I would mot install 8 X 24 porcelan on any floor that is not extremely flat and that I was sure the deflection was correct on for that tile format and type.

Specs determine that it should be flat to within 1/8" over 10 feet....or 1/16" over 2 feet. Those are NTCA Specs.

This would be on a concrete slab. It has had porcelain tile on it for years. It need to be replaced and I am considering a wood look tile. Thankyou for your help.

You will need to grind the floor after the removal of the old tile and thinset.

So given that the floor needs to be grinded -- would you steer away from a rectified tile altogether or is it just that size (the 8x24)? Does the size of the tile matter ie would a rectified 12/x24 or 18x18 be better when you are working with a rectified tile going in where previous tile is being removed?

All floors require a FLAT substrate. You're not getting away from doing proper prep just by changing sizes.

Sophie - thanks for the comment. Wasn't sure if size made any difference. Given that I am having old tile removed and installer will need to grind the floor, would you stay away from a rectified tile all together? (I checked with installer and he said rectified is fine but would like others' feedback on this)

I am on cement slab - looking at large format (18x18) tile for foyer, hallway and hall bath floor. I would like a small grout line for ease of maintenance and aesthetics. If I understand correctly I can get rectified tile and then do larger grout line if needed but if I go non rectified I am committed to the larger grout line from the get go. Is that correct?

Not really. The term rectified refers to the vertical edges of any tile. You can still have a rectified tile that is shaped like a taco chip. Your tiles still need to be checked for flatness, which is another factor in determining grout joint size.

To get the thinnest grout joint size you need 1- a very flat substrate, 2- Very flat tiles, 3- square cut edges tiles (as opposed to the traditionally rounded edges) 4- a skilled installer.

If you add in a tile leveling system such as LASH or MLS you can speed the installation and help ensure flaeness from tile to tile, but they won't make up for un-flat substrates

If you fear grout, check out the Fusion Pro and Flexcolor CQ which are virtually stain proof.

Thank you -- great information which I really appreciate. I am using Fiorancese Nu Travertine porcelain tile -- I do hope they are "very flat"! Any experience with this specific tile? They will be installed on cement slab (where installer will be removing previous tiles).

Take one tile and place it on a kitchen counter face up. Take another tile and place it directly over the first tile, face down. Poke at the corners. Does it rock? Can you spin the upper tile?