I'm planning a concrete (almost zero energy), passive solar home in SoCal where the walls will be ICF. The roof will have some curved sections of ferro cement and some almost flat that will accomodate a green roof of local succulents and native plants and solar tiles. Not sure how that will work out as this point.
Most likely there will not be an attic and all services will be supplied from the garage or a crawl space. Or not...

For the almost flat green, solar tiled roof, would concrete be better or wood, maybe SIP's?
For passive solar design in a semi-arid climate, is thermal mass on the roof crucial or would insulation with a high R value be best?

For the almost flat green, solar tiled roof, would concrete be better or wood, maybe SIP's?

I recommend getting a PE involved for starters that understands dead/seismic loads there. In seismic alot of roof weight is no good due to inertia, and if your soil strengths are low, silt-sandy, limits how much dead load it can see walls combined.

Sounds like a great roof design do you know about CALEarth they probably have alot of resources. Domes have span limitations your PE can also address before you get too far along. Last I checked these guys are being monitored by local jurisdictions as a trial design for code adoption. If they now have code you coulld perhaps use it.
http://www.calearth.org/visit

CA climate varies alot from dry desert to mountains so need more info as far as mass vs insulation. Both have worked just be careful over heating rooms. Here again I bet CA knows your climate well.

Thanks for the tips and the link. The climate is more desert though the summer doesn't get quite as hot as the desert, maybe 15 - 20 degrees or so less. In the summer highs are typically in the 90's though 100's are not unlikely.