Instructions
Press "Build Mode" on your interface and select the roofing options, then delete your roof. This is accomplished by clicking a single button in the roofing menu. Add walls around the perimeter of your house on the second floor in place of a roof.
Select the "Terrain" tool. Click twice to lower the terrain anywhere around your house within two to three tiles away.
Place a foundation at the bottom of the depression created by the terrain tool. Add a one-by-one "pillar" of wall over the foundation. Go up another floor level via the "level up" button on your interface and add more wall directly over the first pillar to double its height.
Press "Ctrl," "Shift" and "C" together to expand the cheat input box. Type "BoolProp ConstrainFloorElevation False" and press "Enter."
Select the "Level Terrain" tool. Click on the top of the pillar and drag it over the length of your second floor. The height of the walls will be cut in half. Place an ambiguous flooring texture over the top of the half-walls to complete the roof.