Screws for 18mm ply

3" No 8s, drill pilot hole and countersink the ply.

 

A couple of 'screw and countersink' experts will be along later I'm sure

Although one of them may be on a 'break' at the moment

Also, ur heading says 18mm, post says 25....... which one is it

 

Are you talking about ceramic tiles? If so, consider using Hardie Floor - concrete based and ideal for tiling. That is, unless you already have teh ply!

 

60mmNo 8 or No10, might get away with 50mm.
2" or 2 1/4" or 2 1/2" in old terms.

 

I would say 4.5mm x 40 or maybe a touch longer.

A pity as the flooring is excellent!

 

pilot hole & screw with 40mm black plasterboard screws. every 250mm.

 

Too short.

 

Perfect.

 

3" was probably a bit overkill for 25mm, if 18mm then I agree 2" or 50mm is spot on - especially if you have them to hand!

 

2"/50mm is fine and if you have a surplus of bugle headed screws they should be OK if you countersink but you may have a problem with the heads snapping off as they aren't designed to go into a hard material.
I don't know because I have never done it and apart from PB I have only seen them used on MFC.
I've snapped plenty of good woodscrews on birch ply.

 

60mm 10's is my two penneth, especially if the joists aren't perfectly in line and the ply needs forcing tight to the joists to prevent movement or squeaking, no. 8 screws don't seem to have the pulling power of a meaty ten......

 

I tend to use 4.5's now for it's extra grip unless I think the wood might split, which isn't going to happen here.