Screed or not underfloor heating

[8762damien]

Hi all 1st post so hope I'm doing it correct
I am tiling a floor in a bathroom and I have laid 12mm wbp plywood and laid electric UHF no the question is do I screed over the UHF mat first then spread tile adhesive and tile or do I just spread adhesive straight over mat and tile also does the plywood need to be primed with Pva? if screed is needed which is best many thanks for relief in advance :smilewinkgrin:

 

[Rookery]

This is liable to failure as the combination of thin ply and UFH will cause movement. You dont say whats under the ply but assuming its s/w floorboards or chipboard I would do it thus:
Remove the plywood. Wipe the floorboards with a damp sponge to remove the dust.
Glue and screw 10mm Wedi/Marmox/Similar insulation boards.
Lay UFH mat/cable.
Cover mat/cable with flexible SLC. Weber Floor Flex would be my choice.
Tile with flexible, cement based adhesive. Mapei Keraquick would be my choice, perhaps with latex instead of water.

 

[Uheat - Keith]

All my quotations carry the following message:-
After the mat is installed, one of the following two ways are used to finish the area:
A) A powdered flexible self-levelling compound is used to cover over the underfloor heating element, then a powdered flexible tile adhesive to fix tiles.
OR
B) A powdered flexible tile adhesive is used to cover the underfloor heating & fix the floor finish.

From feed back from tilers, A) seems to be the choice 90% of the time.

Warm regards, Keith.

 

[Dan]

I'd concur.

See here: Tiling Forum - Installing electric underfloor heating - Step by Step Guide