Some more kerdi questions [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-23-2010, 04:54 AM
I started what i thought was going to be a cosmetic remodel last weekend only to dig up ~4 sq ft of rotted 3/4" TG under a really poor tile job:
1) the builders had applied directly to ply: no backer/uncoupling at all
2) the floor was extremely unlevel and they made up for it with two 5/16" gobs of thinset on each tile (6x6) with absolutely no trowling in the low areas

All of which explain the multiple cracks we had. At this point I've cut it out, blocked in all butt joints to the T&G and intend to it right including SLC and hardi....

However... I used PT around the toilet as i don't want to deal with this again as long as i own the house. I cut two pieces with semicircles to surround the sewer pipe under the flange. Both are just over 12" square and anchored to blocking / truss on all sides with the exception of the seam which is parallel to the sewer pipe. But i did block on either side of the pipe.

That said, i admit I've read a few posts that recommend against using PT under tile at all. There in lies my concern. I am obviously weighing those guidelines against the fact that toilet rings/flanges inevitably leak and once a toilet base is caulked, i am not going to be able to see what going on under there (the fact that the bath is directly over my kitchen makes me that much more uneasy about it). While I understand both the warpage (hoping the small cutouts would offset that) and bonding issues (i have some Mapeprime 1k i intend to use to bond the SLC on both types of decking (doesn't say it can't handle PT in the instructions)

Anyways, am i overly paranoid about the rot issues? I'm not trying to buck accepted guidelines and if you all tell me even in the above application i should rip it out and put in the standard, i will... Any other/better options to address both concerns?

many thx in advance...

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02-23-2010, 05:00 AM
Brad, if were my house, I'd use regular exterior rated plywood. I'd make sure the top of the flange was about 1/4" to 3/8" above the tile surface and well secured to the subfloor. I'd use a single wax ring or a waxless donut. I'd make sure the toilet was level and completely stable when set on the the flange, shimming as necessary to make it so. I'd either not caulk, or caulk but leave a gap at the back. Then, I'd stop worrying about it.

02-23-2010, 05:27 AM
Yup, what Bob said. :goodpost:

Pic of how your flange should be set:

The full article on Toilet Rough-In is in the Liberry HERE (

02-28-2010, 01:45 PM
I tried searching but "lip" isn't an accepted string so apologies if this is a common question.

I just finished ripping out the tile in an existing bathtub/shower. I had three theories when i started:
1) no vapor barrier
2) no backer board but only green
3) no isolation/backer on the floor with tile bonded to ply.

Unfortunately I've been right on all three (well the builder took backer half way up the wall, then greenboard) which explains alot.

Ok so i digress, headed in my wife had one request: keep the tub.

Well this house was built in 95' and it has a kohler tub with no lip (like i had on the last jacuzzi i installed (1/8" thick, 1.5" high or so)) over which to lap vapor barrier and/or backer. If has a slight hump (maybe 1/4" tall at its peak) that begins on the top of the tub (about 2" from the front which has nothing) and runs around most of the sides and back. Do i have any third party options to fabricate / attach a real lip? Without i fear i'm about to put in a lot of money and effort for naught. Do i tell the wife this is a bad idea given the other water damage and convince her to go for a new one?

thx in advance

02-28-2010, 02:24 PM
I had pretty much the same problem... my bathtub was a 72" x 42" cultured-marble soaking tub with a slight hump at the edge of the ledge, but nothing close to what it should have been for an alcove tub. As I suspected, the lower 2-3" of drywall behind the tile adjacent to the tub's ledge was solid black with mold... so moldy, in fact, that I was genuinely concerned about exposing myself to so much (but I'm young & non-wealthy, so I made do with respirator masks, a half-dozen cans of lysol, and a 140cfm exhaust fan that didn't get turned off for a week afterward).

In retrospect, I *grossly* underestimated just how hard it was going to be to replace the tub. From the moment the old tub came out until my first real shower since then (I only have one full bathroom), it took about a month of daily labor (keeping in mind I only had about an hour per day M-F to work on it due to HOA restrictions and the fact that I didn't even get home from work until 6:30).

If I were doing it all over, I would have probably explored the option of leaving the old tub in place (contrary to my earlier fears, the tub itself was almost completely mold-free, and the mold I did find would have been easily cleanable once the drywall came out), and instead have someone like Bathfitters lay an acrylic shell with proper lip over it. I think it would have ultimately cost about the same amount of money, wouldn't have been nearly as agonizing, and I probably would have been happier with the final result -- a tub that looks like a brand new acrylic tub with proper lip, but the solid feel of a cultured marble tub.

