There are various types of roofs built in the real world, and various methods of creating roofs in SketchUp. We show you a variety of techniques in our SketchUp training courses.
For example, to create a skillion type roof as illustrated here, we first go to the Camera drop-down menu and choose the Top view, then the Parallel mode. This allows us to take the roof measurements from the CAD elevation, for instance the roof overhang may be 1 metre on either side and 300 high (in the Blue axis).
So we return to our model and using the Line tool project a line from the angle of the skillion outwards by 1 metre on either side, basically drawing the profile of the roof. Note that projected lines will be magenta in colour. If the lines drawn do not create a  filled planar surface, there may be a gap in the perimeter, so just click around the main points of the perimeter again with Line tool. This should result in a flat plane which represents the roof in profile.
Another reason why a plane may not be created is that one or more of the points do not lie flat on the same plane – you can check this by viewing the drawn lines form the top or one side in Parallel projection mode. SketchUp will not draw a twisted plane using this method; other methods are used if this is the desired objective.
Then Push/Pull (P) this roof profile.to the width of the building. Then add an additional 750mm on each side for the overhang. Finally tidy up the resultant shape with the Eraser tool. Then, as always, triple click on roof to Make Group. Create a new layer named Roof and put roof onto it via the Entity Info dialog box. Repeat this process for other skillion roofs in the model.
Many useful tips and techniques can be found at the SketchUp website. And see many examples of our own clients’ work on our Facebook page.
Other related SketchUp Blog Posts:

• SketchUp background
• SketchUp introduction
• SketchUp modeling
• SketchUp contours
• SketchUp workspace