Which Plywood substrate?

[Boondocker]

Afternoon all,

In the process of bathroom refit I have had to remove all of the old chipboard floor covering.
I am now ready to put down a replacement before laying Ditra Heat and then tiling.
What is the opinion regarding ply thickness?
Would you suggest 18mm plus a 6mm backerboard or a thicker,single layer of 25mm ply?

Also, having looked at all the conflicting advice regarding both Ditra and Ditra-Heat I think my head is going to explode!
I did some unscientific research using some Ditra matt off cuts on to ply to see how well it bonded. Advice, from the tile shop was to use either a flexible adhesive or Mapei Eco VS90. So, I tried the VS90. When I peeled back (easily) the Ditra after 24 hours I noticed that the VS90 was still wet, same as when it came out of the tub. Am I missing something here? How is that going to bond the ditra to the ply? Would it ever cure properly? What do you guys use?

 

[p4ulo]

I'd go with the thinner ply + hardiebacker boards. Remember to sponge off the surface of the boards before laying anything on top to take out the initial "dryness" from them.
Dunno about Ditra Heat, but Ditra just lays with a standard tile adhesive I would have thought it to be the same?
Mapei vs-90 looks like it should work properly from the spec sheet? How think did you spread it?

 

[Boondocker]

I used a 2mm contact adhesive spreader. Probably not enough thickness? Even so it would have taken a month of sundays to go off. I was expecting a good secure bond to the matting on the underside of the Ditra. There was certainly enough coverage when I lifted it.

 

[cam_low]

i would be using at least a 6mm insulation board and not hardie.

 

[Boondocker]

Having already looked at insulation boards such as marmox, orbry etc they claim to offer decoupling as a benefit. So would that make the Ditra Heat a pointless and expensive option as that also is decoupling membrane?

 

[Boondocker]

Just read some guff that might help answer my own question. This Ditra mat stuff should be put down using a flexible modified adhesive whereas when tiling on top a standard adhesive should be used because a flexi wouldn't set as it trapped between tile and membrane and the air cant get to it. How does the air get to the flexi underneath then? Through the ply? No wonder it takes forever to go off which might be the same case for the Mapei Vs90. I thought you could tile straight away after the mat is whacked down?

 

[Stef]

Just read some guff that might help answer my own question. This Ditra mat stuff should be put down using a flexible modified adhesive whereas when tiling on top a standard adhesive should be used because a flexi wouldn't set as it trapped between tile and membrane and the air cant get to it. How does the air get to the flexi underneath then? Through the ply? No wonder it takes forever to go off which might be the same case for the Mapei Vs90. I thought you could tile straight away after the mat is whacked down?

Adhesive (powder) doesn't need air to dry, it cures under a chemical reaction.
If I was doing the job I would fix 18mm with extra Dwangs fitted & then stick down 6mm thermal on top, fit heat mat, slc then tile.
I've not fitted Ditra with that Mapei gear but I've used Ardex & it stuck really well.

 

[Russell brown]

Adhesive (powder) doesn't need air to dry, it cures under a chemical reaction.
If I was doing the job I would fix 18mm with extra Dwangs fitted & then stick down 6mm thermal on top, fit heat mat, slc then tile.
I've not fitted Ditra with that Mapei gear but I've used Ardex & it stuck really well.

Dwangs for anyone South of the border are noggings!

 

[Stef]

Dwangs for anyone South of the border are noggings!

Good lad Russell, forgetting our Dialect up here.. [emoji1][emoji1]

 

[Russell brown]

I was working in Alexandria upto Xmas putting a roof on a old peoples home so picked some of the twang up!.

 

[rj1884]

Schluter recommend sticking Ditra to wood using ardex af200. It's basically like a glue in a tub

 

[Boondocker]

Thanks all so far.
Well, further unscientific testing with BAL SPF mixed slightly looser than usual but still holding a notch didn't work either. Ditra pulled straight off this morning. Not a trace of adhesive on the underside of the mat. Doesn't seem to penetrate the fleece to form a bond. Will try some Ardex next. At least the Mapei had 100% covergae of both the ply and the fleece. It just didn't set.

Stef, thanks for your advice dwangs n all! I may install the way you suggest. Would be slightly cheaper I think. Also, I want to be confident that when the floor is down it's staying down!

 

[Stef]

Thanks all so far.
Well, further unscientific testing with BAL SPF mixed slightly looser than usual but still holding a notch didn't work either. Ditra pulled straight off this morning. Not a trace of adhesive on the underside of the mat. Doesn't seem to penetrate the fleece to form a bond. Will try some Ardex next. At least the Mapei had 100% covergae of both the ply and the fleece. It just didn't set.

Stef, thanks for your advice dwangs n all! I may install the way you suggest. Would be slightly cheaper I think. Also, I want to be confident that when the floor is down it's staying down!

Ditra will definitely stay down with Ardex af200.
It should also stay down with adhesive.

I've used it here to pack out a feature panel on a wall for penny mosaic.
Stuck it up with AnhyFix but I've done it before with SetaFlex standard set & it wouldn't budge once set.