Laying Porcelain Tiles 450x450

[Holohana]

ARDEX 7001 single part flexible adhesive is designed to go straight onto floorboards!

maybe thats an option?

 

[Monsoon]

Is there such a thing a British standard for a surface to be tiled on?

We have building regulations which deal with stability from a structural POV. but thats a long way short of advising on a tiled substrate.

Just get your builder to overboard at a right angle to the T&G with WPB 18mm ply - job done.

How low is the ceiling that the additional 1.8cm is going to make that much difference?

Everything else is a compromise and lacks your tilers warranty.

 

[basshunter]

Go with what gaz suggests.

6mm hardibacker(dont lay it too tight together+span joints underneath best you can) 2part flexible adhesive and flex grout.
This is where i usually end up with clients once all the options are explained.
What size grout joint are you having?