Brooklyn NY brownstones are not built to carry heavy loads on their roofs. So when you install a green roof you need to build for it and know what the weights are.
Here are the numbers from my friend Avra:
Extensive – 10-50 lb/squ. ft
Semi-intensive 30-90lb/sq. ft.
Intensive 80-150 lb/sq. ft.
Here are thicknesses for each:
extensive – 6″ or less
semi intensive – 25% above or below 6″
intensive – more than 6″