Mono-pitch roofs at different heights supported by load-bearing wall

I have a school building to model using CAD input methodology. 

It has a central load-bearing timber-frame wall supporting the ends of two mono-pitch roofs positioned at different heights.  The wall between the top roof and lower roof is exposed and contains windows. 

Below the lower roof the wall is an internal stud partition.  The building is has a common ground-floor with no intermediate floors. 

When I try to model the building and set up the roofs using the 'extend walls to underside of roof' command, the wall is rendered as an internal wall up to the top roof level.  How can I model this wall to be exposed between the two roofs and not exposed internally?  Any help much appreciated.