a note from Paul of Gaia Soil :
"All green roof media, including mine, are porous, so usually retain about half the rainfall up to saturation.
The reason we now virtually always build microswale systems is, if water is held in contact with the capillary materials in the medium, water will be pulled up into the capillary matrix.
Six inches of GaiaSoil holds about 2 inches of water, or 1.25 gallons of water per square foot (7.48 gallons/cubic foot times 1/6th of a cubic foot = 1.2467 gallons)
So will actually expect the GaiaSoil to hold about this quantity if microswales are used (pictured below).
One way to do this is eliminate the drainage matt, and create swales either with the roofing membrane itself (we have used Neptune's zero VOC neoprene WetSuit)"