BAL state that when tiling onto underfloor heating you should use a flexible adhesive eg single part flexible. The build up in my floor from bottom to top is dpm----150mm concrete----kingspan 70mm------underfloor heating pipe clipped to kingspan-----65mm of sand cement screed.

To save on adhesive i was going to use a weber sand cement self level screed to flatten tolerances under straight edge.

what i dont understand is why the 65mm of sand cement screed is not flexible why should the self level be flexible and tile adhesive be flexible? does the self level need to be flexible as well as the adhesive or what?

basically trying to save a few quid, got over 100 sqm2 of tiling to do and want to know if and why it is necessary. i have a feeling it is to do with the bonding side rather than cracking side....cheers

There are some cheaper products on the market that you can use for UFH. Mapei do a product called Mapeker. Which has just been reformulated so that you can use it on UFH. It is cheaper than Flexi adhesive (Mapeker retails at about £20 for 22kg). Bal is generally quite an expensive product to begin with.

yeh cheers will look into that. still leaves the other questions unanswered. any1 shed light on this to clear it up in my head cheers

Thing is, and i very much stand to be corrected, all screed are flexible to some degree before they fail. Without seeing your screed I bet it has a few cracks...even hairline ones if there is no settlement. Well it doesnt really matter if it cracks as its not your finished floor. It will generally expand and contract with the UFH.
Tiles have different tollerences of expansion and contraction hence you need a flexible adhesive inbetween to 'take-up' the differences in movement and stop cracks in addy & grout or worse still in the tiles themselves and boast tiles.

I would use flexi SLC, flexi addy and grout and make sure the screed is well dry before tiling, 6-8 weeks. Plus keep the UFH off for 2 weeks after tiling and then only turn it up at 1 deg per day to full rate.

Do it once and do it right, good luck

What Dave said ^