View Full Version : Shower/tile backer board on old plaster - NEED ADVICE

05-25-2007, 03:29 PM
We're been remodeling a 1940's house we bought that is CINDER BLOCK construction. We're handy but novices. The bathroom is our current project. We removed all the old tile (some of it was soft) and some of the old plaster just fell of the like dust. The rest (what is not under the window) is still rock hard/sturdy. I've attached a picture (its not the most recent but what I have here). SINCE THE PICTURE - I've removed the toilet, bathtub and the rest of the tile/loose plaster. I want to put up backerboard and then tile. Can I use glue/tapcons to attach the backer to what is there without removing the rest of the plaster (which is attached VERY well unfortunately)? Then Ill have a smooth surface for the tile. Will the backer board be ok going over the old tile adhesive (brown stuff in the pitcure). Do I need to try and remove all the plaster, use furring strips, and then put up the backer? I'm torn here and I've read about tons of options. Any experience with this out there? I REALLY could use some clarification.


05-25-2007, 04:11 PM
I have some ideas, but I think you'll get more, and probably better ones if you ask at www.johnbridge.com (http://www.johnbridge.com). I would not want to try to attach the cbu with tapcons. Drilling the holes and then trying to get them to seat in the cbu risks having the leftover plaster let loose.

Chipping the plaster off with the right tool, probably would go pretty quickly. In theory, you could tile directly over the cinderblock. I think you might be able to set Wedi panels directly on what is left, if it has a fair amount of surface area. These panels are fairly expensive, but would also add what may be some very much needed insulation (they are special foam panels with a coating of a special cement product). You can tile to them directly, and you might be able to just thinset them to the existing bits of plaster. Not sure, but it might be worth looking into. www.wedi.de (http://www.wedi.de) is the parent company. They have a US branch, but I don't remember their website - you can get to it from the one above, though.

05-25-2007, 05:16 PM
since i don't know how important heating and cooling are to you, there is a lot i cannot yet say. Where are you located in terms of geography and climate?

you don't need cement backerboard since you have cement already in the cinderblock. You can thinset Kerdi or Wedi, or you can paint RedGard or any other liquid trowelable membrane. By not putting cement CBU backerboard, you save time money and you get a bigger shower. You apply the Kerdi or Wedi or Redgard directly to the cinderblock -- if all conditions are OK, which may or may not be the case until you tell us a lot more.


05-25-2007, 11:05 PM
I live in the Pacific Northwest (Oregon) so things are often damp and cinderblock is not a typical construction around here. Its got stucco siding, also not typical but the house was built pretty sound from what we can tell. We plan on installing a new tub and then we want to tile the surround walls. We had a new window installed (along with all the others) this past week. Hopefully avoiding any future/potential leaks. So if I went with the kerdi or wedi I would need to make sure I got all the old plaster off the blocks, making a flat surface. If I install the kerdi or wedi on the cinderblock, will it go over the tub tile flange as if i was installing CBU, just resting above the tub (not in full contact)?

I really appreciate the responses/discussions.

05-26-2007, 08:02 AM
Basically, yes, you want your waterproofing layer to lap over the tub's tiling flange. How are those walls insulated? If you wanted to add some, then I think the Wedi would give you a one-step insulation/tile backer step, but it is not cheap. If you can afford the space, you could build a wall against what you have, insulate it and the either use cbu or wedi. With the Wedi, you could fairly easily cut a rabbit on the back to cleanly go over the tub tiling flange.

Kerdi is great for showers and is designed to be attached to normal drywall (since it is waterproof, the drywall will stay dry), but I worry about the edges if you use it on a tub/shower. There, if you used Kerdi, I'd think about cbu instead of drywall on the walls - that's what I did in my remodel. But, if you ran the drywall all the way below the flange, then added a strip to come over the flange, that is an approved method.

If you use the Wedi, you wouldn't need or want Kerdi, since the Wedi is already waterproof.

The Wedi site has some good pics and you can download the manual.

The www.schluter.com (http://www.schluter.com) site has some videos of using Kerdi to build a shower that is interesting, too.

You will need to pay close attention to detail when tiling around the window to preclude moisture intrusion.

05-26-2007, 09:53 AM
Wedi... one-step insulation/tile backer step...yes, it will cost four or five times what gypsum board costs, and two or three times what CBU costs, so maybe $50 more than the cheapest method and $30 more than the Kerdi method. It is fast and clean, straight and plumb, and it adds heat and cold insulation too, as mentioned. On one side, thinset it to the concrete surface, and on the other side, thinset it to tile. It the middle it is 100% foam, of the most rigid kind, made by a Dow Corning firm. So, it is waterproof and structural.

