Plasterboard and tile backer

[Barnboy]

Morning all,

I have 2 bathrooms to to plasterboard the ceilings for skimming and put up tile backer on the walls for the tiling.

My 1st floor bathroom is slightly in the roof space so the walls are about 1.9mtr vertical section which then go into a 45⁰ undert the rafters

 My question on this room is, after boarding the flat ceiling, do I fit the plasterboard to the underside of the pitch 1st or the tile backer to the low walls or doesn't it matter to for getting the best finish between plaster and tiles ? 

My second bathroom is a std square ground floor room, here do I wait until the ceiling is skimmed before fitting th backer board or fit it all at the same time? Thanks

[nod]

Ceiling first is easier But doesn’t really matter 

Gypsum’s backer board outperforms cement board and is much lighter and easier to work with Or simple moisture boards