Ceramic tile - how clean does the subsurface have to be?

[Tom7227]

I am replacing some ceramic tile in an entryway.The old stuff was put down on plywood by me maybe 20 years ago. It came up hard and I had to use an air chisel. There are areas where the old thinset didn't come up, maybe 15% of the total area. I have been trying to break that up but it is tedious. The tile I am putting down is 16 X 24 ceramic. How clean does the subsurface plywood have to be? I am close to saying the heck with the old and replacing it. Of course that means that a good solid piece of well seasoned plywood would end up being replaced by new,which probably isn't as well seasoned.My simple mind tells me that the notched trowel will be able to tell the difference in the areas where there is still some old thinset and all will work out.

Advice - keep scraping,R&R,or just go ahead and put down the new stuff?

Thanks for your time.

Tom

[YiGGiN4SLoBS]

I would keep scraping and get the subfloor as clean as possible. I would also recommend putting down 1/2 or 1/4" cement board underlayment with screws over the top of the plywood. THat stuff is $$ for laying tile on top of. Obviously the flatter and the cleaner the sub-surface will help with lipping and leveling the new tile and also help the new stuff stay in tact for the life of the house. hope this helps

[cavalierowner]

Like YiGGiN4SLoBS said, the best method is a new layer of cement board over what you have. Set the cement board in thinset. If that adds too much height (the door won't clear, for example), then I would work real hard to get the old thinset just as level as you can. That larger tile is very difficult to set level and avoid lippage.

You could also do what I did and tear out the old plywood. Just set the skil saw blade the thickness of the plywood, buzz the perimeter as close to the walls as you can, then cut the field in 12" x 12" squares. It should come up fairly easily if its only set with screws or nails (no adhesives) to the subfloor. Then set cement board on thinset on the old subfloor.

Good luck!

[Warner's Point]

I work excusivley with tile and would also recommend using 1/4 Hardy Backer or equivalent set with thinset and then screwed down. I have never set tile directly to plywood but have fixed many floors that have broken loose from the plywood. Durock does have a fairly new product which is a very thin felt like product that you thinset directly to the sub floor. It works great if you have height issues. Rubble Tile or MN Tile and Stone would be a god place to look for it. I have used it in my house and it is working great.

[CAMAN]

Menards has that felt stuff in their tile department, though I don't recall the name. I used it last year on a project so I don't have any durability reports but it went down easy and seems to work well.

[Tom7227]

Project is done. Thanks for the advice. Since I didn't wait for it the whole thing is probably going to fall apart. Had to get it done while the Mrs. was gone. That's what's important isn't it?

[JohnMickish]

Great info guys. I will be replacing the tile in my foyer soon. Original 40 year old tile I believe set over plywood, and yes it has some broken tiles.

Again thanks, and I'm sure I will be asking questions too.

[Sifty]

Hot Hot Hot water does wonders on old thinset when trying to remove it.

[Tom7227]

Hot Hot Hot water does wonders on old thinset when trying to remove it.

So does an automotive pneumatic air chisel.