Trowel size advice required - Tile Advice

[HoorayHilander]

I am a general DIY'er and have previous experience tiling bathrooms and kitchen floors.

My question is which trowel should I go for on a 30cm x 60cm porcelain tile (same tile, different colour) going on a tanked walls (two plasterboard, 2 plastered) and floor (new T&G with 6mm plywood 10cm screw spacing). I have never tiled using porcelain and this size.

Also which hole cutter would you use, I need to cut for 15mm pipes and 50mm for the shower bar/waste pipe.

Any advice would be greatly appreciated, many thanks in advance.

 

[Dave]

6mm ply is a big no no. Use a suitable backer board.
8mm to 10mm trowel will suffice

 

[HoorayHilander]

6mm ply is a big no no. Use a suitable backer board.
8mm to 10mm trowel will suffice

Thanks for the advice.

With respect to the floor, I am reluctant to remove and replace the 6mm ply as it has already been tanked. The ply is screwed down at 10 cm intervals onto brand new T&G, this is also well screwed down. What are the risk if I continue and what are my options?

Also WRT trowel size, should I use a half moon trowel for the floor and square for the walls?