I'm interested to know what you're going to plant on the roof, Rick. In the long drought of last summer I increased the numbers of sun-proof succulents in our dry chalky garden, and planted up several big bowls of sempervivum, which come in all sorts of colours and shapes, and grow babies round the edges with gratifying speed. You do need very sharp drainage in the soil base, in fact they seem to need practically no water-holding compost although I found they needed a mix of about 1/3 John Innes to 2/3 horticultural grit, just to give them enough soil for their roots to get a grip. My big bowls are mainly under a little roof porch over the front door, so sheltered from most of the rain in winter, but I have one outside in full rain, although I've tilted it away from contact with the ground to increase drainage. I believe you can actually buy suitable compost base material for green roofs, made from ground-up bricks etc?