Prepare upstairs bathroom for tiling

BarraOG

I wish to tile my upstairs bathroom. I have taken up the old floor and exposed the joists.

My plan is to use:

• 12mm plywood, screwed down with:
• 60mm screws, partially threaded: https://www.screwfix.ie/p/spax-tx-countersunk-self-drilling-woodscrews-4-5mm-x-60mm-100-pack/8705p
• 6mm Miltipro Tile Backer Board 2400mm x 1200mm
• Adhesive between floor and backer board.

Questions:

1) Is my floor adequate? The distance from the joist to the height of the floor in the hall is 35mm, but despite this should I consider 12mm Backer board instead?

2) What length screws are screwed into backer board and should they screw into both subfloor and joists or just the subfloor?

rob316

Curious why you took up the old floor? You could just board straight over it.

Now that you have though 12mm isn't enough I'd be going 24mm. Too much flex in 12mm if its just sitting on the joists

listermint

No I don't think 12mm ply is strong enough for flooring. It'd be looking for 18mm minimum. 12 has too much flex for me.

BarraOG

I

https://www.boards.ie/discussion/comment/121566473#Comment_121566473

If I was to go with 18mm ply, would the 6mm backer board be enough?

rob316

https://www.boards.ie/discussion/comment/121566576#Comment_121566576

The backer board is only adding a waterproof substrate really, if you have no floorboards now you need to go thick on the plywood, 24mm.

listermint

https://www.boards.ie/discussion/comment/121566576#Comment_121566576

As above, 18mm minimum it's unlikely to flex, depends on condition of joists and noggins. 24mm would be safer option you want to avoid flex on your tiles or shower. That's why leaks occur.