I created a test room to keep it simple. 4 walls, 1 of them 1,5m, another 2,5m, the sides corresponding.

No, not walls, create and select a room.

Icons at the top:
Select Pointer
Pan Hand
Add Wall
Add Room

(Or in German "Raum estellen")

Once the room is created, use the Select Pointer to select the room, then use Tools -> Generate roof...