Clarifications on Tile Over Plaster [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Mike K
05-08-2005, 07:26 PM
I have read for two days the tremendous amount of information on this site. It is great, especially for someone like me who has tackled everything in the house - except tiling. I have a bathtub that never had a shower. I opened the wall to run a show head. The walls are all plaster, nice and smooth (always had wallpaper, no paint - glue is gone) My plan was to close the one wall with sheetrock, put up backer board around the entire tub, and tile. Someone told me that this will not work and I was not sure why but they suggested Kerdi. I read up on that and the one thing that stands out is the notion of applying it with either modified or unmodified thinset. There seems to be two different thoughts on this when it comes to thinset on plaster. (by the way I already have the cement board and not the Kerdi so I thought I was ready to go.) Just trying to figure out once and for all the best way to go..kerdi, or apply backerboard to plaster walls and then tile. Thanks. I am a novice at this. (could you tell?)
Mike K

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05-08-2005, 08:44 PM
Hi Mike,

Just so I understand the situation...are the walls plaster over sheetrock Or are the entire wall mud/plaster?

You mentioned the tub never had a shower head..what kind of tub is it. There are some tubs ( garden tubs) that do not have a lip around the edge to keep water out of the walls. They were never intended to have a shower head for this reason. Pls tell me this tub has a lip around the edge..


Mike K
05-09-2005, 05:52 AM

Thanks. Yes, when I opened up the one wall for the shower, there is a lip around the tub. Looking at a cross section piece of the wall, probably from the 50-60's, appears to be sheetrock or gypsum and over that is what appears to be a good 1/4 inch of mortor of some type and then a skim coat of plaster. No lathe, but it definately is cement of some type in the wall. Regular bathtub as far as I can tell. Right now what I was going to do was close the one wall (the other two walls were not touched), put up hardibacker with sheetrock behind it to build it up to the right width. I was then going to apply hardibacker to the existing plaster walls and tile over.

Mike K
05-11-2005, 07:15 PM
As I was pondering my approach, combined with the fact that nobody local sells Kerdi, I would like to pose a question. How could water infiltrate plaster if I have backer board, and then tile, with it sealed? I am still not exactly clear why this would not work. Thanks
Mike K

05-11-2005, 07:28 PM
Mike,every tile has a certain amount of tendency to absorb moisture,grout also,then the cbu is like a sponge, it tends to suck it in and hold it,as the water molecules get hot, they get smaller, and are soaked into the tile job,thus permeating anything beneath.Now we don't know how much is going to make it to the plaster,but our TCA tells us ,and the cbu manuf. tells us to use a vapor barrier for this reason. :)

05-11-2005, 07:32 PM
Hi Mike, glazed tile is somewhat waterproof but grout and hardibacker are not. All it takes is a pinhole or small crack in the grout to allow water in. Normally it doesn't soak in fast but rather a little at a time over the years. :)

I would put up a barrier of #15 felt behind the backerboard. :)

Mike K
05-12-2005, 08:05 AM
Thank You for the responses. Unless someone screams at me that I am way is what I will do based on the great responses. I already purchased the hardibacker. I will use felt behind the hardibacker. As much as Kerdi sounds good, I can't find it locally and would like to move on this job this weekend. Thank You. The site is very helpful. Mike K

John Bridge
05-12-2005, 05:53 PM
Hi Mike, Welcome aboard. :)

Sounds like a plan. :)

Mike K
06-24-2005, 02:58 PM
My file job I think came out OK. (OK the tiles are still sticking to the walls!) The kerdi worked, I really feel it is sealed well. I would send a picture but as an amateur I would be embarrased. One more question if anyone sees this....I was going to grout this weekend. We in New England are going into a heat wave but I really want to get this done. I am not concerned about my temp :) but applying grout in 100 degree weather. I will have AC going in that area of the house but I am not sure if this will help too much because right now it is not going directly into this bathroom. Is it OK to grout in this weather? Any tricks to ensure it doesn't get screwed up. (Other than hire a tile guy?!)

Mike K.