I have used all these methods, not just Wedi, and I am a thorough researcher too, so i am telling you what i think i know, based on being both a textbook expert, and a bathroom builder. :) .

Scraping off old plaster is the hard part. Wet it first.


05-26-2007, 10:09 AM
well it certainly sounds like the way to go. $50-$100 is worth the time I would spend doing the other stuff. I'm working on the plaster today and its a beast. I was considering clearing off 'strips' and then putting in furring strips. Then I could put up the backerup (glueing to the plaster and screwing to the furring strips). Similar to this post ... http://www.diychatroom.com/showthread.php?t=3175 (look at the picture they posted).

But I won't take a lazy way out so if thats no good so be it. I guess Im still confused about how I would thinset the Wedi onto the cinderblock and then still have it overlap the tile flange without it touching?

Sorry for all the questions, I'm really not as incompetent as I sound. I know I can do this, just too many options!

Here's a more current picture of the bathroom:

Thanks again.


05-26-2007, 10:24 AM
Ok I had a thought, maybe Im crazy. What if I scrape off all the plaster under the window and square it up. Then I install 1/2" cement board on that part so it's flush with the plaster on the other sections. Then I thinset the Wedi onto of everything (plaster sections and the cement board section)? I know the cement board may not be necessary if I'm putting up the Wedi but I'd like to be safe, not cheap. Also I assume I need to put up cement board on the wall with the showerhead which is a wood stud supported hollow wall and then put the wedi on top of the anyway.

just a thought.

05-26-2007, 02:57 PM
no matter what you do, it'll be fine as long as you have a single waterproofing layer, and that that is done right. One only, never two, so no to, let's say for example, a plastic sheet under CBU under Wedi.

That is all you have to keep in mind. A single water (vapor) barrier. Then, yes, you do need to make sure that the water (humidity, moisture, H20, steam, mist, vapor) that gets under the tile and seeps, drips, percolates, slips, downward due to the force of gravity, finds its way into the inside of the tub and not into the studs or cinderblock down at that tub rim level. That explains why the overlapping is necessary and why the little air gap is necessary too. Behind the last tiles there is a little void, emply space, at the bottom, so that water that is there has only air in contact with it and not anything that might wick it back up into the wall. (Whew!)

all the rest is less important, and not mission critical. Whether to fur, to screw, to glue, to attach this way or that, to leave plaster or to remove it, to build out to accommodate some other need, is all up to you. It is best decided by the person delegated to be there. He makes on the spot decisions because he can see what he is up against and he knows best. I leave it up to you.

Having said that, I need to remind you to read up on Wedi instead of jumping to conclusions or assuming things that may not be true. It is a true replacement for CBU (cementboard) and plasterboard, so a little extra Wedi or one thicker Wedi panel is all you need if you want to build out the wall. Have you ever cut CBU bfore? Do you know how much dust it makes? Do you know how bad that dust is? Silica dust, cement dust. When you get your hands on Wedi, you'll be glad I told you to look at it first before deciding things based on assumptions about what it is. Tactically speaking, you'll be fine with Wedi instead of mixing in some CBU. It is not serious. It's just practical, to use Wedi to thicken the wall if you want to thicken the wall.

Although i find it hard to believe that scraping off prewettened plaster is deemed hard work, I can agree with you that it's not a necessary step. Skip that step. You can thinset Wedi onto a rugged surface mix of bare cinderblock and old plaster. Or you can screw it on. Or both.


05-26-2007, 03:54 PM
If you look on the Wedi website, they show some freestanding tiled showers, the only structural part is their foam, cement coated panels. Admittedly, in the freestanding designs, it is much thicker than you'd need on a wall, but it all works the same way. Wedi can be thinset to a compatible surface, or it can be screwed to nearly anything else. It is ready to tile after you use their special pucky you spread over penetrations and joints. It is quick and easy to use, it just costs a fair amount. WHen time is money, you could start with a rough opening and be tiling in probably 2-hours. Spend some time on their website. Look over Kerdi on www.schluter.com (http://www.schluter.com). My guess is that it wouldn't hurt to add some insulation in this area, and there, the Wedi wins out for its speed of installation and ease of use, but again, it is not cheap...what's your time worth? It is a very good product.

05-29-2007, 11:36 AM
Thanks again for all the suggestions. I've spent quite a bit of time looking at both products. Kerdi seems a lot easier for me to get my hands on and I think I'm going to give it a go.