Tile on Floor
 

How soon after I lay concrete can I lay tile?

I was told to lay a peice of poly on the floor and 24 hrs later, if there is moisture under it the floor is still to wet.

I've heard anywhere from 24 hours to 28 days. I guess if I was going it, I'd wait the 28 days to be sure there was no question.

You can do the test as Neal stated, duct tape a piece of visqueen to the floor and see if any condensation appears. Or for about $20 you can get a calcium chloride test from a local flooring supplier . It is a fairly simple process, just follow the directions included with the test.
Tile is very forgiving when it comes to moisture, here in Florida we do a lot of tile work on very new slabs, as early as 28 days! Remember to get an accurate moisture reading it must be done in a controlled environment ( ac running, home dried in etc....)

The standard for concrete drying is 28 days per inch. Therefore a 4" slab should dry a minimum of 112 days. No one follows this but they should. Don't know about tile, but for other floor coverings, any sooner would void the warranty. I have been installing and teaching installations (everything but ceramic) for 40 years.

Not that I doubt you, Sam, but have you never mud-floated a shower wall and then tiled in less than 28 DAYS????? That would mean over three months just to tile a bathroom floor . . . not gonna happen.

Help me understand this in the field . . .

I said no one does it, but that is the standards for other floor coverings.
http://www.cement.org/tech/cct_drying.asp
This isn;t the one I usually use but it lists pretty much the same amount of time. It says a month per inch.

That time span does seam a little excessive, but we are taking about indoors not out side so I can understand it taking longer but not that long.

I said, no one follows it. Look at carpet, for example. The mills say that carpet should be rolled out in the customers house 24-48 hours before installation, that's not gonna happen either.

Try moisture testing and sticking to the manufacturers requirements. On most concrete floors, 4 months isn't long enough. It's their out. If it fails, you didn't follow their requirements, so no warranty.