What stops the water?

You forgot the sealer.

You're relying on a properly-installed and well-maintained floor. Properly-installed means all the layers were put down as specified by the manufacturer, without gaps or cracks. Well-maintained means applying sealer every couple of years (more often if heavily used), repairing any chips or cracks, and mopping up spills as soon as they occur.

None of the layers you mention is actually waterproof except the tile, and that's only waterproof if it's glazed. (Everyone feel free to point out some type of tile that I don't know about.) The sealer fills small cracks, including the pores in the grout, but it wears out.

I can tell you from experience that water gets through or around tile. I recently relaid a bunch that came loose from plywood because of a leaky roof. (The client didn't want if fixed right, because he plans to tear the building down in a year or two.)

Grout is not waterproof. Some water will get through it. My understanding is that backerboard absorbs some water, which then evaporates into the wall space and through the other side. I was told NOT to use a vapor barrier on the back of the backerboard to avoid mildew problems. It traps the moisture in the backerboard. I assume the same would be true for floors.

There are products like Redgard which can be applied to the backerboard to waterproof it before applying the tile. I assume that any moisture absorbed would then have to evaporate back through the grout.

Epoxy grout is probably a lot closer to waterproof, but more trouble to put on, and more expensive.

A mop.

Rely on the towel and floor mats you hang to dry.

Sounds like you are doing the right job. Don't worry about excess water, its the nature of the beast. Just use a towel.

Water will stop at the lowest point. I don't know how much water you're talking, but if you have alot, it may get under the baseboard and flow under the tile and such anyway.

Women are like tile. If you lay them right the first time, you can walk on them forever.

why is there so much water on the floor?

You can try the Schluter water proofing method under the tiles, but if you have that much water maybe you should slope the floor down to a drain and add a "dam" across the door threshold to keep the water in.

hopefully concrete backerboad and use cement to seal all seams

My understanding is that you want at least 1 1/4" of wood under the backerboard to get the necessary stiffness.

So what about tiled walls in the shower.. the water impacts directly on the tile and grout...

Mark

Uhhmmm? That's what I was talking about.

Grout is water-resistant, but not waterproof. It can stand up to 15 minutes of water, especially on a wall, but not to hours of exposure.

Shower floors have a waterproof pan underneath to stop the water that gets through the grout. Walls rely on you getting out of the shower and letting them dry.

It's _still_ a good idea to seal the grout in the shower.

No kids (or dogs), huh?

-- aem sends....

I did a little checking in some of the tiling books at the local Big Box store and many of them advocate a membrane be put down on the bathroom floor to help prevent water getting to the subfloor. Is this really necessary or is it overkill?

Based on some of the floors I've seen, it's not overkill.

A client called just today to have six one-inch tiles put back down on her shower floor. That's normally a quick fix--chisel off most of the old thinset, then put it back down with either thinset or mastic.

I removed the old tiles, which were lying loose in place, and found a hole full of sand. The old concrete has decayed down at least two inches. Of course, they don't have the money to pay for the correct fix (put in a new base, pan, etc.), so I'll get out the junk, fill the void with concrete, and put the loose tiles back with thinset.

This is a good reason to use a membrane.

Thanks. Just to clarify, I'm inquiring about the bathroom floor not the floor (pan) in the shower. Does this change the answer?

Only a little. The last _major_ tile repair I did, the tile was laid on a plywood subfloor. A ceiling leak had caused the thinset to release from the plywood. A membrane would have prevented that. A ceiling leak is pretty much the same as a sink leak or a teenager who can't figure out how to mop up water after a shower.