Heated floor dilemma - Tile Advice

[Regis Quality Tiling]

Hi everyone. I'm doing a bathroom floor over existing floorboards. I'm also adding a 1.5 sq m electric heating mat before tiling with 30 x 30 granite tiles. I'm applying thermal board which I'm sticking with adhesive. Been told don't need screws or tape as adhesive will do. My dilemma is whether I then self level or just tile directly onto the matting? If I don't level think I will struggle with my levels across the floor. If I do use self leveller first, how do I make sure it doesn't flow through the holes left accessible for pipework coming out of the floor? Should I batten small areas around ? I've used self leveller before but only on ground floor solid floors
Any advice thoroughly welcome. Thanks,
Nick

 

[J K Tiling Cornwall]

Using screws with washers along with taping the joints would be belt and braces. Just go around with duct tape and cover any gaps or fill with tile adhesive, I usually put a piece of batten across the threshold and duct tape the inside face 👍

 

[Regis Quality Tiling]

Thanks. Do I need to prime the thermal board before applying the self leveller?

 

[J K Tiling Cornwall]

It never hurts to apply some dilute primer to the boards. Would allow to completely dry before fitting the underfloor heat mat 👍

 

[Regis Quality Tiling]

Final question - hopefully! 30cm square polished granite tiles. Going on top of slc and heated floor. What size notch trowel for this. Would normally go with 12mm aware notch. Too much??