Tiling in a room with suspended and concrete floor, with under floor heating

[Martin Harrison]

Hi. We are tiling a ground floor kitchen that is half suspended floor (100x50 joists on 300 spacing, with supports every 1200mm) and half concrete slab. The concrete slab has just been poured and once insulation has been added, it will have UFH pipes buried in the screed top layer. The tiles to be laid are 600mm 11mm thick ceramic.

I was originally looking at this site to try and understand what the best subfloor would be for the UFH. The UFH system the builder is looking to use is Nu-heat. The pipes are laid in 15mm of gypsum board, so it isn't structurally strong. As my joists are small, it sounds like I need some good reinforcement before laying the UFH to prevent bounce. I want to do that without impacting the finished floor level too much - which would otherwise have an impact on the door thresholds to the rest of the house. What is considered correct - plywood, what thickess, ditra matting, priming etc? The builder is suggesting doubling up the joists and bolting them together. He's been advised that the original floor boards could then be put back down but I think we should put plywood, screwed and glued. Can anyone advise?

In reading many of the forum comments it also looks like I may have an issue where the suspended floor transitions to concrete - as i won't be able to eliminate all movement in the wood? Is that correct? If so, what's my best option here?

Thanks

 

[SJPurdy]

As I understand it you are putting the Nu-heat system over the whole floor ie the new concrete part and the suspended wood part. I am not familiar with Nu-heat product so can't help but if you put in some relative floor sizes then you may get other replies.
Normally if tiling over two different substrates (concrete/wood) even if over-boarding and/or decoupling mat is used a movement joint is required in the tiling (+gap in overboard and mat) directly over the substrate junction. The same is true for a suspended floor where it passes over a supporting wall. I do not think that adding insulation layer and then a further screed on top means that it can be treated as a single floor do to the uncoupling at the insulation layer - maybe someone else knows definitively.
Joists at 300mm spacing with min. 18mm plywood (and sufficient noggins) is regarded as suitable for tiling if it provides a rigid floor with no deflection/bounce. Dimensions and lengths of the joists are dictated by building regulations I would have thought but remember in your case they need to support the additional weight of the underfloor heating screed.
Sorry not able to help much but maybe as started the replies more helpful ones may follow.

 

[One Day]

Nuheat should be able to provide you a full, bespoke specification.
The job I'm working on has used it throughout in various guises and I'm amazed just how much paperwork they provided! Everything is crossed and dotted.