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Montie Zoughet a retired UN Arid Plant Ecologist who designed an
built green roofs overseas, conducted a four month cost benefit
analysis of green roofs for the central valley and found Green
Roofs to be 25% LESS effective than planting trees near the
structure. We were asked to make 5 minute presentation to the
Sacramento Chapter of the USGBC. This is a summary of our
findings. 

Problems:

Currently, materials costs are prohibitively high in the US, as
compared to overseas
Green roofs are currently being over designed & over built in the
US for practical, large scale application here. 
These Designs are not easily applied to existing construction,
many are heavy.
Green Roofs need and will retain moisture and may damage the
structural integrity of the roof over time.
In a side by side cost benefit analysis, planting large trees near
the house to 
shade the roof is more energy efficient by a factor of 25%.
All green roofs require maintenance, a hidden cost.
The US lacks incentives for retrofit-construction to existing
buildings.

Solutions:

Montie selected and tested the proper green roof plants for this
region.
He has an idea for a modular solution, but we lack sufficient
funds to privately underwrite the manufacture, installation and
testing of this concept on a large enough scale.
Montie believes that a low intensity, modified version of a green
roof may be possible by planting selected plants at the corners of
a roof and allowing them to grow across the roof.