Click modify walls tab modify wall panel detach top base.
How to extend wall to roof in revit.
Attach walls to other elements.
Shape handles are not available for surfaces created by openings placed in roof faces.
Select the individual elements from which to detach the walls.
Select an edge of the roof that you want to join and then select the wall or roof to which you want to join the roof.
By default when you pick a compound wall with multiple layers the floor sketch will extend to the core boundary of the wall.
In the drawing area select the walls to detach.
Drag the shape handles to extend the roof face.
If desired you can specify an offset using a positive or negative value.
When you select the roof face roof shape handles display.
If necessary use the viewcube to spin the design to facilitate selecting the wall or roof edges.
Click modify tab geometry panel join unjoin roof.
See hd version of video tutorial here.
On the view control bar for visual style select shaded.
In the drawing area select the roof face.