Platforms disappear or roof appears when placing walls again

Hi there,

I want to make two diagonal platforms against my wall. I can build the walls again, but then my "roomdefinition" goes away and the roof on another wall appears into the room. If I redraw my walls again (I go over every existing wall in the room) the roof disappears again, but the platforms go down and I can't raise them up again.

Does anyone know a workaround?