Levelling Cement Board Over Joists

[LuthersDad]

We're planning to tile the floor of the bathroom in our c.1900 property. Currently there's lino over floorboards which are uneven. As we are ripping out and replacing the bathroom I plan to pull up the existing floorboards and lay 18mm T&G Cement boards directly onto the joists in order to maintain the floor at the same level as the hall floor.

One concern I have is whether the bathroom floor itself is currently level (I've not checked as previous owners have lifted floorboards and not screwed them down so it's all over the place) and how to resolve the issue if it's not.

What's best practice? "Pack" the cement board to achieve the level or lay the cement board and then use levelling compound to get the level.

Also with T&G cement board am I correct in assuming that there still needs to be a 3mm expansion gap between wall and cement board edge? Wall will also be tiled so we'll probably replace the existing plasterboard with backerboard as there's several damaged areas.

 

[blup]

Once tne lino is off and the boards are examined, the floor may not be as bad as it first appeared. But if it is significantly out you could use wood or plastic shims on the top of tne relevant joists or even sister them.

 

[slawit]

I know you said you want to keep the floor height the same to run through with the hallway, but could you afford (in terms of height) to put the cement board down then lay a thin self-leveling screed just to finish it off.

 

[cdbe]

Is 18mm t&g cement board even a thing?

Edit - just seen them mentioned on another thread so clearly they are! Must be quite heavy I guess.

 

[LuthersDad]

I know you said you want to keep the floor height the same to run through with the hallway, but could you afford (in terms of height) to put the cement board down then lay a thin self-leveling screed just to finish it off.

Do you mean once i've lifted the floorboards? If yes, then possibly, if you mean over the top of the floorboards, no i'll end up with around 30mm difference between the floor levels.

 

[LuthersDad]

Is 18mm t&g cement board even a thing?

Edit - just seen them mentioned on another thread so clearly they are! Must be quite heavy I guess.

Come in 22mm too. 18mm are around 18kg for a 1200mm x 600mm board

 

[slawit]

Do you mean once i've lifted the floorboards? If yes, then possibly, if you mean over the top of the floorboards, no i'll end up with around 30mm difference between the floor levels.

Yes I meant take the floorboards up, lay the cement boards then just a thin layer of screed to level up.