Help!: Ditra matting - optimum depth

[Neilm]

Hi Guys, very amateur diy tiler here seeking some advice.
I'm halfway through my extension - separate builders, plumbers, sparks etc rather than one company. Now at stage of looking to start floor tiling. I was a bit daunted by the floor size (32 sq mtrs to be done with 600 x 600mm porcelain tiles), so the wife and I decided we'd get a tiler in. He came highly recommended. First thing he did was slag off the existing floor screed.
The screed was uneven and finished between 30 - 45 mm below the desired finished floor level.
When I spoke to the builder about this he said the level was correct as we should be putting down ditra matting on the screed before we tile. This was the first I'd heard of this (then did some net searches). I said to him then that surely this was too deep anyway. He came back with porcelain tiles 9mm + adhesive 10mm + matting 5mm + adhesive 5 to 10mm + latex floor leveller 5 to 10mm.
To me these figures seem all wrong and would cost a fortune in adhesive.
I would have thought tiles and adhesive 12mm, matting and adhesive 8mm and floor leveller max 5mm. Maximum 25mm.
He is absolutely insistent that we must be using ditra matting but the tiler says we don't need it at all.
Questions.
1. Do I need to use ditra matting?
2. What is the optimum depth when using ditra matting?
3. Is 15+mm of adhesive too much?

Many thanks in advance for any responses.

[Alan.P]

1. Do I need to use ditra matting?

Not always

2. What is the optimum depth when using ditra matting?

I normally use a 6mm serrated trowel to fix the Ditra, once rolled out and flattened out, the mat plus the adhesive won't be much more tha 2-3mm

3. Is 15+mm of adhesive too much?

Yes, it might start off at 15mm if he were using a 15mm serrated trowel but by the time the tile goes onto this it will be much much less.

Best bet, lay a self leveling compound to the desired depth ( or get the builder to do it if it's his mistake ) most fixers will want to do the prep work themselves.

Ditra mat (or similar) can be used if there's a risk of lateral movement, or just to be on the safe side, you don't mention any form of under floor heating ?

Hope that helps.

EDIT: and the builder sounds like he's trying to fob it off.

[Dave]

Hello Neil..

If the screed is new and not totally dry yet, then the Ditra could be used as a vapour equalisation membrane to allow the screed to dry and no sweat and blow the tiles off the screed.

Ditra is 3mm thick and goes on probs 1mm adhesive bed.. and then tiles will bed on a solid bed of say 3mm to 4mm as long as the screed is flat there is no need to build up any higher.

For the builder to finish 30 to 45 mm low is a bit OTT to make up with adhesive and tiles...

A tiler might use a 10mm trowel for the adhesive but this beds down as you push the tiles into place, so should only be around 3 to 4mm solid bed.

So in total even if ditra was used ,then around 15mm build up with a 9 mm thick tile.