T
Traxman
You could secure the floorboards with screws at 15mm centres then possibly use ditra matting. Give Schluter systems a call I am sure they can advise.
best way to learn if unsure is buy a bag of SLC and have a practice in your own time. Make sure you have a nice big floor trowel for the big areas. Don't let the customer convince you that SLC is the miracle cure for floors that run off. Its not for that. Its for slight lumps and bumps in floors, to get them smooth for tiling / flooring.
If you are going to use SLC on wood, make sure the wood is primed first, and you use a flexible type of SLC.
Just tell the customer to cut 12 mm off of one of their legs.
I always think that if the SLC is to thick, it will start to crack / shrink.
If a floor needs to have 20-30mm down before tiling, then a sand/cement screed needs to be considered. A nice 4:1 damp mix feathered off and devil floated.
Using SLC to cover large areas at this depth will cost a fortune. 😛