A Roof is a Terrible Thing to Waste
As the buzz around urban agriculture reaches a exciting pitch I frequently hear students, friends and associates ask “what about growing food on the rooftops in San Francisco?” It does seem odd that in our super dense city, as any quick trip up to Coronoa Heights or Twin Peaks will demonstrate, we have relatively few rooftop gardens or green roofs. As a permaculture enthusiast when I think about rooftops it is difficult for me not to see the abundance of opportunties resident in our urban canopy to grow food, yes, but also to sequester carbon, harvest rainwater water (fog too?), harvest sunshine, heat water, house bees or, at least, reflect light. I will attempt to elaborate on the concerns and opportunities regarding rooftop gardening in San Francisco, but if you care to investigate the potential of rooftops from a wholistic perspective I encourage you to familiarize yourself with the work of Bay Localize, specifically the Rooftop Resources Project. Bay Localize’s publication – Use Your Roof provides an excellent summary of many of the considerations for designing with rooftop abundance in mind.
Access and safety (Title 24) issues aside, the main concern about rooftop agricutlure in San Francisco seems to be additional load capacity and structural integrity of the the buildings and rooftops themselves.
Many, but certainly not all, buildings in San Francisco were not designed to carry snow loads (snow loads not being common) and are designed with varying degrees of allowances for additional loads. Intensive container agricutlure on rooftops is considerd a “live” load (soil settling and water moving around) and needs an allowance of 50 to 100 pounds per square foot (psf) live load allowance (it has been demonstrated that rooftop hydropoinc agriculture can be done with additional loads of as little as 16psf) (Severson, 2009).
It seems unlikely, with specific figures unknown to me, that many San Francisco rooftops are equipped with the strucutural requirements to host intensive urban food production.
That said, it seems to me that many industrial buildings and parking structures, etc must be equipped with appropriate strength and, of course, retrofit to strengthen rooftops is also a possibility (estimated by one real example in the Mission District to cost $40 per square foot ($17K for a 25′ x 18′ one story garage).).
So what to do with our rooftops. Of course it depends on the roof. One possibility that seems to beg attention is solar heating of water. When we assess the carbon footprint of the neighborhoods of San Francisco we quickly find that heating water through mains gas or electricity is a major contributer of greenhouse gas emissions. Perhaps surprisingly we find relatively few installations of solar hot water hearing in San Francisco compared to photovoltaic electricity generation, so far. I anticipate this will change in the near future and I am looking forward to seeing some permacutlure designers form a worker owned cooperative design and install business to meet the forthcoming demand.
Photolvoltaic (PV) electricity generation is, demonstrated by the solar SF map, seems like a reverent use of of our rooftops. Case studies show that we can generate an abundance of electricity for household use from our collective roofs all the way out to Ocean Beach despite our fog (and seemingly low-light conditions). PV is of course perfectly compatitble with rainwater harvesting (as is solar water heating). Seems like a function stack to me.
The rooftop rainwater harvesting potential of San Francisco is enourmous given the amount of rooftop acreage. The obvious challenge becomes storage capacity. Creative integration of cisterns into our urban landscape (above grade and below grade) will be imperative to our designs, however, we will likely ultimately find the ground to be our best storage. San Francicso of the future may look like a city of micro windmills pumping groundwater to be filtered and treated for household potable use (and irrigation) with household cisterns creatively integrated (safely) into the built envirnment with greywater incorporated into every structure with an appropriate landscape to discharge into. Maybe someday most of our garage spaces will be workshops or spaces for rainwater cisterns as automobiles move towards feedstock for micro wind turbines.