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.
Create an odd shaped building.
An l shaped building in this article.
Type the thickness of your parapet and then press enter.
Modeling parapets on flat roofed buildings is easy.
With the offset tool click the top face of your building.
Use the pencil tool to draw a straight line that goes off of the building you created at a perpendicular angle.
Click again somewhere inside the same face to create another face.
Then click on the protractor tool.
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