Fix stucco underlayment beneath old ceramic tiles?
December 26, 2005 10:51 AM   Subscribe

DIYfilter: We're re-tiling the kitchen countertop. Underneath is a one-inch thick stucco-like material, some of which broke and came out in chunks with the old tile.

The stucco-like material is one inch thick, chicken wire on the bottom, and appears to have been "poured" or applied -- wet -- in-place. A particular area must have been internally weak, because some large chunks broke and came out when I tried to pop the old tiles off. How can I fix this short of removing the ENTIRE underlayment and replacing it with a cement backer board over the whole counter? Can I just replace the broken area with 4 layers of quarter-inch wonderboard stacked-up and fill the gaps with cement? (I'm not even sure it's possible or practical to cut wonderboard in jagged shape?) Should I get some stucco mix and try to "re-pour" the bad areas?
posted by fuzzy_wuzzy to Home & Garden (5 answers total)
You can fill in the holes with 'stucco mix', and it will probably be ok. I think it's probably some kind of filler cement they used before. Pulling out the rest of the stucco and replacing with cement board is the better way to go, however--it'll be a much better surface for your new tile, as well as last longer. You might have some problems with tiles cracking or not supporting weight when you fill in the stucco holes.

Note: plywood would be ok as underlayment, too--unless this is constantly exposed to water, ie shower.
posted by lester at 11:32 AM on December 26, 2005

I would not try to feather in more of that junk. I agree with Lester you're better off pulling it out and starting from scratch. I know it sounds like a pain but then you'll have a level, solid surface to lay the new units on--essential for any tile project.

Not knowing the total thickness you're trying to achieve here so I'll omit those dimensions. I would first lay down some construction grade plywood, "knotty" side up. Then I'd put down a layer of WonderBoard or Hardibacker (I recently used the latter and found it easier to work with. It cuts like drywall and you don't get those crumbly edges like the Wonderboard gives you) set in a thin layer of Thinset. I also like to tape all the joints of the backer board with mesh drywall tape and mud those joints with some Thinset too.

And screw everything with some galvanized or exterior grade coarse thread screws. Nails tend to "pop".
posted by SawBeck at 3:19 PM on December 26, 2005

Try self-levelling concrete, used for exactly this sort of thing on floors. You mix it very wet and pour it in. It levels itself out, filling in every nook and cranny. Works like a charm.
posted by johngumbo at 4:16 PM on December 26, 2005

Best answer: It's mortar. Professionals frequently use mortar instead of overbacker board to install tile. It requires some skill to apply, but allows you to do things like slope the tile so that your counters drain into the sink. This shows how to prepare, mortar and set tile on a countertop. You might consider just repairing that small area using mortar, instead of cutting out and replacing with backerboard.
posted by electroboy at 4:25 PM on December 26, 2005

Response by poster: eb, that link to was exactly what the, um, doctor ordered. I've decided to try and repair the broken areas, rather than replace the whole thing. If I remember, I'll update this question in a few weeks to document my experiences and results.
posted by fuzzy_wuzzy at 9:05 PM on December 26, 2005

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