Flattening CBU Walls [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-08-2003, 11:24 AM
Hello everyone, my name is Dan (but that name was already registered. For some quick background, I recently started what was going to be a minor bath remodel. Unfortunately, I only more recently found this forum. I've been reviewing the posts here for a couple of days now and found it to be loaded with great info.

Anyway, long story short, while tearing out some shower tile I found some water behind the wall which led to a complete tearout of a full mud job, 4'x5' shower down to the studs. Man does my garbage man hate me!!!! So far I have put in the pre-slope floor and PVC membrane up the walls about 9", I have also completed the CBU installation.

Which brings me to my question. I did not shim out the studs or notch the bottom where the membrane is folded as I found yesterday on this forum shoud be done. I put a straightedge on the wall last night and found, whatdayaknow, my walls are not perfectly flat. The worst case spot is shows a dip of almost 1/4" but most of the "unflat" areas are maybe an 1/8". I really don't want to tear out the CBU and start over so is there any material out there that I could use to fill in these depressions and then just screed using a straight edge to get everything nice and flat???

In case anyone is interested, the reason the original installation failed I think was a comination of things.
1) The PVC membrane was laid directly on the plywood subfloor with no pre-slope.
2) The weep holes in the drain were clogged (no gravel or spacers around them prior to floating the floor.
3) The PVC pan only ran a couple of inches above the level of the floor. The mud job was on top of greenboard which was sitting almost on the pan. Sooo, when the water got into the floor mud and had nowhere to go, the greenboard was quite happy to suck up as much as it could. See how much I think I've learned from this forum!

I guess its good that I found this now before any major damage was done but man this stuff is a lot of work. Thanks for any help you can provide and I'm sure I'll be back with more questions...


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01-08-2003, 12:06 PM
Welcome to the forum Dan.

Looks like you've been doing your reading for sure. Stick around for a short and JB and the rest of the real tile pros will be along to give ya the straight skinny. Happy Tiling :)

John Bridge
01-08-2003, 06:02 PM
Hi Dan, Welcome. :)

Listen, if you screwed the cbu up, I would advise you to take it down and do the notching. If it's nailed, I understand your reluctance to do that.

I get in trouble all the time around here for recommending thin set to flush out dips in walls and floors, but I've done it for years. I would use the same thin set you intend to use to mount the tiles. (What type of tiles are you using?)

Mix it just a little stiff and trowel it onto the wall with the flat side of a notched trowel. Use a board as a straightedge to rake if flush. Keep the board wet as you work. Have a bucket of water and a sponge handy to repeatedly clean the board. Thin set is sticky stuff. In the quarter-inch area you might have to do two applications a day apart. Lightly smooth the thin set when you have it where you want it. Keep the trowel clean and wet as you do this. The thin set will, of course, want to stick to the trowel too.

That's the easiest and cheapest fix I can offer. Your walls won't be perfect, but they'll be better. I'll repeat, the best way is to take down the cbu and make everything straight.

01-09-2003, 08:14 AM
Thanks for the advice John. Actually I went home last night and had already decided I would take down all of the CBU (it was screwed not nailed). I tend to be more toward the perfectionist side of things and knew that if I didn't correct it and it showed up after tiling that it would bother me every day when I stepped in the shower. Luckily I have another bath in the house so I'm not really under any time constraints to finish this one since I only have time to work on it on weekends and a couple hours during the week.

I'll have to check the brand of thinset I'm using. I got it from the tile store and it's in a black bag. I know it's not Custom. It does have the latex modifier already in it.

Another problem with my walls besides the folded pan was that the last job had used construction adhesive in addition to screws to attach the greenboard (under the mud walls). I've chiseled and scraped as much as I thought was reasonable but it looks like I might have a bit more work to do there before re-installing the CBU. Or would I be better off just laying down a heavy bead of consruction adhesive myself to help even things out before re-installing the CBU?

What keeps bothering me about this shower is that from what I've read, the guy who installed it did a quality job with the full mud walls and everything EXCEPT no pre-slope under the pan and he didn't keep the weep holes open. Did this guy skip the pre-slope just because he didn't want to have to let it dry overnight and come back the next day?? I realize time is money but to me, pride in what I've done (and knowing it is done right) is just as important. Although... I did notice on the Oatey instructions for the PVC liner, it showed the liner placed directly on the subfloor with no pre-slope. Go figure.

Again, this is a great forum. Thanks for the help and I'm sure I'll be back with more questions as I sloooowly progress.


01-09-2003, 08:22 AM
Don't use construction adhesive as a filler under CBU. It will remain flexible forever. Instead, use shims of lath, or even strips of tar paper (felt). If you have a high spot on only one stud, you can plane or sand that down.

01-09-2003, 10:49 AM
Here is another workaround. Just take down about 24 inches of the walls down at the bottom. Peel off the membrane from the walls and using a jamb saw, chisel, whatever cut a series of cuts about three eights of an inch deep. Make them consistent and about half inch apart. Using a sharp wide chisel, knock out the kerfs and you have knotched the studs for the membrane.

Re-attach the membrane with nails, staples whatever at the top. I use roofing cement or butyl to attach the membrane flat and tight against the cutout.

01-17-2003, 12:44 PM
This is kind of a long post so I'll start with the questions and if you're interested in how I got to this you can read the detail.

1) My curb is deck mud but seems solid. Is this OK?
2) Can I add anything (mud, cement) to the top of the curb to make it taller?
3) Can I use a 1/8" thickness of thinset on top of cbu to get a doorway opening the correct width?

