Watch this tutorial to find out how to make a basic roof in google sketchup the easy to use 3d modelling tool.
How to make a flat roof in sketchup.
Create an odd shaped building.
Click again somewhere inside the same face to create another face.
Use the pencil tool to draw a straight line that goes off of the building you created at a perpendicular angle.
Modeling parapets on flat roofed buildings is easy.
Type the thickness of your parapet and then press enter.
By using a combination of the offset tool and push pull you can make a parapet in less than a minute.
Be sure that you are looking pretty straight on to your image.
Then click on the protractor tool.
Easily creates roofs with various parameters in sketchup plugin name.