Urgent advice needed - Tiles on wet system underfloor heating PLEASE HELP

[deadsquirrel]

Hi all, I need some urgent advice as I am going to do some work today and need to make some quick decisions.

The situation so far:
Kitchen 5m x 5m Suspended floors.

The beams are quite thin and had supporting wall 1/3 of the way from one end so quite a lot of deflection. So, we went under the floor and built a wooden frame to support the 2/3 of the remaining beams. Much more stable now and almost no deflection at all when I jump up and down on the floor.

In between the beams we have some buttons which hold a 50mm insulation boards all tightly fit with supporting sections underneath the links. On top of the 50mm insulation board we have the plastic UFH pipes which are clipped down and that is approx 20mm under the floorboards. We are planning on using a dry mix 1:8 cement sand to fill that void. (I presume kiln dried paving sand is best for this mix?)

The problem: The floorboards are useless and are in very small sections all over the place so we decided to remove and replace with 18mm plywood. Unfortunately, we bought structural Wickes plywood which is not marine grade or WBP. Will that be ok or should I replace it this morning? If I need to replace it with WBP, do I still need to seal it? If so, is it best to seal it completely or just underside and edges? Any advice would be gratefully received but Wickes open at 10:00 today so I need to make some quick decisions on this so please let me know what you think.

I am hoping to tile directly on this ply using 60x60 porcelene tiles. I dont really want to add anything else on the ply to increase the

Many thanks,
DS

[GirlRacerRed]

anybody able to help the member with this query?

[kilty55]

hello dead squirrel,i wouldnt tile direct to that ply no

if it was me i would overboard with 6mm cement boards although i know you said you dont want height increases

or replace with wbp ply and also install a suitable membrane

[deadsquirrel]

Thanks for the reply. Since my last post i have:

replaced the ply with WBP ply
Sealed sides and underside twice with wood sealer

We have mixed 1:8 cement sand mix and pured it on DRY (littrally completely dry as the sand was kiln dried paving sand). The we leveled it so it is perfectly level with the top of the beams and screwed down the WBP ply. Is that correct or should i have mixed some water into the mix?

Thanks,
DS