Ply for tiling bathroom floor

[Yanni]

I'm tilling a very small bathroom wooden floor in a small terraced house here in Manchester.

The floorboards have been down for more than 100 years and they seem ok.
I want to put a ply down to tile on top instead of going straight onto the floorboards but I do not want to create a big step with the corridor outside the bathroom.

I'm wondering can I get away with a 9mm ply or do I need to go for at least 12mm to make sure the floor stays flat ?
The bathroom is located upstairs.

Thanks for your help.

 

[Richard C]

12mm WBP ply is really the minimum when overboarding; screw through into the joists (but find any pipes first) every 200mm & not just onto the floor boards.

 

[robbro77]

If you do not want a large step try 6mm 'NO MORE PLY'. I have just put it down in my kitchen diner (15m2) so i could tile on top as it is floor boards under. It makes a very solid base and add some (all be it not much) insullation to the floor. It is slightly more expensive the Ply but as the area is so small i guess a few quid will be ok. Very quick to put down and cut into shapes for around pipes etc.

If you dont care about a step up or down go with Ply as it is easily available.

 

[jefoss]

have you thought about replacing the floorboards?

 

[Lower]

I lifted the boards and replaced with 25mm wbp ply before tiling in our bathroom. Floor is so much more solid its amazing and as the old floor boards were around 22-25 thick in there isn't the big step you'd get with overboarding.