On 15/05/18 08:24, Kevin H wrote:
On Monday, 14 May 2018 19:54:27 UTC+1, wrote:


I prepared a floor for tiling for a friend more recently. I
pulled up the floorboards and laid an 18mm plywood floor in whole
sheets. I didn't do the tiling, but no tiles have cracked on that
floor.

they will.

They may not.

I had tiles over 18mm chip that cracked on a 3mm bed of quick set, I
replaced them with tiles on a 6mm bed of flexible. They haven't cracked

I don't think it is as simple as that. I depends on the spacing of
the joists under the floorboards and the length of the floorboards -
i.e. how much deflection is there. Put a glass of water on the floor
and jump up and down. How much does the water (or glass) move?


Yup.
On the other bathroom with a greater span I went to significant lengths
when building shower and bath enclosures to tie the floor to the wall to
lower flexu That also worked on a bigger depth of cement (the floor
was not level).

I agree though that 18mm would be absolute minimum.

The depth of the floor is less imporatant than what it rests on. 6x3
herringboned at 400 centers is pretty rigid.



If the additional
height could be tolerated I'd be using 25mm ply (which I have done in
a bathroom refit). Either way ideally overboard the ply with cement
board (6mm) and then tile onto that ensuring full coverage of
adhesive.

As I said, you are not thinking this through logically. What matters is
not te skin, but the structure on which it rests

Far better to double up joists and herringbone if the floor is bouncy
than use a one inch ply over


--
Its easier to fool people than to convince them that they have been fooled.
Mark Twain