Fibre cement floor to replace timber section

[danger005]

Just a quick question...
Anyone know if it is ok to fix 6mm Hardiflex to 18mm compressed f/c on the floor? I can't see why you couldn't but wanted to be sure. Also, would it be ok to just screw the 6mm sheet onto the 18mm sheet with say, 20mm f/c screws?

The reason... after ripping up old floor tiles I discovered a section of rooten timber (under where the bath was) so I've ripped out this section which spans 3 joists, leaving a cavity in the floor about 1900x750. I was going to lay down a compressed f/c sheet to fill this cavity, thus bringing the floor to the same level as surrounding. Then I was going to lay 6mm f/c over the timber flooring and thought about putting it on top of the cfc to maintain the same level throughout.

Ta

[echnidna]

Why not pack just the joists so the top of the CFC is at the correct finished level.