Thanks everybody. Cambridge, Massachusetts has interesting regulations and we have to come up with innovative ideas to meet them. The building occupies the entire site and there is a garage under, so they don't want a tank below that (in groundwater), and can't spare any spaces on the garage...
I have to retain an arbitrary volume of water on the roof of a building. By the local regulation, the total volume of runoff from the site, for storms up to and including the 25 year storm, cannot exceed the volume of the 2 year storm. Infiltration is not an option, because the building covers...
