Installing rooftop garden?

I live in SF and have a flat roof. I’ve wanted to garden in my (small) yard, but I don’t get a ton of sunlight due to nearby shady trees. It recently occurred to me I could maybe install a garden on my significantly-more-sizable roof, which would also provide much-needed cooling insulation during our increasingly hot summers. I looked a little at local regulations and it seems it might be possible for me (I also have a spot in my house that would perfectly accommodate a stairway up to and through the roof). I’m wondering how (insanely?) expensive this might be and also if anyone with experience has pros or cons to share. 
 

thanks for any input!

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RE: Installing rooftop garden? ()

I don't have costs to give you but the idea seems like a good one if you can get it to work. One thing to note is that roofs are typically only designed for minimal loading and aren't typically designed for gardens (soil weighs a lot!). Depending on how large of an area you would want to use for your garden you will likely need to beef up your roof framing and possibly the foundation as well. I recommend getting an engineer to look at your house. They can tell you how large you can go without affecting the existing framing/foundation.

RE: Installing rooftop garden? ()

Roofs aren't designed to bear weight, and soil is heavy.  A couple of options to lighten that roof-top garden:

Consider using planters rather than covering everything with soil.  This would work better for the plants because you could provide them with more rooting depth at the same time reducing the aggregated weight.  It would also make repairs to the roof easier.

Second, make sure you have a low-density soil mix. Years ago the SF Chronicle did a series on a roof-top garden where weight was an issue, and they used a potting medium of perlite and compost. Perlite is very low in weight.