By using a combination of the offset tool and push pull you can make a parapet in less than a minute.
How to make a flat roof in sketchup.
Be sure that you are looking pretty straight on to your image.
Type the thickness of your parapet and then press enter.
With the offset tool click the top face of your building.
Watch this tutorial to find out how to make a basic roof in google sketchup the easy to use 3d modelling tool.
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.
An l shaped building in this article.
Modeling parapets on flat roofed buildings is easy.
Then click on the protractor tool.