Screws needed to screw 25mm wbp plywood into joists

[Aston]

you need 60mm screws on 25mm ply. they need to have some decent bite into the joists.
the screws should be corrosion resistant too..

galvanised screws are not seen as a good option anymore due to the fact that the zinc can break down..the whole building industry changed its format on galvanised materials from about 1990.. angle beads, strap hangers, fixing, screws, cavity wall ties all were recomended to be stainless steel or corrosion resistant in exterior or moisture sensitive areas.

 

[Wood's better]

Here what your saying, then with your explaination, why not use ordinary screws to fit Hardi and not very expensive ceramic coated screws?

 

[Scott]

I use drywall for hardi 12.5mm and have never used hardibacker on floors.

 

[Wood's better]

Dean,

All very good quoting this and is all true. However, the origianal post says that the origianl boards are being lifted and replaced with 25mm ply. Surely this is very good practice?

The ideal here would be to then overboard with 6mm hardi do you not agree?

R