Hi,
Firstly thank you to Dominic at Diamond Tiling for answering a lot of my questions and directing me here.

I have roughly 70sq/m of suspended floor and appx 30sq/m of concrete floor which all need tiling over a wet under floor heating system.

On the concrete floors I have laid down 4" of Celotex insulation and was planning on running my pipework over these boards. Then I'm intending on screeding over this with a dry mix before my tiles are laid onto that.

The suspended floor joists have all been battened underneath and I have laid down the insulations boards in between the joists to leave me with appx 60mm to lay my pipes and screed or have an air gap. My question is this,

1) Do I screed over the pipes, all 60mm of it and then lay the tiles on top of the screed and joists?
2) Do I leave a 60mm air gap on top of the insulation and ply over the joists (I was thinking 18mm ply but have been told that this is far too thick)
3) or do I lay 60mm of screed and then ply using 6mm backer boards with some type of membrane underneath to decouple the tiles from the floor underneath.

I've read up on similar posts and 3 sounds about right but I'm concerned that I may get cracking in the grout and movement in the tiles especially since some areas will go from being a suspended floor to a concrete floor all in the same tiling zone.

Any help would be much appreciated.

Cheers

I'm not the best guy when it comes to screeds and whatnot so wait for further advice on that front. But I think you'll need an expansion joint through all materials and layers where the substrate changes from concrete to suspended floor.

Thank you Dan,

I've got a few tilers booked in to come down and give me quotes for the entire job but this seems to be a bit of a grey area with different methods preferred by different tilers. I'm just trying to make an educated guess as to which tiler is on the right tracks.

Hi Indy, as I said on the phone yesterday ply on its own is not the best tiling surface, I would add lots of strength with extra noggins before fitting 18mm ply and then 6mm backer boards which will give a much improved surface, a de-coupling membrane would also be a wise choice.

I am currently tiling floors with exactly the same situation as yours, the screeded floors which is approx 80m2 have been done as you are doing,
the wooden floors were piped with wet ufh between the joists then filled with a dryish cement screed to the top of the joists this was done between the plumbers and builders area 30m2 approx. where the screed and the wooden areas meet there is expansion joints.
(I asked for them to put down 18mm ply and then i would put hardi backer on top)
But when I turned up they had installed 25mm ply they were concerned that the tile backer would interfere with the heat transfer threw the tiles.
So I am now installing a de-coupler matt over both the wooden and the screeded areas and putting in expansion joints where they are already. The tiles I am using are porcelain and vary in size 600x300mm and 100x1100mm and 200x1100mm in different rooms of course.
Hope this helps you out.

Thank you for sharing your experience peckers, I definitely appreciate any advice on this one.

Is there a particular de-coupler matt that you used (any particular brand)? Also which make/size of expansion joint did you use?

Sorry for all of the n00b questions but I can't afford to get this one wrong. Outsourcing the job has sorted the 'finish' issues out but I'm a tad concerned that movement may still be come back to haunt me. As I'm the builder on my own job I'd love to do a good job.

Cheers
Indy

The de coupler is the dural make an the expansion joints are 10 mm wide which follow threw the same size as which was put between the screed and wooden areas. I must point out that you don't want bounce in the wooden floor as a de coupler membrane won't overcome. This

Originally Posted by peckers

The de coupler is the dural make an the expansion joints are 10 mm wide which follow threw the same size as which was put between the screed and wooden areas. I must point out that you don't want bounce in the wooden floor as a de coupler membrane won't overcome. This

Thanks mate, the Dural membrane could be just the ticket. I've had a quick read here and it seems appropriate for my job. I'll ask the tilers for their thoughts on it before I rush out and buy the stuff.

I do feel I am making progress now.

Best of luck