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zaphod 04-21-2014 05:26 PM

Tub Surround Installation Question
Hi Everyone,

I'm working on a tub surround and I've been searching and searching, but I can't seem to find the answers to a few simple questions.

Here is what I am doing: It's a simple tile surround above a whirlpool bathtub. I have installed Perma Base down to the tub flange, but not covering it, directly onto the studs, with no vapor barrier behind it. I now plan to install Kerdi Membrane on the surface of the Perma Base, the Kerdi will overhang down over the tub flange, and then I will install the tiles onto that.

My question is: With a Kerdi membrane installation, do I still use the fiberglass mesh tape on all the joints? and if so, is this installed in 2 separate installations? In other words, do I install mesh tape on the seams and corners with thinset, then install the kerdi with more thinset? or just put the mesh tape in the thinset used to hold ther kerdi on.. or do I even use the mesh tape if I will be using Kerdi?

One thing I did find out from reading through things is that I don't want to create any humps in the wall, so thats what got me thinking, of perhaps doing both the mesh tape and kerdi at the same time, because then I won't end up with thinset on top of more thinset.

any advice would be greatly appreciated.

Houston Remodeler 04-21-2014 05:31 PM


Welcome to the forum.

1- no need to with kerdi
2- See #1
3- See #2
4- See #3
5- See #4

Richard Tunison 04-21-2014 05:36 PM

You are quick tonight Paul1.

I'll go play some golf...........:nya:

zaphod 04-21-2014 06:10 PM

1 Attachment(s)
I wasn't too sure about #3, but #5 answered my question nicely LOL. Thanks for the fast response Paul1

That sure simplifies things. Ok, next question:
See the hopefully attached photo... the walls behind the tub seem pretty straight forward, but what do I do with my strangely shaped corner shelf thing? I am guessing I will need to cover it with Kerdi in sections: starting at the bottom and working my way up, and overlapping the kerdi. How much of an overlap do I want and is this overlapping going to cause any issues with the tiles or is the kerdi thin enough that it won't hurt anything?
and while I'm getting educated on the subject.... do I want to wait for each section of kerdi to dry before installing the next, or do I just keep going, and install more on top while it's still wet?

Sorry if this seems extremely basic, but I am a complete novice when it comes to tiles

Houston Remodeler 04-21-2014 06:16 PM

0- Remember what I said about numbering your questions ? :tazebro:

1,2, & the rest of them - As long as you maintain a 2" overlap you fine. Buildup reduction takes practice and forethought. Sometimes we compensate for buildup by building benches smaller and niches larger. You can do all the kerdi then start tiling immediately. No need to wait between pieces of kerdi or kerdi and tile. Pretty cool huh?

Ceement board is very thirsty. You'll have a much easier time if you thin down some Red Guard 1: 4 with water and apply that over the CBU before starting the kerdi.

Watch this video to see how thin the thinset gets mixed.

zaphod 04-21-2014 06:22 PM

Yes, Very Cool!! Thanks again for the help and advice. I'll go watch the video.

zaphod 04-22-2014 05:17 AM

Question #7: Should I attempt to cover the 3 walls with one piece of Kerdi? The sizes are 34x22 66x22 18x22

Houston Remodeler 04-22-2014 07:33 AM

It's been my habit to run one piece of Kerdi horizontal all the way around hung to a height of 33" to allow for a healthy overlap on the floor. This will help eliminate some of the more troubling wall build up. The floor build up is easier to handle. Plus you can tile the floor after tiling the walls and still have plenty of area to overlap the floor Kerdi.

Somewhat carefully typed on teensy cell phone keys.

zaphod 04-22-2014 10:55 AM

One piece sounds like the way to go.

