Attaching plywood

[learnerDIY]

Hi

Thanks to the last person who has done a guide to laying tiles - however as a complete novice could someone help me with a few queries - sorry if they seem a bit dumb:
1. I am going to screw 12mm waterproof ply to my shower room floorboards before a tiler comes to lay ceramic tiles - Do I have to screw the ply into the joists? and do I have to lift the floorboards to find where the joists are or is there an easier way? Is 12mm too thick - the timber merchant suggested 9mm? (All the old suite is being removed by the way).

2. In another bathroom I think the floor is concrete but it already has plyboard over the top (although it might be hard board - what's the difference?). If its ply how can I tell if its waterproof ?- the people who lived here last were real bodgers so can't rely on them having put the right stuff down (they even painted a wooden supporting beam grey to pretend it was a steel beam!) although it doesn't appear to be rotten anywhere.

Many thanks for any help.

 

[gcol]

1.
No you only need to screw the ply to the floorboards. Try and get screws long enough to get a good purchase in the floorboard, but no so long that they protrude through the bottom of the floorboard or else you risk piercing a pipe or cable. 25mm screws should be fine.
12mm is being on the cautious side - you can usually get away with 9mm if your existing floorboards are in very good condition and are fixed well. When I wrote that tiling guide I was writing it in a way that anyone that followed the instructions should get a successful result. If your floorboards are uneven then you want to be using 12mm. It's up to you what you use but for the sake of 3mm, if you're not sure I'd recommend sticking with the 12mm sheet.

2.
Hardboard is a bit like a cross between cardboard and MDF - one side is coarse (like a shreddie ) the other side is smooth (like.... a smooth thing). You'll be able to tell the difference easily - just look and see if there's any woodgrain visible. If you're still not sure, try and scratch it with a screwdriver - if it splinters it's ply, if not it's hardboard. Get yourself to B&Q and have a look at the timber section at the different materials - ask one of the muppets to show you hardboard and exterior ply. Often WBP(exterior) ply has a pinkish colour but this is not always the case. There may be a stamp mark on the surface somewhere that might tell you what it is. While we're on the subject, are you intending to tile over this hardboard/ply? The reason I ask is that if there's concrete underneath, you'd be better tiling onto that directly.

 

[learnerDIY]

Thanks very much - you are a star