My biggest complaint about the new (acrylic) tub is the fact that it's just not rigid and solid underfoot the way the old tub was. It doesn't visibly deflect, and I was able to stop the random creaks by filling the void underneath with 6 cans of "Great Stuff" foam, but it still just doesn't feel solid. I'm not sufficiently unhappy to burn another $3,000 doing it over, but I do feel like I've taken a step backwards compared to what I used to have.

02-28-2010, 03:39 PM
If you search for "tiling flange" or similar you'll see lots of info. Here's a few to get you started.

Where are you located? Pro or DIYer? :)

Jim wood
02-28-2010, 04:41 PM
Brad if you are this far why not do it right and get a new tub. You can tile to that tube but chances are in the end it could leak (again?).

02-28-2010, 05:41 PM
Thanks for the advice so far...

Dana - central Florida and i'm a DIY-er, but give the job the "pros" did on this house, well :)

Jim, well i've thought about (even hinted), but the mrs likes the cast iron one we got and claims it's the easiest cleaning tub she's ever had.

02-28-2010, 06:13 PM
I missed that it was cast iron Brad. Those little bumps are typically all you get in the way of a 'flange' with those things, so I'd not attach an after market flange in your case.

Depending on how you plan on waterproofing the new shower, the details of the seal at the tub may vary somewhat, but will involve caulking at the tub. In multiple locations. You'll caulk the waterproofing to the tub, then after grouting you'll also seal the 1/8" gap under the bottom of tile to the tub with 100% silicone caulk. Do both of these properly and you'll never have to worry about leaks. Ever.

So, how you planning to waterproof that thing? :)

02-28-2010, 07:13 PM
Funny you ask... i'm still reading every other post i can find but:

Originally i planned on a plastic vapor barrier then hardi
But I'm now thinking about no plastic, just the hardi then kerdi

The latter has a couple of benefits in my opinion including the fact it lets me experiment with kerdi before i tackle my master bath which is more complex with the same. I know people say i don't need the backer with the kerdi, but i own it and i feel like it would offer a little more security (i'd wipe it down with a sponge / spray bottle)

fyi: I've been reading a post by John himself that states modified thinset should bond the kerdi to the tub surface if i wanted to go that route too... I'm torn between that and your proposal with caulk.

I'm also reading through posts on transitions. I'll have drywall / paint ceiling (caulked to tile) and one outer corner where the wall protrudes and one where backer would meet up straight with drywall. In the corner case, i should be able to get a 3" bullnose right to the edge (thinking or putting corner bead (plastic - current steel is rusted badly). On the other case, the bull nose would be mostly on the drywall (but outside the main shower)


02-28-2010, 07:20 PM
Check out the Kerdi Shower Installation Handbook if you haven't already. Link over there -->

There's a detail in there for attaching Kerdi to tubs. With Kerdi, I'll typically have 3 layers of caulking. Between drywall/CBU and tub, between Kerdi and tub and between tile and tub.

Good on ya for doin your homework. Have you checked out the Liberry yet? Lots of good info in there. Link in dark blue bar above... :)

03-02-2010, 06:17 AM
Been reading this forum and bought john's book...

I am one of those who just wants that extra assurance of backker board even if i go with Kerdi... I have the hardi already, but need to decide on the mortar.

I got a nearby Lowes, and HD is quite a bit of drive. Will Kerabond (mapei's "premium dryset mortar") be ok instead of the slightly modified Versabond in this app or with the sponged down hardi still soak it up too fast? I could swap out to durock though i would think that would be pretty good at sucking up water too.

thx in advance

03-02-2010, 06:21 AM
I am not a pro but I'll tell you what someone here told me for a similar installation.

Coat CBU (Hardi in my case) with two sprayed coats of SLC primer before applying the thinset. I am using Ditra-Set from Bostik

Brian in San Diego
03-02-2010, 06:29 AM

Kerabond is a good thinset for kerdi installation. Avoid the Keraset or the Keraflor. If you use Versabond you'll void the warranty but will still get a good job. If you don't want to go the SLC primer route you will need to liberally dampen the CBU before attaching the kerdi. Make sure your thinset is mixed on the loose side. (don't follow the directions for water to powder ratio).


03-02-2010, 11:06 AM
I've got a quart of the mapei primer for plywood substrates for their SLC...

Will that work and does it have to be sprayed? can't roll it?

03-02-2010, 11:15 AM
Probably will work the same way. I do not know that you couldn't roll it, I was just restating advice I was given by one of the pros on here.