8) would it be best to install it in sections, perhaps apply thinset to one wall and around one corner, install it that far, then thinset the majority of the long wall, install it to that point, then thinset the remainder and finish the installation, OR just apply thinset to the whole thing and install it all at once? total size is just under 10 feet x 2 feet

Lakee911 04-22-2014 11:01 AM

If your thinset is going to start skinning over (i.e., drying), before you can get it covered, than work in sections.

zaphod 04-22-2014 02:28 PM

1 Attachment(s)
I got my Permabase coated with a 1:4 dilution of Red Guard. I think it came out pretty well. Now that I know that little trick to keep my thinset from drying out too quickly, I think it would be much easier if I coated my permabase outside on a pair of saw horses before I screw it to the wall. I ended up making a pretty big mess because it's so thin it just wanted to run down the vertical sheets, but I kept rolling it around until the sheets were coated with it. Unfortunately it did run underneath my making tape and onto my tub surface, as the masking tape didn't really stick to the fiberglass all that great, but it all cleaned up nicely.

9) what's the best way to keep the thinset off my fiberglass tub while I'm installing the kerdi membrane?

zaphod 04-24-2014 07:48 AM

I'm getting ready to install the Kerdi membrane on my tub surround.

10) What kind of mortar do I want for this? There are So many kinds and the people at my local stores just don't know, they don't even know what Kerdi membrane is, since they don't sell it, I had to special order it in. There are so many kinds of mortars all labeled as 'Thinset' I'm really confused here.

Here is what I have:

I'm wondering if I need something more like this:

Any advice would be greatly appreciated, I have zero experience with this.

zaphod 04-24-2014 09:27 AM

Perhaps this is what I need?

Richard Tunison 04-24-2014 09:36 AM

That is the proper thin set to use with Kerdi.

Richard Tunison 04-24-2014 02:32 PM

Just realized we are close. I used to live on Oak st. by the High School.

zaphod 04-25-2014 08:29 AM

just to make sure, the one labelled floor tile mortar is the right one?


Yep, I know where that is, I'm building my own house (literally) on 5 acres in an agricultural zoned area near sunshine grove road. I'm right next to the expressway, in fact it cuts off 0.1 acres of my property. I have a yahoo group about it here:


Richard Tunison 04-25-2014 09:06 AM

Yes, that is the right product.

zaphod 05-01-2014 06:10 AM

A quick question about installing kerdi membrane on my fiberglass tub surround. 11) What do I do where the kerdi overhangs the tub lip? Do I just make the thinset go over top of the lip.. not sure how that would stick to the fiberglass... or should I just put the thinset on the perma base then perhaps silicone the kerdi to the tub lip later? any suggestions?

Richard Tunison 05-01-2014 06:18 AM

Yep, adhere it to the flange. I'd use this product instead of silicone.

PC7060 05-01-2014 06:36 AM

The link Richard provided is the self leveling crack sealant version of Sikaflex, good product but kind of runny for use with Kerdi.

I use this version of the product for tile work.


This post on my thread shows kerdi band and corners installed with Sikaflex.

Richard Tunison 05-01-2014 06:45 AM

Thank you P.C.!!:yo:

PC7060 05-01-2014 08:17 AM

No problem, Richard, glad to help. :)
I'm not so helpful with questions on thinset, if the answer isn't versabond I'm sunk. ;)

zaphod 05-02-2014 05:16 AM

Thanks for the info and link.
a few questions:

12) how should I apply this? Should I attach the kerdi to the permabase with the thinset first, then come back and try to get the sikaflex between the kerdi and the tub lip... or should I attempt to do both at the same time?

It looks like you only use this on corners that don't overlap at least 2" I have a few 36 degree corners, one inside, one outside, I can't think of any way to deal with that angle other than to just cut the kerdi to the corner and overlap it... the overlap will end up being zero right at the corner and increasing the further it gets from the corner. It seems like using sikaflex on these odd corners would be a good idea.

13) is the procedure for attaching kerdi with sikaflex the same as using thinset? Do I use the same size V notch trowel.. etc?

