Tiling what size of screws ?

[juco]

What size of screws to buy for screwing plywood to wooden floor?

9mm ply being screwed to 18mm floorboards.
thanks

 

[Richard C]

What sort of wooden floor is it? What sort of ply is it? 9mm isn't thick enough, 12mm is the minimum for over boarding. You should screw through into the joists not just into the top of the original floor. Acrylic prime the underside & edges of the ply before laying.

There are many things that can catch you out when tiling a suspended timber floor. I would advise you read the Tiling Sticky & Forum Archive posts before doing any more work or buying materials, it could prevent you making disastrous & potentially expensive mistakes. It’s also important to use only quality trade tilling materials of the correct type for your tiles & tile base; cheapo own brand & DIY stuff is mostly crap.

 

[juco]

Its wooden (not chipboard) joists are 16 " and there is no bounce in the floor so was advised 9mm was sufficient.

 

[Richard C]

Whoever advised 9mm ply is sufficient has given you poor advice, it’s marginal at best. The BSI & all adhesive manufacturers’ recommendations for ply over boarding is 15mm WBP minimum, however, in most cases 12mm is sufficient. Joist are 16” pitch but, just as important, what size & span are they?

If you use 9mm ply you risk tile failure, you’d be better using a decent 10mm tile backer board but it’s your decision!

 

[juco]

Thanks for the advice.
The span is about 10 ft with a brick support arount the 6 ft mark.
will go for 12 mm I reckon.