Cement backer board to help level a floor?

PermaChanged · 8 Oct 2019 at 15:10

After years of renting I finally bought my first flat in late August – hurrah!

First job is to rip out an aging, cheap and awful ensuite bathroom and replace it – I’ve gone from an 800x800 shower which was mounted on a riser kit (over 6 inches off the ground!) to an 1100x900 40mm off the ground, with the waste running through the floor. The riser kit was hiding that the floor is out by quite a bit – about 20mm in height over a run of 1200m. I’ve used packers and a good mortar bed to mount the tray level, and the built in run off in the tray is directing waste water perfectly fine.

However, I want to lay the floor now but I need to level it. Would using cement tile backer board be advisable to take out the majority of the slope? I planned to use a 20mm board for roughly 400mm, a 12mm board for roughly 400mm and then to pour self leveller over the lot – is this feasible / advisable, or would I be better off doing similar but with plywood in different increments?

AHarvey · 8 Oct 2019 at 15:26

Honestly, if I was to do it I'd be looking at a solid piece of plywood or marine plywood and use wooden battens across the floor to level it out.

Place the battens every 6 inches or so for a nice solid base, wood on top, then your choice of floor.

You'll need to check everything else too including door, skirting etc.

Mercenary Keyboard Warrior · 8 Oct 2019 at 15:40

Are we talking ground floor or upper floor, or actually specifically is the floor solid underneath

Do you already have some sort of step into this room (up or down) as by adding a fair amount of height you could introduce one

200sols · 8 Oct 2019 at 15:50

Solid floor, use a compound like Mapei ultraplan, floating floor or joists use wooden frame to bring floor level, overboard with ply then lay flooring, if laying tiles overboard the ply with hardiebacker.

PermaChanged · 8 Oct 2019 at 16:07

It's a chipboard floor straight on to the joists (the portion under the shower tray is marine ply) - I really want to avoid lifting those boards if at all possible.

Mercenary Keyboard Warrior said:
Do you already have some sort of step into this room (up or down) as by adding a fair amount of height you could introduce one

No, no step up or down - the entrance is the level which I'd like to maintain, and raise the back wall to be in line with.

Mercenary Keyboard Warrior · 8 Oct 2019 at 16:11

Lift old floor, level joists with packers, use marine ply as new floor, seal (particularly edges) and lay flooring.

200sols · 9 Oct 2019 at 08:46

Hope the chipboard doesn't run under any stud walls either

PermaChanged · 9 Oct 2019 at 09:06

Thanks for the responses guys, raising the floors seems like the best and most sensible option to replace with ply, but also a harder job for a novice DIY'er like myself - only one way to learn though, right?

200sols said:
Hope the chipboard doesn't run under any stud walls either

It does run under one very small one, about 1000mm in length, but the floor actually stops short of the back wall where it slopes down towards :confused:

Fortunately the joists run perpendicularly to the slope, so I won't need to touch the stud wall side!

Mercenary Keyboard Warrior · 9 Oct 2019 at 10:22

Rather than hard I would just say its time consuming really.
But i think you will get a much better job by doing so, and should you need to get under the floor at a later date it will be far far easier (depending on covering of course) than having a layer of cement backer board and compound

PermaChanged · 9 Oct 2019 at 11:43

Mercenary Keyboard Warrior said:
Rather than hard I would just say its time consuming really.
But i think you will get a much better job by doing so, and should you need to get under the floor at a later date it will be far far easier (depending on covering of course) than having a layer of cement backer board and compound

All very true and relevant - and knowing my plumbing skills getting under the floor may be required