mosaic tile floor has weak spots

December 1, 2008 6:54 PM

My mosaic tile floor is crumbling. I suspect its because the floor gives a little under weight. If I repair it, will the same thing happen again? Or, what else could I do to avoid giving up on it completely?

I put together a mosaic on the floor of a small open area and the bathroom, (cement glue, grout, small pieces of heavy floor tiles), but recently random tiles which seem like they should be squished solid in the middle started coming out, and then a weak spot is formed when the ones around the hole can lift up.

It feels like the surface of the floor below the mosaic (which unfortunately is not concrete but I;m not sure what it is) is not 100% immobile, and tends to move a little under weight (probably similar to stepping on an older wood floor, pressing less than a centimeter down under weight).
I am guessing this causes the grout to somehow break in between the firm tiles and the ones that have the possibility to move when stepped on.

I don´t think I have the resources/ money/time to pull up the mosiac and the material beneath it to resurface and redo the mosiac completely. Most areas of the floor are completely secure and dont/wont need repairs.

Is there some way I can redo these trouble spots where a couple pieces lifted up and make them more firm or able to overcome this problem?
If not, what could I possibly do to salvage my mosaic or floor, aside from calling it a complete failure and putting something flexible like linoleum on the floor?

posted by nzydarkxj to Home & Garden (6 answers total)

Did you install concrete backer board before laying the tile? We tiled the floor of our half-bath last year, with the help of Home Depot's DIY instructions.

After removing the old carpet and pad, down to the plywood subfloor, we screwed down concrete backer board to create a solid level surface for the tiles. I think if you skip that step, the tiles/grout will crack due to slight variances in the subfloor surface when they are walked upon. Don't really know how to fix the problem after the fact, unfortunately, as you would need to install the concrete backer board below all of the tiles.
posted by ktpupp at 7:12 PM on December 1, 2008

In addition to concrete backer board, there are now several products (rolls of what look like press-formed plastic or cork sheeting) for sale for tile underlayment that claim to be able to tolerate slightly more movement than traditional installation techniques. I've not used them, but have seen them for sale at Home Depot and similar stores. And there are thin-set mortars that claim to allow a bit of give, too.

But that doesn't solve your problem without tearing them up and redoing. One possibility might be to remove the grout (not so hard as it sounds, though it makes a big mess -- you can use a hand grout-saw or an attachment for a Dremel tool), glue down the loose tiles with either tile adhesive or construction adhesive, and in place of grout use grout caulk -- you can buy it in all the same colors as regular grout, and with the same sanded texture. That might give you the flexibility you need, at the cost of a lot of trouble and some expense.
posted by Forktine at 7:17 PM on December 1, 2008

Best answer: My tile-setter/floor-layer husband says the following: "If you have movement, you are screwed." Not too constructive, unfortunately. He says that the grout caulk might help a bit, but tile does not accomodate any movement at all, so even with flexible grout, it is a matter of time before the tiles bust loose. Instead of cement backer board, which is expensive and adds height, add a layer of Ditra membrane to the subfloor before tiling. Instructions can be found on the Schluter website. Good luck: this sounds frustrating.
posted by ms.v. at 9:01 PM on December 1, 2008 [2 favorites]

Yep, this installation is done for. FWIW, cement board alone wouldn't have solved the problem, and Ditra probably wouldn't either. The floor underneath tile has to be absolutely solid. The floor framing must be rigid and the subfloor must be a stable material, firmly attached to the joists. Cement board is good for adhesion and water resistance, but isn't mandatory.
posted by jon1270 at 3:25 AM on December 2, 2008

Best answer: Agree with others- cement and grout is pretty much incompressible, but it has very little tolerance for strain.

What I'd do:

1 - If possible, add joists to the subfloor. Get in the basement and just siamese (screw them together) joists to the existing joists. If you can, add them in a way that actually adds structure to the floor- get them up over the supports in the basement. But at least, get them under the bathroom so that while the whole floor might bounce a little bit, the whole plain of the floor doesn't flex. They did this on This Old House a few seasons ago for a second floor workout room- they actually added a sandwich of steel and a second joist to every joist. The floor was rock hard and had no flex at all.

2 - If that's not possible, just reinforce the snot out of it. Pull the floor completely out, even the subfloor, right down to the joists. It's probably 5/8 or 3/4 thick. What I'd do is put some construction adhesive on the joists, lay down a layer of 1/4 plywood, then some construction adhesive, then another layer of 1/4, more adhesive, then a layer of 1/4 backer. (Use whatever thicknesses necessary to get the floor to the right height. Use a ton of screws for fastening. This should stiffen the floor enough to maintain the tile.

3- If that's not possible, I would re-lay the tile using a flexible caulk as the glue that holds it down, and then get (or mix up at home) flexible grout. Use a water soluble caulk and mix in some fine sand so that it looks right. This will allow it to give and flex without wrecking itself.
posted by gjc at 5:51 AM on December 2, 2008 [1 favorite]

There's a reason they used to box out and pour four inches of concrete in older, tile-floor bathrooms. Any movement, and it's ixnay on eepingkay the iletay.
posted by notsnot at 7:28 AM on December 2, 2008