Have run a little experiment on realtime shadows...
building in attachment 1: 1unit thick sloped roof thickness - 1unit wall thickness
notice poor roof shadow. appears as a light leak.

building in attachment 2: 6 unit sloped roof thickness - 1 unit wall thickness
shadows appear as intended.

building in attachment 3: 1unit flat roof thickness - 1 unit wall thickness
Roof and interior walls have terrible shadows. interior wall allow light leak.

building in attachment 4: 6 unit flat roof thickness - 3 unit wall thickness
Shadows almost correct but still small roof shadow leaks

building in attachment 5: 7 unit flat roof thickness - 3 unit wall thickness
Shadows appear as intended.

Any models that do not follow the 3 unit wall and 5 unit roof minimum requirements run a risk of not rendering shadows as expected in realtime.
Be aware this is only an experiment run for my own benefit. Not to be interpreted as info from TGC.
I just thought other modelers might also benefit from the results.

The unit requirements here appear to have no effects on light mapping shadows.