To create a dormer model required walls and additional roof.
How to make different kinds of roofs revit.
One of the most important elements of any flat roof is the membrane.
Then follow these 2 steps.
See hd version of video tutorial here.
Choose a relevant level for a bottom face of the roof.
This tool is located in the modify tab.
There are a lot of different combinations that could hold the membrane in place like glue or nails gravel or rubber.
Tutorial on roof construction modeling in revit.
Define roof parameters like overhangs slopes offset from level rafter cut.
In architecture tab select roof roof by footprint tool and choose desired roof type.
Use join unjoin roof tool.
Use it to extend the dormer roof to the main roof.
To embed this screencast choose a size.
Use pick walls tools and hover over a wall press tab to select all joined perimeter walls.
9 join roofs add opening to create dormer.