Renovation of a small guest toilet underway. Since two of the walls, which are concrete, are a bit uneven, I want to level them out with renovation gypsum board (so 6mm) before tiling. The plan is to attach these with gypsum mortar. But do I also need to screw the boards in place? I think a point is that it also holds the boards while the mortar dries. What kind of screws if so? Or should I skip the screws? Thanks in advance!