On Sat, 10 Sep 2005 06:57:25 -0700, butch burton wrote:

If the floor is going to flex
enough to the grout to crack, the tile will eventually too. With 12"
tile
I like a 1/4" grout line. I'm doing a bunch now, in fact.
snip

In the areas with tile, will have 3/4" T&G exterior grade plywood,
glued and screwed down to manufactured joists consisting of 2x4
connected with a metal web with web being 12" high. In the areas with
tile - will attach 3/4" OSB UNDER the 3/4" plywood with screws and glue
to reinforce the floor to keep it from flexing under the tile. Is the
latter necessary - will a 24" span between manufactured web joists flex
too much. Don't want to go to the expense of putting down a double
thickness of decking everywhere. Maybe simpler -easier but it would
cost a lot more.

Disclaimer: I'm a homeowner, not a pro...

I think that may flex too much. The specs for tile call for
at least 1-1/4" subfloor (on 16" joist centers, I think). My house has
3/4" plywood over 2x8's 16" O.C. Where I tile, I add 1/2" Hardi-Backer
across the beams (not aligned with the ply). That is put down in thinset
and screwed every 8". I also use the thinset that's designed to allow a
little flex (FlexBond is the brand name, IIRC) for mounting the tile. It's
expensive, but not nearly as much as a cracked tile. I also shimmed and
glued under the subfloor, where possible, to try to eliminate any movement
I could.

My downstairs bathroom and laundry seem to be OK after a year. I'm now
doing the foyer coat-closet and upstairs 1/2 bath. Finished the
harti-backer yesterday and cut all the tiles (toilet is a PITA). Today is
tile day. Fun, fun, fun. ;-)

--
Keith