Select the lines of the roof outline and right click over the line.
Revit make a flat roof.
Select finish roof on the create roof footprint ribbon.
Attach interior walls to the roof.
Go to the new level for the roof under floor plans on the project browser.
Your now have a flat roof.
Add roof lines with a slope and an overhang defined.
Make sure you have selected roof by footprint from the drop down tools.
This option turns off the roof slope extrusion for the line.
In the drawing area angle icons identify which lines of the sketch define slope.
Add a flat roof.
You need to make sure you are in a floor plan view in order to use the roof by footprint tool.
To create a sloped roof in revit modify the footprint sketch of the roof by selecting and specifying the slope defining edges of the roof.
When editing the roof sketch select the edge or edges you would like to slope and select defines slope on the options bar.
Select toggle slope defining for each line of the roof outline.
You can also use toggle slope defining on only a few of the lines to create different roof profiles such as a gable.
Select the architecture menu and then roof.
The second option in the list is toggle slope defining.