As a Pro user you could consider making two Scene tabs with identical plan views.
Do no use Sections at all, use Layers
One tab would be with the Roof Layer switched ON and one with it OFF.
In Layout link the SKP.
On a page paste a view port for the roof plan [with shadows].
Copy that vp and change its scene the the floor plan [without shadows].
With the Roof plan below the main plan the shadow should show correctly, although the floor plan is visible.
If there are big roof overhangs that might be seen consider having the real roof layer switched off, but a copy of the roof group on another layer with all of its faces painted in a fully transparent but shadow casting material [PNG texture] so you still see the shadow without the roof ?
This simple example SKP [v2013] might help - left hidden lines on so you can see the transparent roof…
It contains the fully transparent but shadow casting material used on the that roof copy…