When you create a new project, the Application 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.
If you search for Velux windows in the Components panel, you can see that those windows can be applied to sloping roofs only: starting from Pegasus, indeed, you can place them on horizontal ceilings too. That is a very useful feature also in order to allow natural light entering the scene from the top. As you will see in the chapter related to Rendering techniques, sun and natural light are very important for a realistic illumination of the scene.
Note that if you add a sloping roof the consecutives walls, the Application will automatically merge them around the corner.
This article is valid up to version DomuS3D® 2018