I would say, it would be nice to use values roof:shape=
skillion,
pyramidal,
gabled and
hipped
for walls with area=yes.
Example how could it work:
http://www.openstreetmap.org/way/278768458
and the picture for that:
https://commons.wikimedia.org/wiki/File:Wieluń_Kosciol_swMikolaja_03.jpg
