Mass are quite easy to make but using this method might be require a lot of works.
How to make roof on revit.
Click architecture tab roof drop down roof by footprint.
Select toggle slope defining for each line of the roof outline.
On the draw panel click pick walls.
The second option in the list is toggle slope defining.
On the properties palette set the underlay value to 01 store floor.
Your now have a flat roof.
If you try to add a roof on the lowest level.
Create a roof by footprint display a floor plan view or a reflected ceiling plan view.
On the draw panel select a sketch or pick tool.
Select the lines of the roof outline and right click over the line.
To edit roof properties before.
Changing slope angle and using slope arrow.
Click architecture tabbuild panelroof drop down roof by footprint.
First you can create a mass and create roof by selecting the mass faces.
Get these project files and all advanced 1h courses.
I will not discuss much about this for now.
Select finish roof on the create roof footprint ribbon.
If your design is quite simple you will not want to spend to much time on your roof.