First you can create a mass and create roof by selecting the mass faces.
How to make roof on revit.
On the draw panel select a sketch or pick tool.
The second option in the list is toggle slope defining.
Mass are quite easy to make but using this method might be require a lot of works.
On the draw panel click pick walls.
Select the lines of the roof outline and right click over the line.
Changing slope angle and using slope arrow.
This option turns off the roof slope extrusion for the line.
Select toggle slope defining for each line of the roof outline.
Select finish roof on the create roof footprint ribbon.
If you try to add a roof on the lowest level.
Draw a flat roof.
Click architecture tabbuild panelroof drop down roof by footprint.
To edit roof properties before.
Click architecture tab roof drop down roof by footprint.
Create a roof by footprint display a floor plan view or a reflected ceiling plan view.
If your design is quite simple you will not want to spend to much time on your roof.
Your now have a flat roof.
Get these project files and all advanced 1h courses.