Matthew Lewis Super helpful, thanks - so it's on piers, not a slab? Hard to tell from the photo. I assume they are set at grade? How deep is frost line where you live? I live in frost-free Berkeley and am prepping to build something quite like this, but was told I'd need to pour a slab to manage the load due to our clay soils - would love to know the pier depth as it would be easy for me to match that (easier than pouring a slab!). And makes sense on the 3/4", I'd assumed that would be required with all the weight; seems like it's basically engineered like spanish tile roof?

On the weight calculation - 2500 #s seems low to me? What is the growing medium? I've seen a lot of these designed with slate chunks and 4 inches of slate on 120 SF would be quite heavy ... but curious? Thanks a ton!

Graham Design and Construction I'm no green roof expert, so take what I say with some skepticism. The goal is to keep your growing medium as light as possible. That's why I used lava rock for the base layer. Also, compost is fairly light as well. It's the sand that's heavy. I believe all the growing medium was about 2000 lbs. and we loosely figured it could absorb about 500 lbs of water. We also calculated additional weight for snow load.
You'll have to research what kind of growing medium you'll need depending on what you intend to grow on the roof. Our mix was designed to grow sedum plants which like dry soils.
As far as the footers/piers, I can't really comment too much on that b/c each part of the country has different rules/laws regarding that. I usually dig footers at around 30" and the soil MUST be undisturbed compacted soil. Also, if you're in California you'll have to consider earthquake proofing your structure. Contact your local building permit office for help (you'll probably need a permit for the job anyway).

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