This room needs to be the shape and size of the desired roof so you may need to combine several empty rooms together.
Sims 4 add another floor without removing roof.
To add a flat roof to a building you will need to add an empty room on the upper level.
Once there is a room on the upper level you can add floor covering and a wall trim to the outer edge.
Although the sims 4 has made building houses better than ever it s still a bit tricky to make your dream home.
Here are a few tips to help you graduate beyond just making a box with windows.
Hey in this sims 4 video you see how to delete walls floor doors and how you can remove the ground.
Zoom out the camera select the removing tool hammer from the upper panel and click on the roof.
Hit page up on your keyboard to move up to the second floor view then build the walls floors etc from there.
Building another floor if that is not enough for you or you just want to build your house upwards instead of sideways you can build another and then yet another floor.
On your lot use either the basement room tool to create a basement or navigate to the basement level with the page down button and use the standard.