His exact words were "HArdi really stinks to do Kerdi on, you could get bit from it. but go to HD and buy the LevelQuick primer. $6 and put it in a bug sp[rayer and spray your hardi with a few coats. this will help with thinset bond and also stop the hardi from being thirsty." And to let it dry a few hours before applying thinset/kerdi

Houston Remodeler
03-02-2010, 11:35 AM
We put kerdi over hardi and cbu all the time. Never noticed any problems except the need to pre-dampen before thinsetting.

03-02-2010, 07:28 PM
1) OK, i've read what i can on transitions, but i'm not finding exactly what i'm looking for on how to handle an outside corner/wall on a tub/shower combo. The old install took 2" bullnose vertically to 1" shy of this 90 degree corner. That one 1" was wall papered with a sheet covering the whole wall. I need to redo this as the steel corner bead is rusted, and the greenboard behind it mildewed and damaged.

In new install, i plan to use 3" bullnose (more of a mosaic, not glossy) which they sell at lowes. Can/should i take the kerdi all the way to the edge (overlapping a new corner bead - thinking white plastic to avoid rust since i suspect this will get water. Or do i take it an inch or so to the edge and drywall compound the corner up to same thickness?

2) are the seal-MVs universal to any single valve? I have an old (but i think common) Delta valve.

3) i am thinking of trying to get one large vertical sheet on the side walls (33" wide) and either two large horizontal or vertical pieces on the main length over the back of the tub. And then seem with 5" band at both the long seam and the two vertical corners. Is that ok? Are the KERECK 2.5? high on each side or 3-5/8" such that i should go with 7-1/4" band?

4a) Looking at the ShowerHandbook on Schluter's site, they seem to show a narrow band to seal the main kerdi to the tub. Is this just due the difficulty in getting a single large sheet the right length?

4b) as per my prior post, i have cast iron tub with no lib/flange. I agree with the recommendation i got there and with johns' book that i need to address the "legs" (figure 34) but in my case, the 1/4" hump on the cast iron stops shy of the leg's vertical transition. I plan on taking the advice of doing a ~1/4" fold and bonding to the tub itself with thinset (after putting in backer rod and caulking the CBU gap behind it). I assume that a slight vertical gap between the backer and the tub is ok going ?

5) if i want to use 3" bullnose instead of base board and use kerdiband to bond the 1/4" backer to the drywall (again 5"?) is there any trick on getting the tile even where there would be a 1/2" or so at the top of the bullnose w/o the band?

6) I'm using SLC on the plywood before laying the 1/4" backer as it's pretty unlevel. I have two boxed of the backer screws - will they self tap through the SLC? (slc would be ~1/4" max thick in the lowest areas)

thx (again) in advance

Houston Remodeler
03-02-2010, 09:07 PM

Have some pics to make sure we are chatting about the same things?

1- I like to put kerdi in all the wet areas. If you are tiling outisd ethe shower like a picture frame or door casing, then there is no need to wrap the kerdi around the corner, unless you get water running down the wall there. If you are wrapping the kerdi around the wall, skip the corner bead all together as Kerdi doesn't like making sharp turns.

2- Pretty much, I don't use them though

3- As long as you have at least a 2" overlap, you can hang the stuff any way you want. I prefer mine in one long continuous horizontal piece. Whatever floats your boat, skill level, room size, and attention span.

4- IMHO schluter has made great efforts to make their installation instructions as generic as possible for nearly anyone (of any skill level) to get things waterproof. If you can hang a single sheet and hit the mark, go for it.

5- You totally confused me on that one. Pics? drawings? mock ups?

6- yes, with ease.

03-02-2010, 09:27 PM
5) You don't need 1/4" CBU behind the bullnose base, just stick it to the sheetrock with thinset. The Kerdi Band will not add enough of a gap behind the bullnose base to matter, just grout the gap between the bullnose and wall or caulk it.

Of course I'm assuming I understood the question correctly!

Houston Remodeler
03-02-2010, 09:29 PM
Why the kerdi behind the baseboard?

Am I in left field by myself ?

03-02-2010, 09:36 PM
Brad, it'll sure help if you'll keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered, eh? :)

Houston Remodeler
03-02-2010, 09:38 PM
Thanks CX, but I still have no idea what #5 is......:confused:

03-02-2010, 09:40 PM
Me, too. :)

03-02-2010, 09:59 PM
I could see it in an application where the shower curb is in the middle of a long wall. Assuming Ditra is under the floor tile, I would run Kerdi Band between the ditra and wall the entire length of the floor for waterproofing. In fact, that's exactly what I did in my bath.

