Hi, I'm selfbuilding, hope someone can help with chosing adhesive for our floor.

We have black limestone slabs to lay over underfloor heating pipes (concrete slab base, 50mm kingspan and wet UFH pipes in sand cement screed) and need to know which is the right adhesive to use for it.

The floor slabs are uncalibrated between 25-35mm thickness.

We've been told we must use a flexible adhesive as the UFH will cause heating and cooling, there are many products on the market which makes it so confusing.

Also as the slabs are thick and the pipes are bedded in screed why does the floor adhesive need to be flexible, assuming the screed is not?

If it has to be flexible is there anything we can mix with sand cement to make it flexible (such as the bottles of latex available?) as we will need to have something we can lay at variable thicknesses to get the floor level?

What would be the most sensible solution?

Any help or suggestions are gratefully recieved.

thanks in advance
SBS

If you wat to use sand and cement then you can add SBR to the mix.....Or use a thickbed flexible tile adhesive...

The screed will have some sort of modifier in it...

The adhesive needs to be able to cope with thermal expansion or the tiles will debond...thats why flexible is needed and flexi grout as well...

Also remember to allow screed to dry out : I think about 1 day per mm of screed is the standard.
Then before tiling, fire up heating and over the course of a week bring up temp ( usually in 5 degree implements ) to maximum operating temp, then allow to cool before tiling, this will allow for any initial shrinkage of the screed, movement or cracking to occur before tiles are laid.

thanks Dave and Leatherface!

We too are laying limestone over wet UFH. A friend has done a similar job but has experienced cracking of the tiles in every doorway. He used a glass reinforced cement screed whereas we intend to use a liqued screed. Should expansion joints be fitted in doorways to prevent expansion/contraction being passed on tho the brittle tiles?

You might want to consider using a decoupling membrane under your tiles.

yep they need laying on a de coupling membrane how big is the area?

Total floor area is 100 sq M, partly block and beam and partly reinforced slab. Do you think that an expansion joint will be required between the two different base materials or will the decoupling membrane be sufficient?

you will need expansion joints between 24 to 30 feet runs as well as the matting