You can delete a dormer roof in a couple of easy steps. If the dormer has walls, they need to be deleted separately.
To delete a dormer:
Click on the edge of the dormer roof to select it.
Press the Delete key on your keyboard, or right-click and select Delete, or select Edit > Modify Element > Delete.
If the dormer has walls, select one of the walls, then Shift+click to select the remaining walls. Use the Delete tool to delete the selected walls.