing a good job in that I’m not sure of the steps. From what I’ve pieced together the procedure would seem to be: 2x4s go directly on top of and make contact with concrete; wrap membrane around 2x4’s but not underneath; wrap a wire mesh around the membrane; nail mesh into place; mud curb so that it comes out flush with the drywall on either side of the curb on both the inside and outside of the shower. What kind of mud should be used? Are these steps incorrect?

2nd method. Kerdi curb. I seem to be somewhat lucky here as I’m anticipating the finished walls, where the shower is to go, to be about 58 ½ “ apart. My biggest concern here is whether it is possible for the curb to come undone from the concrete slab with time? The curb is only held in place by mortar.

Would I be making a mistake by going with the Kerdi curb option in my situation?


Thank you.

In John Bridge's book on Kerdi, he mentions problems he had with moisture wicking from the concrete into the 2x4 curbs he made since he wrapped them tightly with Kerdi. If I remember correctly, he now simply lays brick as a curb. I've used 2x4 every time, and never had a problem, but it doesn't mean I never will. You could try putting a piece of felt paper between the curb and slab, and securing the curb to the door jambs. I would use three 2x4's, securing each one as you stack it. You will still want to wrap the curb with something to attach the kerdi to, such as a CBU or drywall. Kerdi is not supposed to be attached directly to wood.
The Kerdi curb is always an option, I've just never used it since it's easier for me to cut a 2x4 down to size. Also, something makes me think the Kerdi curb might not be as sturdy.

Thanks for the response Kman.

I did some further investigation and ended up purchasing the JB book. Further to your point regarding
attaching Kerdi to wood, he says that framing lumber doesn't provide a good bonding surface for
the membrane. I've also learned that Kerdi curb should be attached to concrete using unmodified thinset and that the curb cuts without difficulty.

Thank you.

Hello,

Just some questions regarding the outside walls of the shower in the bathroom.

1. Would it be ok to not use moisture resistant drywall on the non-shower walls if I go with full wall tile wainscoting?

2. Would there be any benefit to using Kerdi membrane in behind non-shower wall tiles over either regular drywall or moisture resistant drywall?

3. Am I correct in assuming that the bathroom, non-shower, ceiling will need moisture resistant drywall?

Thank you.

In my experience, drywall is drywall. If it gets moisture, I use Kerdi or Redgard and thinset (as opposed to mastic). If you think the area you are tiling is going to see significant moisture for whatever reason, paint two coats of Redgard on it. You could go with a CBU, but if it's already got drywall, then Redgard would be cheaper. A proper exhaust fan will take care of most of the condensation, so in my house, I have the fan positioned just outside the shower, and I never have any significant condensation on the walls or ceiling (just on the HVAC vent once in a while). Use a good quality ceiling paint and that will retard any moisture from wicking into the ceiling.

Thanks Kman. Got it.

I phoned Redgard and had a pleasant conversation.

I’ll have to decide how much wainscoting to go with in the bathroom. My thought is to go full wall, but I’m not totally sold on that yet. Any thoughts?

Back to Redgard. He indicated that Redgard is to be kept down 1 inch below the wainscoting and is not really appropriate for painting over. He suggested, as you did, two coats. Also, he recommend starting off with a primer, 4 parts water to 1 part Redgard. Tile is to go on with modified thinset.