Ply, UFH and decoupling mat?

Seeing some of the recent comments about laying UFH onto ply flooring & use of decoup mats I thought I'd ask a few of my own questions...

I'm planning the re-fit of our en-suite. It's approx 2mx2m and we're going to tile the floor and walls with 30x60 porcelain. We've taken a liking to these in the attached photo (from Avanti tiles in Milton Keynes - £40/m).




I'm not going to be doing the tiling myself - will be asking one of you fine folks here to do it when the time comes :smilewinkgrin:

The floor is currently chipboard and I'm planning to take that up and replace with 22mm T&G Ply (with extra noggins) and insulation board. I'm not sure yet to use a heating mat or just snake a wire over the floor, but does the floor need a decoupling mat fitting? I'm conscious of the overall height of the new floor, once you add together:

22mm (ply)
10mm (insulation board)
2mm (thinset)
3mm (decoup mat)
5mm (SLC)
3mm (tile adh)
12mm (tile)
=57mm.

I do expect a transition from the bedroom floor level, but at this rate it's going to be close to 30mm (assuming the chipboard is 18mm + carpet/underlay).

Also, if the decoupling mat is needed, where would it go in the "stack"? Above the heating wire/SLC? If that is the case wouldn't it act as an insulator?

Any thoughts?

Cheers - Kev

Hi Kev, the complete hieght may be 57mm but you've taken the original chipboard out so can minus that amount. so will only be about 8mm.
i've attached diagram of what the "stack" should be. View attachment UHF floor1.doc

Plywood is quite a good insulator as it is, so you could lose 10mm height if you require..?

Your floor isn't that big and to me it looks like you are going for a "belt and braces" approach. You will be fine using a 6mm insulator on ply and the slc only need to just cover the ufh wires not an extra 5mm on top. If it was me i would probably not use a decoupler with porcelain on such a small floor. just my opinion though, others may disagree.

I agree with grumpy. You really dont need to decouple on such a small area, especially using porcelain. Decoupling mats are relatively new to the tiling world and are only really required over larger areas and when natural stone is the finish. Im a bit sceptical about the whole issue of decoupling, certainly agree that some situatiuons rise when its needed but for many years it was never around. Maybe people are too worried about possible failures and fit as standard but I always go by what I see as a fit substrate to tile on and take it from there.