Right you helpful lot. I am tiling a bathroom floor soon. Got to rip old tiles/ply up before hand which i dont see being too much of a problem, so loose they move when you look at them (will get some pics up, for a laugh). Just wanted to ask you guys for some tips/advise.

1) will use stud finder to find joists, any wires/pipes. (assuming floorboards ok) Screw down floorboards to tighten up. (what screws, type and size would you recommend?)

2) Mark up top side of ply (18mm), where joists will be. Prime underside and edges with SBR. (Should I prime floorboards?)

(3) Flexible addy and screw down ply. Along joists (Every 100mm?)
same question applies to screwing rest of ply down to floorboards, (what screws for into floorboards and though joists?)

(4) All screws to be just slightly countersunk, do I just leave screw heads showing.

(5) Lastly If using 2 pieces of ply, what do i do about the joins? and when tiled should i use expansion joints.

Thank you all in advance.

Forgive me for all the questions, fairly new to the game and as Im still working my other job I cant get experience in as fast and as often as I would like.

If you can get any board up take a look to see where pipes/cables are. Check the defelction of the floor as its failed once with ply for a reason. What thickness are you taking up?

Try and add some noggins if you can, its fairly simple to do if you havent done it before and will only need one line of the them.

You dont need addy under the ply r to prime the boards, just screw it down minimum of 200 centres. I usually use screws long enough to go through the ply and into the subfloor but not long enough to pierce the bottom of the floorboard/chipboard.(1 1/4) That eliminates the hitting pipes etc. I then use longer 13/4 to go into the joists.

Leave a small gap about 3 mm between the joints and fill with silicone. Stagger the joints on the side if you are using more than one sheet. You could overboard it all with 6mm hardi backer but could give you a height issue.

Check with you addy manufacturer to see if you need to prime the surface of the boards as some dont need it. Obviously make sure you use a flexible adhesive and grout

Good luck with it