PC7060 05-02-2014 06:02 AM

12 - I'd put the thinset on the cbu using 1/4" notch and then spread the Sikaflex on the tub flange with a 1" flexible putty knife. Then apply the kerdi band using a 6" blade or the flat side of your trowel to even out the thinset under the kerdi (no tiger stripes).

Use the putty knife to make sure the kerdi band is fully embedded in the sealant and you are done.

13 - see above for tools and methods.
Re the 36 degree corners, are these on the tub surround too? Probably OK there where the tub flange helps control the water. I'd be concerned if you doing that in the water proof area of a shower or bench where any leakage is unacceptable.

Got any picture of the problem area?

zaphod 05-02-2014 07:44 AM

3 Attachment(s)
Here are pictures. my house has 10 equal sides arranged in a huge decagon, making each of the corners 36 degrees. This little shelf is just something to fill in the corner with, it will have cabinet doors under it, mainly so I can get in there if I ever need to access the pump. This is a Jacuzzi tub that is set up to be a bathtub ONLY, there is no shower head, just the spout to fill the tub... there is a corner shower only in the same bathroom just on the other side, so we didn't need 2 showers in there, and this way we don't need to have any kind of shower curtain or wall on the end of the bathtub... it's just going to be open. The outside 36 degree corner, I'm really not too concerned with because if any thing ever did get behind there, it's open on the bottom and it could dry out, and it's above the bathroom floor which wouldn't hurt if it ever got wet... and chances are if there was water by that particular corner, the floor would be wet anyway. It's the inside corner that I'm mainly thinking about... however, not overly concerned because it's not in a shower area, and it won't be getting soaked on purpose ever. I did put a slight pitch on this shelf toward the tub, so hopefully any water on it would run into the tub anyway.

I would still like to do the best job I can waterproofing this, so I'd appreciate any input on how to handle these 2 non-90 degree corners. I'm trying to think of the best way to do it without causing too much buildup that might be a problem later.

PC7060 05-02-2014 05:10 PM

I think I'd use something like Redgard or Aquaban to seal the joints and the odd sized corners similar to Jr's example in the link below.


zaphod 05-06-2014 12:36 PM

Thanks, that looks like the way to go with those odd corners.

I got some Sikaflex as recommended in the link provided, and I'm getting ready to try to install my kerdi membrane over the tub lip, but now this brings up another question:

14) how far down do I bring the kerdi? there is about a 3/8" radius between the vertical and horizontal part of the tub edge, but part of that will be behind the kerdi because my premabase sticks out a little past it. Do I bring it down all the way to the horizontal part, or do I bring it most of the way down, and perhaps leave a gap under it?

15) while I'm asking.. how far down should the tiles go? I think it might be asking for trouble to attempt to bring them all the way down to where it is horizontal, because the fiberglass tub may not be perfectly flat.

Richard Tunison 05-06-2014 12:52 PM

Bring the kerdi down so you can comfortably get it glued to the vertical flange,,,,,,no need to get down to horizontal.

If the tub is not level you will want to make the layout so that you are cutting the bottom row of tiles to conform to the tub.
Leave an eighth in. gap for 100% silicone caulk.

zaphod 05-06-2014 01:41 PM

ok, thanks! I put a level on the tub and it is nice and level, and mostly flat, looks like a total variation of less than 1/16" it's hard to measure.

16) should I make the gap under the tiles a minimum of 1/8" so where the tub dips down slightly I'll have a little thicker gap? The thickest part would be less than 3/16"

I also have a few places where I need to build up something on top of the permabase... there is one edge where the drywall is about 1/32 thicker than the permabase, so my bullnose tiles would have a problem, and another where the corner of my shelf is about 1/8" short of where the corner should be.

17) should I fix these areas with something like a layer of thinset before I install the kerdi, while I'm installing it, or after I install it? I'm thinking I might want to do it either before or after because it looks like I want the thinset pretty thin to install the kerdi and need to be working fast, not fooling with corners.

