Molding joined at the corners

[Ash]

Click OUTSIDE but near the exterior wall (any wall), that shoud select the "Outside room" i.e. whole house will be shaded, then click down the edit bar on made room molding polyline. adjust the hight then go to the other Tabs in the DBX to set you molding profile.
the key point here is to select the whole house by clicing outside the building parameter.
You can repeat this process and add the molding at the door level, then using the edit tools in Plan view, you can break the molding around the doors or windows.