Grass roof... they have a slightly bigger one at Ford Rouge River:
"The roof of the 1.1 million square foot (100,000 m²) Dearborn truck assembly plant was covered with more than 10 acres (40,000 m²) of sedum, a low-growing groundcover. The sedum retains and cleanses rainwater and moderates the internal temperature of the building, saving energy. The roof is part of an $18 million rainwater treatment system designed to clean 20 billion gallons (76,000,000 m³) of rainwater annually, sparing Ford from a $50 million mechanical treatment facility."
"The roof of the 1.1 million square foot (100,000 m²) Dearborn truck assembly plant was covered with more than 10 acres (40,000 m²) of sedum, a low-growing groundcover. The sedum retains and cleanses rainwater and moderates the internal temperature of the building, saving energy. The roof is part of an $18 million rainwater treatment system designed to clean 20 billion gallons (76,000,000 m³) of rainwater annually, sparing Ford from a $50 million mechanical treatment facility."
http://www.youtube.com/watch?v=lW0G6zNStTE