I would not utilize buffalograss in a green roof application. It’s major issue is that it has a narrow precipitation window: too dry and it will expire like any plant, too wet and it will be out-competed by invasive species. Buffalograss also prefers dense clay soils contrary to the lightweight sandy soils you typically see on a green roof. I would check with a structural to determine the weight of your green roof profile if you changed your soils.
Buffalo also has an extended “browndown” period and a physical texture that many find unappealing. I would possibly utilize buffalo only in areas where it would work naturally (i.e. shortgrass prairie ecosystems) and in areas that would not be seen or used. If you’re looking for aesthetically pleasing, functional grass on an extensive roof, I would look at another grass type.
