When you create a new project, DomuS3D create a ceiling for you. The standard elevation of the ceiling (and the height of the walls) can be modified in the preferences (options) of the Application , as you have seen in the Introduction chapter. Such a ceiling is horizontal. If you need to create sloping roofs you can move the cursor on the internal side of the wall you want the sloping roof to start from, then right click the cursor over it and select command Sloping roofs -> Add.
You will open the Sloping Roof properties, where you can set the roof dimensions.

If you made any mistake, you can change that roof again by clicking the right mouse button on the sloping roof line and select command: Sloping roofs->Properties (or double click it with the left mouse button). Sloping roofs->Delete allows you to remove the sloping roof and get back to the horizontal (standard) ceiling.
Note that if you add a sloping roof the consecutives walls, DomuS3S will automatically merge them around the corner.