Hi
I got a lot of useful advice over my last post, thanks guys:

To Ditra or not to Ditra...

I had reached the decision, as suggested by Stewart, which was:

Originally Posted by Stewart

I would always use a decoupler with stone tiles. If the floor is as ridged as you say, why not use 6mm backer boards then the decoupler, then the tiles. If you use 12mm ply you will effectively void any guarantees from the adhesive manufacturer.

So I was going to put down No More Ply over the chipboard floor instead of the 12mm WBP, then Ditra over.

Now I am being told by different suppliers that if I use Ditra then I should be able to forget the Hardibacker/No More Ply and just put Ditra down over existing sub-floor (pretty darned solid 22mm green flooring chipboard).

What are your opinions on this? It would of course be nice to avoid the extra £600 or so plus labour that it's going to cost me to put down No More Ply but more importantly gaining that lower floor height would be great.

Suggestions welcome, thanks guys, a really useful forum well supported by very generous contributors!

Paul

you need to understand that the no more ply is doing a completely different role to the de-coupling membrane !!

the no more ply or backerboard is providing a stable surface on which to tile!!
it is a cement based board which adheres better to adhesive..plywood in recent years has become problematic due to many reasons...tiling on wood compared to a cement based product has always been a challenge in terms of guranteeing work but lately due to many poor standard imported ply, overboarding with a backerboard has been the logical solution

the de-coupling membrane is there to counter act 'lateral movement'...its role is to take the expansion and contraction stress away from the tile via the independant movement of the de-coupling membrane layers

its your choice/gamble if you want to do away with the no more ply....but the de-coupling membrane imo and in many others on here, is a must for a professional installation....

at the end of the day, you pay your money and take your chance..

Hi AstonTiling, thanks for your reply.

Yes, I fully appreciate that there are two different jobs going on here, thanks to the advice I've received on the forum over the last couple of days. I have decided to use Ditra for certain.

My issue is that I had previously planned to cross-board with 12mm ply to further bolster the floor. Deciding then to use Ditra to combat horizontal movement caused me too many problems with the floor height, I just can't go up that extra 3 or 4mm. Hence the decision to ditch the 12mm plus and use 6mm NMP as a substitute extra deflection strengthener and combating any possible shear movement at joints etc.

The surface is at the moment pretty damned stable, and appropriately primed (SBR, not PVA, another lesson learned here). But my gut feel was that a little more strength can only help.

What confused me (and given your comments and what I've learnt from you and other guys here I think you may find shocking) is this comment from suppliers of my stone floor when I asked the question about matting, they seem to have totally the wrong end of the stick...

Dear Paul,

Sorry for delay, I have been looking for the best advice, consulting with my Technical Department.

I understand that there is a floor of 22mm T&G is overboarded with 12 mm WBP, and the question is whether to fix limestone tiles directly over the WBP.

In an ideal world this should present no problem as this is very similar advice given by Weber, the adhesive people, except that the thickness of WBP is 15 mm minimum. Having said that, I have seen project go wrong due to wrongly overboarding, therefore most adhesive companies recommend matting. Dittra would be very expensive, then I would recommend Dural as an alternative. Yes, the thickness is between 4-6 mm but if the tiler is not experienced, this would be my recommendation.

Just on another note, it is not the tiles that break on floors, it is always the substrate that moves and breaks tiles. Therefore it is imperative that movement is limited to nearly zero to make sure tiles don’t break due to movement after fitting. I would perform this test to see if matting needed: fill a glass with water fully and jump up & down several times while the glass (which is completely full) is on the floor next to you. If the water spills then you need matting.


Yours faithfully,
etc.

Now I fully agree with everything up to the very last short sentence "If the water spills then you need matting."

This implies that some vertical deflection will be absorbed by the Ditra, and I guess it was this (and similar comments from others "If you're using Ditra why overboard?"
I would venture that this is dangerous talk, by suggesting that Ditra will provide help with vertical deflection problems.

Oh well, think I'll go the NMP overboard + Ditra route anyway. As a listed building project at least I can get VAT zero rated which helps the extra bill...

Thanks again.

The last part of the statement is misleading, as a decoupler only helps with lateral movement. The prep you're going with is the best way in my opinion, and the supplier needs to be educated a little more in the correct fixing methods and what the products he recommends actually do. Maybe you should send him a link to the schluter site. Good luck with the install and a few pics would be nice.

Will do Stewart, will post some pics as soon as done. Thanks.