Houston Remodeler
03-02-2010, 10:11 PM
I just wrap my kerdi down the back wall, under the shower pan, up and over the curb, down the front and out into the floor 12 " or so.

03-02-2010, 10:24 PM
Style preference I suppose? Not sure it matters, I like to waterproof the whole length of the wall containing the shower. I guess because I could.

03-03-2010, 04:00 AM
Will try to get a couple of pics for #1 / 4 once i have some daylight

on #5 sorry for the confusion, but basically i'm not putting backer on the non-tub walls but i will have it on the floor. Snets is correct in the interpretation. i was just going to use kerdi band to seal the backer to the vertical drywall (as in the ditra example) and use the 3" bullnose instead of 3-1/2 wood base board. But i was proposing to do it all walls like where snets' sink/cabinet are and then maybe redguard the backer on the floor just for some additional water protection - in case toilet overflowed, etc. Is that overkill given bath is upstairs? I guess i would need to deal with the tub meeting the floor too would propose the 1/4" fold there at the bottom, butting up the floor tile and caulking?

p.s. sorry for multiple posts. In my day job (software dev), we're sticklers for one issue per bug/thread/request... What you're asking goes against my very being, but hey i'll try :-)

03-03-2010, 09:24 AM
two pics that hopefully show what i'm getting at...

Houston Remodeler
03-03-2010, 12:02 PM

#5 - Ok I am up to speed and am nodding my head. Very good idea, I like overkill.

The curved tub edge is also a normal situation. I like to kerdi fix that hole under the cbu so the kerdi fabric has something behind it. I also like to use one piece of kerdi fabric with the shape of the tub cut out so that the fabric itself has some sturdiness to keep it from getting pushed backwards into that gap.

03-03-2010, 12:54 PM
excellent - thx...

So on that corner detail, how far would you take the kerdi... I don't want to wrap the corner as you were hinting at. I just want bullnose on the main side wall. Would i:1) take kerdi shy of the bead, 2) take it under the bead 3) over the bead? (especially given i have to redo both bead and the backer all around anyway)

If #2, do i just let the drywall compound overlap the kerdi?
If #3, will it anchor to plastic and/or compound ok?

03-03-2010, 01:02 PM
Brad, I'd Kerdi everywhere there is to be tile and nowhere there's not to be tile.
p.s. sorry for multiple posts. In my day job (software dev), we're sticklers for one issue per bug/thread/request... What you're asking goes against my very being, but hey i'll try :-)I know we do things differently than many other sites, but we have so many people answering so many questions on a project that we just gotta keep it all together. Otherwise there would be duplicate upon duplicate of "what do you have here?" and "what is your plan there?" and "which one of these are you using?" to get clarification enough to give responsible answers to some of the questions.

Works for us. :)

03-06-2010, 07:53 AM
OK - i'm hanging the hardi today around the tub. I took out the corner bead and all green board up the the adjoining drywall on the short end of the wall you see in above photo.

I'm planning to take hardi all the way to the end... Then corner bead over drywall and hardi joint. Do i screw the corner bead to hardi with rock-on screws or drywall screws?

On the other side wall where hardi will butt up to drywall, assuming i can use regular tape and drywall compound to make the seam?


Brian in San Diego
03-06-2010, 08:08 AM

If you are tiling out to that edge I would not install the corner'll interfere with setting your tile. I would not use drywall screw anywhere they might be exposed to water or moisture.


03-14-2010, 06:55 PM
OK, i primed and leveled the plywood decking with SLC... I've got 1/4" hardibacker all cut and ready to lay down. Was going to thinset and screw it over/through the SLC and into the plywood.

Highest point of the SLC is probably 3-8"+ (maybe 1/2") as i had a really bad diagonal/downwards slope in one direction.


1) i've been reading several posts here and elsewhere that say SLC should be poored over the backer and not under it. There is no way that would have worked in my case. I am pretty sure the slopes/changes in flooring and the rapidness of it nearly would have cracked the backer (cutting it would have been on a seam in the subfloor).

2) i could also go with ditra as an alternative, but have the hardi and am tempted to stick with it unless there's really a reason for spending the extra at this point...

3) my local Lowes no longer carries Kerabond (just keraset/kerafloot and the ultra stuff)... Thinking i should go pick up versabond for both the Kirdi->backer, tile->kerdi as well as the floor with hardi-> slc and tile->hardi. thoughts?

thx in advance...

03-15-2010, 09:24 AM
any takers?