Filling the cracks will become a lifelong project. The cracks aren't the problem. It's what's CAUSING the cracks that needs to be attended to, or the cracks will keep coming back. Now, that could be anything from the subfloor starting to rot, to the wrong thinset being used and now it's beginning to let go, to improper installation methods being used when it was originally installed. Your best bet would be to get a local contractor to come and take a look at it.

Bill V.,

You're right. But I wonder, since they're hairline, if replacing the grout with a batch mixed with latex additive might help. What's your off-hand thought on this hair-brained idea? Just wondering.

- Homebound

Okay-- here's the thing-- I've had people ask about taking it even another step further, and replacing the grout with epoxy grout. I'll tell you the same thing I told them. You will NOT stop movement where it's occurring. You can only take it into account. Most grout already contains some kind of acrylic polymer, so it's a dead issue, but just for the sake of argument, let me follow this where you were taking it (assuming you're looking to make the joints harder and tougher to crack). If you accept the given that I put forth, that you're not going to stop movement, then you'll understand when I say that movement, just like electricity or water, will find the path of least resistance. Right now, that's the grout joint. Now, the only way to make the grout joint hard enough where it won't crack is to make it harder than the tile. But if you do that, guess what's gonna crack NOW? Remember-- you won't stop the movement.

You need to find the source of the problem and fix it, or it'll keep coming back, and sooner or later, it's going to start getting worse.

Mixed with latex, I had figured the grout joint would be more flexible, not harder. When I did an exterior repair mixing thinset with latex from DAL tile (no water), the grout still seemed rubbery after it was cured. I figured the grout might also have a tough more resilience, but maybe I'm wrong.

Correction...."have a BIT more resilience" (not "tough").

Use that acrylic grout that is used with DuraCeramic, that remains flexible.

I know it isn't fixing what is causing the grout to crack(movement), but it will resist if not eliminate the grout cracking.

I've done a couple of repairs with minimal grout cracking by scraping out the old and mixing the grout with a 50-50 mix of water and Keralastic liquid (Mapai).....more flexible than most latex additives and it's worked well for me. As Bill pointed out, the underlying issues really ought to be addressed, but for the $, this can be a cheap fix...