Decoupling requirment?

**jackrussell31** · 09-09-2011

HiI am tiling the floor of a wet room/bathroom with 457x305x10mm Limestone. The base is currently 18mm chipboard (screwed down) on 1st floor joists. The area is T-shaped - 3.4 x 1.8m with a 0.6 x 1.8 spine running between the bath and 1500x1500 Fundo Wedi tray. This tray is recessed to the joists (18mm plywood base and sump PVA'd) leaving a 25mm step once fixed with 3mm cement based adhesive.I intended to insulate with 10mm insulation board on top of 4mm ply (screwed down) and then screw down 11mm marmox to level the floor with the Fundo tray and provide tanked base (also tanking walls with marmox).So the question - I read in this forum that a decoupling mat takes out lateral movement of the wooden floor. So inlight of the fact that I have a laminated sub surface and marmox top (prior to UFH and tiling) do I need a decoupling mat, which I assume goes on top of the marmox and doing away with the 4mm ply so that I can maintain a level.I hope that all makes sense - happy to clarify if not.Many thanksJack

**Rich** · 10-09-2011

Hi mate,

Im not to sure what you are planning to do with the 4mm ply. If the floor is chipboard (and in good nick) then my advice would be to screw it down as much as possible to remove any deflection and then over board with a backer board. Tile backer boards come in many different formats so I would imagine that you will be able to find a board at the right thickness. If not you would be better off raising the height of the floor slightly with a suitable SLC rather than mixing to many materials. And if cost will allow it, I would use a decoupler as well

**jackrussell31** · 10-09-2011

Thanks Rich - the 4 mm ply was a space filler along with 10mm insulation soard to bring the rest of the floor up to the same height as the Wedi tray - got to fill 25mm (wedi tray 40 + adhesive 3 - chipboard recess 18). I already have 11mmm Marmox backer board hence the requirment to backfill with insulation & ply (10 +4). I want to do this properly as laying Limestone so happy to use decoupler. Forgive the dumb question but I assume the decoupler goes on top of the Marmox backer board - that way I wont need the 4mm ply filler. Cheers Jack

PS - the P5 chipbaord is getting tatty from being lifted to insulate between rafters so intended to replace with 18mm ply and PVA before laying the 10mm insulation board and 11mm marmox on top.

**jay** · 10-09-2011

P.V.A. Versus Primers might be worth a read

**Rich** · 11-09-2011

If you have the PVA ready, throw it out. It is no good for tiling at all. I wouldn't use the 4mm ply, you will be adding a layer that is really unstable. The decoupler can go over the marmox boards. As said above if you are still to low then you will be much better using a self leveling compound to bring the height of the floor up