To Ditra or not to Ditra...

[DHTiling]

Your area for tiling is actually bigger than is recommend in BS5385 for tiling without an expansion joint onto that timber floor but a membrane used and i would do it without a joint.

The problem you have here is a timber substrate and then another timber overlay.. This all will expand and contract at a rate that that stone will not cope with, without some sort of protection..

Now you could use a CFT2 - S2 Adhesive and install stress tapes over the plywood joints..( Slip joints) this would comprise of using Dural Dura-flex sealing strip, this should allow the small amount of stress without cracking the tiles... but this is only a possible measure and not a fail safe like using Dural Ci.. which will cope with the lateral stress associated with timber floors.

Now if you want to have this floor last many many years then you must listen to the members.. A membrane is the way to go and if height is an issue and you want this stone floor then swap the ply for Hardibacker if the floor is solid to start with..

No matter which way you look at it, there will be stress and you only want to do this floor once.. so do it right first time... if you use the S2 adhesive and the Dural Ci then that ply thickness isn't an issue..but deffo no thinner due to moisture..as really BS 5385 recommended is 15mm

 

[HimediA]

i noticed before that you said there are supporting piers under the floor. is the floor a 5mtre span, or has it got supporting piers under the joists every 1.5mtres. i think that may be a different story if it does. it'd mean practically no vertical movement, and maybe the 2 tilers who said you don't need decoupler are right. probably need thicker ply though.

Floor is approx 5.5m wide and yes, it does have brick piers every 1.5m or so. Largest span maybe around 1.8m, but in that small area I doubled up on the 4 x 2's before laying the chipboard floor.

And I think that with the 22mm green 9x2 flooring sheets well screwed down and 12mm ply over I really don't think vertical movement is going to be an issue (there won't be any).

Horizontal movement is my only real concern... and I note what others are saying...

 

[HimediA]

OK guys, think I have decided what to do.

a) Swallow the cost of the 12mm ply I have in my barn.
b) Lay a backer board (6 or 10mm)
c) Lay a membrane (Ditra or Dural)

Hopefully get a good night's sleep in the knowledge that my new and very expensive floor is going to stay looking good....

Questions (yes, I know, more)...

1. Hardibacker or NoMorePly? Any real difference between these types of boards?

2. Gut feeling is to go for a thicker board, ie Marmox 10mm? or is that overkill and will my 22mm T&G flooring chipboard + 6mm backerboard + membrane be fine? Would be handy if it was as less height useful.

3. Is backboard usually put down over a layer of adhesive, or screwed down or both?

4. Recommended locations to purchase backerboard and Ditra/Dural?

Thanks all.