Why can't I lay tiles on an uneven floor?

Question

My buddy and I have gotten into a bit of a dispute on facebook, here's what he asks:

I keep hearing tiling an unlevel floor will result in tiles coming up. This reason makes no sense to me. The only reason a stable but unlevel floor will have tiles come up is if the bond between the cement and floor or tile fails. This failure should have NOTHING to do w/levelness of the floor, correct?

My response goes something like this:

Here's the thing: Unless you use a LOT of paste underneath the tiles (which won't dry out properly so will crack fast) then you will have a potential for a lever situation once the paste that's under an uneven one does dry out (because it will). Then that will cause disruption under the other tiles. Can't you just lay another level of backerboard to try and get things mostly level?

The more the floor is uneven the more you will have to use extra cement, which means it won't dry out at the same time. Here, I'll go ask the pros for you to see what you can do since apparently you don't like my idea of asking them.

Am I on the right track? What can he do to reduce the risk of the tiles coming up if the original floor is uneven?

FWIW: He's refinishing an older townhouse that he purchased in Houston, TX (in case age/location makes a difference)

UPDATE:

after feedback from Tester101 (much obliged) the actual problem is a wavy and uneven concrete floor. It's for the living room, so there's not really any real risk of water.

Answer 1

If the floor is concrete the best thing todo is scorch the uneven floor with a grinder making deep grooves in various directions. Clean with water jetting loose dust away. Mix some new concrete. Wet the floor again and screte it to a level But dont make it smooth just leave it slightly rough maintaing the level. Pour water on as per concrete bag reccomebds. Usually 48 to 72 hours. Let the concrete dry for another 48 to 72 hours. Then using tile paste and the tile tool (the one that is straight on one edge and has teeth on the other) lay the paste.

If it's a wooden moving floor. Like suspended on wooden beams and when you walk on it the glasses in your cupboard rattle then you place long as possible boards to have a flat even surface that is as continuous as possible. Make sure to find out which boards to use. I can't remember the name now. Then using the same tile paste lay your tiles. You might want to use chaulk or silicone to grout. But using normal grout is ok. It could just make hairline cracks or sometimes just a section would shatter. Just need to regrout. This is the minus side of unstable floors.

I have tiled 1 place on a place in Europe a house built in 1st world war. Some german barracks where everything vibrated when you walked on it. I used that method 11 years ago and no complaints till today.

Answer 2

You can lay tiles on an unlevel floor (consider outdoor locations where you need water to run away from the home). What you shouldn't do is lay tiles on an uneven, settling, or unstable floor.

Edit: As Tester101 said, you're both right, my point above was the same "unlevel != uneven" comment.

If the problem is an uneven floor, then cement leveling is the fix I know of. It sounds like ppumkin is describing a similar solution, and there are lots of howto's online that can describe it better than I can.

If the floor isn't stable (soft spots), then installing a layer of cement board first will firm it up.

And if the problem is that the floor is still settling, then you need to find the source and fix it before attempting to do any tile work.