Assess and prepare floor
Make sure that the floor will be capable of supporting the expected load with minimal deflection. It must be stable, well supported, ventilated underneath and level.
The sheets should be of exterior grade plywood and at least 18mm thick. It may be necessary to increase the thickness if heavy loads are anticipated or if the joists are spaced more widely than normal. Replace any defective sheets and fit noggings between the joists beneath any unsupported sheet edges. Prime the back and edges of plywood with weber PR360.
Screw the sheets to the joists/noggings every 200 - 300mm, leaving 2mm to allow for expansion. Fill the gaps with weberjoint silicone sealant to prevent them being filled with tile adhesive when fixing the tiles.