Purpose of ply

[Ruth]

Plan: laying stone tiles on wooden floor and small concrete area at lower level. I understand that the ply provides horizontal rigidity, reducing horizontal movement, the more layers of ply the more this theory holds (18mm ply). Could I use two sheet of 9mm ply instead of 18mm, from a practical perspective it would be much easier to handle? If the joints were staggered and the two sheets glued propery, both screwed to the boards below (is 200mm spacing correct?), could this be even better than a single 18mm sheet since there would be no gaps between sheets and the whole area would be a single solid layer. This is used in boat building to achieve a rigid structure. Or is there is a good reason for using a single sheet with a small expansion gap (is 3mm approx suggested?). This leads on to the question of decoupling membrane, I can see their merit when you predict some movement of the subfloor, would you always use over ply? With my double sheet idea would I eliminate movement and therefore no need for the mat?

My second query relates to tiling on tiles, I know this is scorned on but the ceramic tiles currently down in the small concrete area have been there forever and seem incredibly well stuck down. The levels would be easier too. They have a textured surface, other than giving them a good wash with sugar soap, is there any need to scratch the surface to provide a key? Would they need a different adhesive to that used on ply?

I much appreciate your opinions and words of wisdom

[Dave]

Ply is for Vertical movement.. but with the vast amount of cheap import ply then i prefer cement boards for stability..

As for the elimination of a matting then you have to think about lateral stress issues.. this is what can cause cracks as well as deflection cracking.

[Ruth]

I see, does that mean everything is about reducing lateral deflection, and there is no horizontal movement?
I strengthened my floor following the forum's advice, I have tiny joists (4 inch) at standard intervals so shortened the span (now < 1m) with block piers and concrete lintels from below.

[mikethetile]

hi Ruth

it defeats the object if you overboard twice with 9mm as the boards will slip on each other

[Dave]

I see, does that mean everything is about reducing lateral deflection, and there is no horizontal movement?
I strengthened my floor following the forum's advice, I have tiny joists (4 inch) at standard intervals so shortened the span (now < 1m) with block piers and concrete lintels from below.

No!, you have to combat deflection ( vertical movement).. Horizontal is lateral and that is secondary movement.,,

[Ruth]

Sorry, I'm confusing myself, I did mean vertical defection as the main movement to prevent. Are you suggesting that there will still be a significant horizontal movement within the ply? I was suggesting using a really strong glue (not standard wood glue) between the two boards to prevent one moving over the other as this would have to be key in the plan!

[Dave]

lateral expansion will apply no matter wot glue you use ruth.., this Can be combated with seam tapes but not 100% a cure.