ditra mat query laying on polylumb overlay UFH and ply

[thegwads]

just a quickie
starting a job monday, and they have the polyplumb overlay stuff in, which, from what i can see is 12-18mm ply, with grooves cut out, and the pipes fed through. reflectective foil keeps the heat going the rigth way, up. the system is screwed down to the chipboard floor, and joists below, and over the top 6mm ply has been ;laid, and scred every 150mm if not more. all solid as a rock etc.
will be laying porcelain brick bond over the top, but had a chat with another tiler on the job who won't be using ditra matting, and myself, who thinks i should to counter lateral expansion/contraction of the ply overlay.
what would and how would you guys prep this floor?

my thought, flexi tile adhesive ditra down, after priming the ply with sbr/acrylic primer
then lay as usual

other chap in other bathroom just going straight on with spf adhesive. he also refutes the need for ditra matting , saying it won't work on this system, as you would have to stick it down with contact adhesive (the stuff schluter now sell), and that would become de-boded with the heat

got me doubting my tech knowledge know, as i am pretty sure you lay all ditra matting with single part flexi tile adhesive, and going on to ply will make no difference surely. and with the ufh system, surely we are more likely to experience lateral movement du to heat fluctuations

really interested in thoughts guys.

[Sir Ramic]

I dont like the 6mm ply part.

[thegwads]

same thing i thought, however, the base which includes the ufh, is a 12, or maybe 18mm, not sure ply board, with the grooves cut out for the pipes, and the 6mm overlay merely acts as a solid capping if you catch my drift. so in reality, i would say it is safe to presume the thickness of the ply, over the chipboard/floorboards, is at least 18mm, if not 24mm.
i recently tiled over the polyplumb overlay ufh thermal board system, which i had misgivings about, and turned out to be solid as a rock. and this system was not overlaid with 6mm ply, but placed ( adhesived) some norcross anti fracture membrane (a poorer version of ditra in my opinion) straight over the top, and it has been spot on.
my main question is whether ditra matting is unsuitable for the application as a above, when in my eyes, it is the perfect solution for a heated ply floor??????
this new ditra matting adhesive, which i guess schluter have introduced as a money making excercise - is it any better than using spf, which has always been specified in the past, or is there a reason that a decent spf adhesive would not be suitable over a heated ply floor???
these new overlay systems seem to be causing confusion, and indeed i have had seperate answers from seperate distributors, about whether ditra is a good answer?
one said that ditra has to be stuck down with their own contact adhesive (glue), and that if this is on a heathed ply board, then there are issues that it may become de-bonded.
another retailer said prime the floor as per spec, stick down the ditra with a spf, and tile as normal. no chance of de-bonding with a good single part flexi additive, ufh or not?

would love to hear some guys responses, this is quite new territory using the overlay stuff
cheers

[Sir Ramic]

I still disagree about the 6mm ply. This will be subject to warping ( no not time travel) 6mm ply is 6mm ply unless it were fully laminated to the ply below.

I did one of these polyplumb floors the year before last but the channels were cut into some special boards not ply. I used flexible SLC over then Dural Ci (similar to Ditra) over that.

[thegwads]

cheers sir ramic, i too just did the ones in the thermal boards, but these ones seem very different, sure they are in ply.
lets for arguments sake say it is covered with 12mm ply then, as i may recommend, would ditra still be the correct thing or not needed?

[Sir Ramic]

The problem being is that I hate tiling onto wood...end of ! So i wouldnt even use this method, if 12mm was laid then i will still insist on tile backer boards before tiling but then you have a greater thickness which could impede the heat getting to the tiles.
Ditra is great but its not a get out of jail card.