Hello
I am having to replace a floor 13metres by 6 metres in my house which is in France...cracked ceramic tiles stuck on a blown chipboard floor with warped and rotten joists cut in to 35cm square wooden beams. The floor covers lounge and kitchen NO bathroom...that will be a different thread / problem.
Current plan is to rip out the whole lot repace the beams with RSJ's cut in new joists which will probably be 70mm by 200mm (French size timber) on 400mm centers with noggins and then wbp 20mm ply the lot.
As I am not a tyler i checked on the internet and thee are a lots of posts which suggest using hardiboard aquapanel or similar on top of 18mm ply..it seems that these will eliminate all the timber movement....although with 200mm screw fixings I cant see how this board can do anything other than follow the plys movements.
The bloke in our local DIY store reckons its good emough to prime the ply and then lay the tiles directly onto the ply with a flexi cement adhesive..my plasterer reckons the same...
Any advice.....i cant afford to get this wrong
Thanks

Hi Rusty John, as you say 20mm wbp will probably be OK, make sure you prime the back and sides 1st. Check with adhesive manufacturers as to priming face of ply. As you say you can't afford to get this wrong, why not go the whole hog ( belt and braces) glue and screw backerboard on top

Originally Posted by Rusty John

Hello
I am having to replace a floor 13metres by 6 metres in my house which is in France...cracked ceramic tiles stuck on a blown chipboard floor with warped and rotten joists cut in to 35cm square wooden beams. The floor covers lounge and kitchen NO bathroom...that will be a different thread / problem.
Current plan is to rip out the whole lot repace the beams with RSJ's cut in new joists which will probably be 70mm by 200mm (French size timber) on 400mm centers with noggins and then wbp 20mm ply the lot.
As I am not a tyler i checked on the internet and thee are a lots of posts which suggest using hardiboard aquapanel or similar on top of 18mm ply..it seems that these will eliminate all the timber movement....although with 200mm screw fixings I cant see how this board can do anything other than follow the plys movements.
The bloke in our local DIY store reckons its good emough to prime the ply and then lay the tiles directly onto the ply with a flexi cement adhesive..my plasterer reckons the same...
Any advice.....i cant afford to get this wrong
Thanks

That's 78sq metres of tiling! So a lot of weight and plenty of moisture in the adhesive to affect the plywood.
If I was doing the job I'd either lay a decoupling membrane or the tile backer boards to reduce the chance of cracking every 1200 and 2400mm.

The advice on the backer board isn't right because it won't eliminate movement, it will move with the ply underneath, as Timeless John says, a de-coupling membrane is a much safer solution on top of the plywood.

Originally Posted by Ken Bruty

The advice on the backer board isn't right because it won't eliminate movement, it will move with the ply underneath, as Timeless John says, a de-coupling membrane is a much safer solution on top of the plywood.

A de-coupling membrane is for latteral movement/crack isolation. It will do nothing to help deflection. Excuse me, if 20mm wpb is being laid correctly, then a backer board can only help imo. We are not talking green screeds, cracked screeds, we are talking wooden floors.