Subfloor for tile in Bathroom

hammrtime

Hi - looking for advice from a professional plumber / tiler please to confirm ideal bathroom subfloor thickness.

Doing my first full bathroom reno at home. Old home, joists 31cm on centre (currently being checked, strengthened, sistered where necessary). Knowing the floor build up accurately before hand is important for me as I need this to plan where the hole driller will put the new toilet & basin waste holes in 2 foot thick granite walls. Not something I want to get wrong.

Current plan based on internet research is exterior grade ply layer, insulated tile backer board layer, U/F coil mat (probably Klima from Screwfix, 3.5mm) and 9mm porcelain tile (tiles 200mmx200mm).

What thicknesses should I be using for the ply, backer board and allow for adhesive? 18mm, 6mm and 4mm?

Is ply and then backer board the best option when starting from joists, or two layers of backer boards?

Thanks.

Phil.x

I've added extra noggins between joists and used screws as they were squeaking beforehand, then original 18mm floorboards and 6mm backer board, under shower tray waterproof and tape over the section of backer board.

2 years later all is fine, no leaks, no cracked grouting, no squeaking tiles and floor is not too high compared to floor outside bathroom.