Richard Tunison 05-06-2014 01:55 PM

16. Pushing 3/16in. is,,,,,,,,,,,in my opinion, just to big and unsightly. I like a small 1/8 but that's just me. If you like it,,,,,,,,,,I like it. :biker:

17. I'd do the minor repairs with thin set before the kerdi and of course let them dry.

zaphod 05-07-2014 05:58 AM

1 Attachment(s)
Anyone have any idea how to fix this? What's happened is one of my studs in the bathroom is way smaller than all the other studs.. it's only 5-1/4 instead of 5-1/2. I did not notice this when I put up the drywall, but when I went to put in the tub, there was a huge gap on this one stud, so I shimmed it out so my permabase would overhang the tub lip correctly. I've checked this carefully, the permabase IS nice and flat, but the drywall above it is bowed in on the small stud. I measured it with a caliper and it's right at 1/4" at the worst part. at the time, I didn't think the shim I put in there was all that thick, but I guess it was. at this point, I can't remove the permabase, as it's glued to the studs. I"m thinking I could fill this in with something.. perhaps about a foot up the wall and feather it in... then re-texture it, then repaint it.. but

18) what would work for this? Drywall mud? Thinset? something else?

19) would some mesh reinforcement tape help keep the patch from cracking or coming off?

zaphod 06-04-2014 05:56 AM

Ok, I finally got my wall fixed... and just got the kerdi installed. Now I am wondering....

20) what is the easiest way for me to install my tiles to the Kerdi on the wall? I suppose I could use thinset for this, but I'm wondering if there is a better way that will not make such a huge mess... maybe some kind of adhesive in a tube?

Richard Tunison 06-04-2014 07:07 AM

I can't see where you have mentioned the size of the tiles but as long as you are not using a small, sheet mounted tile you can apply the thinset to the tile on a flat horizontal surface and then stick it on the wall. I would still skim coat the kerdi with the flat side of your trowel before adhering the tile to the wall.

zaphod 06-04-2014 07:27 AM

They are 10" x 14" tiles with 2" x 10" bullnose tiles on the edges.

PC7060 06-04-2014 09:42 AM

20 - Thinset only in wet areas. No premix, definitely no premix :sick:

zaphod 06-09-2014 09:01 AM

21) What size spacers should I use for my 10" x 14" tiles?

Richard Tunison 06-09-2014 09:07 AM

Take about 6 tiles and stand them up vertically on their side. (on a flat surface)
Put a straight edge across the tiles and measure the variation from high to low. (if there is any) Whatever the measurement is you will times that by 3. IOW if you have a 1/16in. difference in tile sizes, you would use a 3/16 spacer and so on.

zaphod 06-09-2014 12:49 PM

I checked the tiles as you recommend, and I can't see any difference between them. I do notice that they are all slightly wider at the back (glue surface) then the front (glazed surface). but the upper corner of each tile hits my straight edge all at the same time.
Is there some minimum grout line I should use in this case? I noticed at Lowes they have spacers 1/16" 3/32" 1/8" and up from there.

Richard Tunison 06-09-2014 01:01 PM

If your tiles are that well sized, then it's up to you but certainly 1/8in. would be my minimum and for a DYI'er I'd bump up to 3/16in. just to be on the safe side. Besides, a 3/16in. joint looks great with your tile size.

zaphod 06-09-2014 01:26 PM

Ok, Great! Thanks for the advice. I don't have many tiles to deal with.. the largest section is only 5 tiles wide by 2 tiles high, but I think I better stick with 3/16 lines, as it seems to be more forgiving and I'm a newbie :wohoo:

23) What is normally done in the corners? do I run one tile all the way to the corner then put the adjacent one one grout line away? that seems like it's asking for trouble because I think my kerdi has a slight radius in the corner... or do I make both of them stop short so the grout line is right in the corner? I'm thinking I could stick a spacer in the corner diagonally and at the surface it would be the correct spacing, but I'm just not sure how it's normally done. I have one 90 degree corner and one 36 degree corner.

24) what is normally done with the corners? are they grouted or siliconed?

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