To create an attic, please follow these steps:
1. Add a room where you wish to create an attic;
2. Hide walls where you'll be having a shed roof;
3. Using the Magic Cube, create a low wall and place it on top of the hidden wall;
4. Use the Magic Cube to create a shed roof, tilt it according to your needs, and place it over the small wall.
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For more details check out that video: