We lost our stick-frame house in the May wildfire in Santa Barbara and are considering rebuilding with Rastra Block. We have some big incentives with the county and our insurance company to build a similar house using the same footprint. The house had a flat roof with parapets and a veneer of stabilized adobe. We used conventional 15# building paper over the shear panel and behind the adobe and had no leakage problems in the walls or around openings. We used bituthane over the tops of our parapets. Although we only get about 20" of rain per year, it all comes in a few months and in very big storms. We are concerned about waterproofing Rastra block with a flat/parapet roof, and a masonry veneer. By the way, if you are wondering how this house burned, the failure was in the veranda beams run through the adobe into the interior framing. Peter K.

i would consider regular cementicious 3 coat stucco, over paperless metal lath directly over rastra, then paint to maintain water proof seal (including the top of the parapet). with a concrete roof (ie insuldeck) i would construct all "vigas" out of cantelevered concrete beams (wrapped in decorative wood, or fire proof hardie fiber cement trim boards). then i would use aluminum framed impact glass laminated windows, which should make the envelope pretty fire resistant.
to create the "viga" look you could remove the foam part of the insuldeck after concrete cure and use the 24" o/c beams as the are, just clad them....

the slab roof can be sealed with liquid urethane or acrylic concrete deck coatings...
no flammable sheet membrane needed in this case...

i have just raw concrete on top of my own house and have not yet sealed it...
some shrinkage cracks, but no leaks...
they are flat and pitched roofs in florida, 5 years after the hurricanes now...

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A composite ICF is a good choice, and there are others besides Rastra. Lath is not needed, and many synthetic stuccoes are waterproof. A post and beam ICF like any composite ICF will not, likely, support a concrete roof system. Check out the other ICF for floors and roofs, like BuildDeck, Lite-Deck and AmDeck.