Thank you all again for your expertise. I sleep better at night knowing I'm doing things right, or at least I would sleep better if I could stop thinking about what I need to do next and any things I need to watch out for.

The detail:
Finally getting started on the shower again after a little delay. I've got the CBU up (again) and floated the curb and shower floor but was just reading another thread where the gentleman was having a problem with the sides of his curb pulling away from the top. I guess he wasn't the only one who missed the fact that the curb is made from masons mix because I used the same mud that I used for the floor. I don't have any problems with it breaking apart though. It seems to be solid and in fact I was kneeling on it when floating the floor the next day. Now I'm worried though. It's solid now but will the curb stay that way after it starts getting wet? I can't tear it out since the floor is in place now so if it will eventually break up is there anything I can do to keep that from happening like skim coating it with thinset or masons mix?

On another note, I am trying to re-use my shower enclosure (door on one wall and large piece of glass in a frame that rests on the tub deck) and to do so means I need to get the width of the doorway, and the relative height between the curb and the tub deck pretty much exactly the same dimension as before. Problem is the earlier installation was a full mud job so thickness was not the same as with cbu. I'm close but my doorway opening is about an 1/8" too wide and the height between my curb and tub deck is probably a 1/2" too big. Can I use a flat 1/8" layer of thinset on the jamb side of the door to make up the difference? Then when that is dry set my tile as normal. And for the curb, can I add 1/2" of mud even though my initial curb is set up? Or cement? Anything?

Obviously I should have thought of that earlier and I could have taken more care to make sure I had my dimensions set but in the thrill of getting all the cbu up I just had to get on to getting the curb and floor in.

01-17-2003, 02:23 PM
:) It's easy to heighten a curb, check this link:

John Bridge
01-17-2003, 04:07 PM

On the pre-slope it could be that the guy didn't know any better, but it's more likely the pan was done by a plumber and the tile man had no control over it.

I keep meaning to get hold of somebody at Oatey and have them change their instructions to agree with the codes. I write it down and then forget about it. I'm writing it down again. ;)

There is usually at least an inch of slop in the width of a shower door frame. Check it out. On the curb height, do it like Dave M. did. Bond a thin layer of thin set to the old mud and then add the new. It'll stick. If your deck mud curb has held together thus far, I wouldn't worry about it. Deck mud becomes very strong. It's just that most people can't get it to hold together until it cures out. You must be somebody. ;)

01-20-2003, 08:42 AM
Thanks guys for the link and response. John, my wife says I'm somethin too! Just haven't figured out what yet. :)

The shower door width does have some slop in it but I am trying to avoid drilling more holes in the frame if I can. I did make some progress this weekend on the project, unfortunately I'm still a long way from tiling. :( I've got some drywall work to finish up, then paint and I still need to tear the tile off the floor and re-level it, then tile and trim). I was thinking of breaking things up and start tiling the shower but I need to clean up before I start that. I'm hoping to be finished with the whole thing in the next few weeks!!!! Still a lot to do though. Thanks again for the help and one of the most informative web sites I've ever come across.


John Bridge
01-20-2003, 06:13 PM
I think that's the way to go. Get all the grunt work out of the way at once. Then it's all down hill. (Mostly) ;)

01-21-2003, 07:32 AM
Got things cleaned up a bit last night but still need to remove the tile on the main floor so I've still got some grunt work to go.

Thinking ahead though, I'm planning to tile the ceiling in the shower with 6x6 ceramic. What is the best way to support the tile while the thinset dries? I was thinking of cutting a piece of plywood the size of the ceiling and propping it up with some 2x4's. Is that overkill? Do I need to support them at all. I was thinking that if I mixed the thinset a little stiff maybe it would help hold things in place without the additional support. I did see the pictures with the heavy duty bracing for the LARGE ceiling tile (I think "panels" would be a more appropriate term) :eek: I'm pretty sure that's a little more than I need.


01-21-2003, 07:41 AM
This is One area where John and i have crossed horns a couple times. Mastic works very well for a cieling and there will be no need for any bracing at all. Use a type 2 mastic Mapei makes a very good one.Use a 1/4 x 1/4 sq notch trowel and push the tile up into the mastic.suction will hold it for forever. allow 36 hours to dry before grouting.
The only time i use thinset on cielings is for marble, granite, or a larger format tile. I have i am sure a thousand feet or more of cieling tile stuck with mastic thats still there. Use thinset for the walls though OK :)

01-21-2003, 08:16 AM
Boy, now I am confused. I thought mastic was a no-no for anywhere near water. Obvioulsy the ceiling isn't going to get soaked though. Is the type 2 a water or moisture resistant version? It appeared that the tile I removed from the ceiling was stuck up with construction adhesive. It was up for 8 years and I never had one come loose so I guess anything is possible. I'll have to check and see what Home Depot has for mastic. I'd also be interested to hear John's response.

John Bridge
01-21-2003, 08:05 PM
As much as I love to speak badly about ma- m- mas- - - mastic, it will work on the ceiling. It will not get wet

However, you can do the same thing with thinset. You can glue the tiles to the sheetrock and walk away. The odds of six-inch tiles falling down is pretty remote. Make sure you get good suction and don't have a lot of material between the back of the tiles and the sheetrock. You'll be fine.

And Todd,

You know we don't lock horns. Knock down dragem up, maybe, be we don't lock horns. :D