Reason for the WBP ply

[flange]

I was just wondering what the purpose of the WBP ply is when doing floor tiling in the bathroom? Is it to make it completely flat or to spread load across floorboards or to ensure that none of the individual floorboards move under the tile?
I'm asking because we've laid the ply and in a couple of places where the edges meet up there is a small difference in height. This is most noticeable when you rest a tile on hte ply across gaps, basically the tile can wobble a bit and i'm wondering if we need to sort this out in some way?

 

[Deluks]

WBP because it's less susceptible to water damage than say chipboard (wet zone) It's also more dimensionally stable than floorboards, and when screwed in place tends to stay put. Which is important when tiling because any movement in the floor will cause the grout to crack. You definitely need to sort out any height diff. but if it's only very slight then you might get away with it as the tile adhesive can make up the difference, and I mean very slight. This all assumes that the ply is at least 18mm and has been screwed down every 6 inches. and that all joints are supported.

 

[gcol]

You only need 12mm ply when overboarding. 18mm ply is for when you are replacing the floorboards and just having ply down. It is also an absolute minimum, personally I'd use 25mm when replacing boards.

 

[Moz]

personally I'd use 25mm when replacing boards...

same here

 

[par1sons]

i have used 18mm ply on my bathroom floor but my floor area is only 4sqm. I suppose if the bathroom was bigger then thicker ply would be needed. Having said that, my floor has been down now for years and i have not got one single crack in any of my tiles neither in my grout. I did put extra noggins between the joices as extra support but in pulling up the old floorboards it aloud me to do the plumbing with ease. Having used 18mm it meant i never had slight step from my bathroom to the landing, any bigger than the slight step would have been noticed and with having kids this isn't advisable.
AndyP

 

[JohnD]

I'm all in favour of spending money but when most floors are laid in 18mm chipboard, I don't see why 18mm ply, which is stronger and resistant to damage, isn't sufficiently better. It also keeps the threshold and skirting levels the same.

 

[Moz]

JohnD
Its the weight issue with tiles ...
as I an Gcol will offer advice to DIYers here , better to be over cautious than not enough ....
then they lay stone on the floor an the 18mm isnt strong enough ........

even if diy-charlie Has strengthened the substrate joists /an treated them ...

the subbase isnt suffice an can "belly"
Grout starts cracking , its a flipping nightmare all that money wasted ...