Does size matter (that much) when it comes to backer board?

[Blue]

I've had to take up half of my floor tiles including the 9mm ply that was underneath. I'm wanting to replace it but am thinking about using a cement board/no more ply option instead. Would a 10mm option work if I used slightly less adhesive? Any help is appreciated.

 

[Dave]

Hi , what sub floor is your tiling onto. ?

 

[Blue]

Hi , what sub floor is your tiling onto. ?

It's on floorboards

 

[Dave]

It's on floorboards

ok just checking, you could use backer boards but it could creat a joint at which one underlayment expands at a different rate to the other. I’d be inclined to lift and renew all.