Screws for WBP Subfloor Under Tiles

[volkswagen]

Which screws should I use for the above? I have bought these http://www.screwfix.com/p/goldscrew-countersunk-screws-3-5-x-25mm-pack-of-200/17189, but I'm worried they might rust.

Do I need to use stainless steel?

Edit: The link doesn't seem to work. They are Screwfix Goldscrew and it says they're "Zinc & Yellow-Passivated for Double Corrosion-Resistance", however in the catalogue there's a guide which says internal dry use only.

 

[ree]

No. Just use what you have.

 

[volkswagen]

Thanks ree, appreciate your advice.

 

[AdamCH]

Screwfix Gold are fine but 3.5 x 25mm aren't really up to fixing ply to joists. Use something more meaty.

If you are overboarding a subfloor then you'd be better advised to use a tiling board (Hardi / Marmox etc) and not plywood which will expand and contract beneath your tiles and crack the grout.

 

[volkswagen]

Thanks Adam. I've not bought the ply yet, so I'll have a look at the products you suggested.