I am about to lay 600x600 porcelain tiles on to 12mm ply which is over boarding 18mm floorboards is there a certain way of laying ply eg opposite way to floorboards and is it advisable to glue & scre

opposing direction to floor board screws roughly 200 x 200 squares then pva over this .This process will stop to much movement and reduce the likelyhood of cracked tiles good luck

Use exterior grade ply or backer boards both as good as each other. backer boards are handy if you can't raise the floor to much. I screw every 150mm in grid pattern using a level to mark the lines in pencil at looks a lot neater and helps to get the screws uniform. Just one tip don't use pva to seal the floor before tiling use a good acrylic primer and make sure you use a good quality flexible adhesive and grout that is compatible with porcelain as some aren't. Hope this helps Chris.

Just screw the ply every 200mm in every direction then pva it before tiling. Temple carpentrys advise is utter rubbish and is a typical example of people on this site giving opinions on matters they know nothing about.He should stick to what he knows which is certainly not tiling.

Hi, It is common practice to lay the ply in an opposing direction to the floor covering underneath. It usually lends extra strength to the existing floor if done in this way. I couldnt advise as to whether you need to use both screws and glue together, it depends on the floor underneath. If you glue, you can be sure there will be little, if any, movement. Screws could be used initially to keep the boards in place whilst the glue cures, but you would only need a few in each board i think. Anyway, Good luck. Phil Temple Carpentry

Are you sure you have to use ply? Minimum thickness would be 15mm and whilst ply is used a lot,it is not the best material for what you are doing.Go into your tile shop and ask about hardie board and other alternatives.It will save you some height on the final finish floor too.

Best practice is to use a tile backer board rather than ply. It's a cement based sheet that can be cut easily.

I am with the real tilers on this one, screw the board down every six inches in a grid type fashion, ensuring that all screw heads are sunk. Then prime with an Acrylic primer for example BAL APD diluted with water at 5:1 (off the top of my head) then ensure you use a flexible adhesive suitable for porcelain.

Always use a decoupling mat on suspended floors.