The Sweet and Sour of Green Roof Urban Agriculture

We have discussed in previous posts the urban heat island effect, and how green roofs are working to reduce the effects of rising temperatures in concrete jungles.  It goes without saying, however, that green roof technology is not 100% worry-free, or that it doesn't come with a laundry list of obstacles.  Today, we will discuss some of those concerns and how they can be reduced or avoided for beneficial outcomes for everyone.

Most green roofs are made with engineered materials over native soil and are not made for growing vegetables, but merely as a leafy green that reduces heating and cooling costs in buildings (Wittinghill 319).  As such, there is concern about whether rooftop gardens can sustain constant yearly replanting of fruit and vegetables.  A simple solution to these fears is rooftop container gardening, wherein plants are grown in cubes and pots laid out in strategic patterns that allow gardeners to easily plant, feed, and water them.

Job S. Ebenezer, President of Westerville, Ohio's program Technology for the Poor, offers up the following ideas in coordination with pots for creating a rooftop garden in spaces where a green roof might not be a practical solution:

• Plastic Wading Pools
• Used Tires
• Feed Sacks
• Bricks Laid Out to Build Planters
• Wooden Boxes

Here, Job lays out the basics of starting urban using cost effective materials that can in turn yield sustainable crops for all involved in the work and care of the resulting garden.

The cost of installing a pre-fabricated green roof seems among the top concern for many proponents.  Green roofs average the same maintenance cost of their conventional counterpart, but can cost about $32 more per square meter (Whittinghill 317).  As such, it's important to understand the start-up costs versus the amount that can be saved in utility bills for heating and cooling.  There's a ton of math and other number-stuff that goes into this and I'm afraid I'm drastically stupid at match.  But these guys aren't, and I've borrowed their power-point to do the explaining for me.  On average, people who install green roofs can look for a range of 2-39% energy savings in shaded areas, and up to an 87% energy savings cost during the summer when the amount of heat transferred directly into a building from sunlight is absorbed instead by the green roof seedum (Whittinghill 315).  

With a typical downtown area comprised of up to 85% of commercial businesses having flat roofs (Whittinghill 318) it would seem like a no-brainer that urban areas should all be jumping on the green roof bus (powered with smart options like electricity, of course).  There are, however, certain factors to take into consideration.  A typical engineered green roof is comprised of multiple layers:

Did you like the part where they said to NOT stand on a freshly installed roof and then showed the guy STANDING ON THE FRESHLY INSTALLED ROOF to water it?  I chuckled.  

It's obvious that even a recently constructed roof should always be analyzed to make sure that, if it wasn't appropriated for this use to begin with, it can sustain the extra load it will have placed on it to ensure a costly collapse doesn't happen, as seen in this picture below of the Aquascape Design headquarters in Charleston, South Carolina:

Urban agriculture, in general, can have unforeseen consequences, particularly when growers take up in vacant lots where a home once sat.  Primary to this are metal contaminants in soil leftover from a variety of industrial sources (Whttinghill 318).  While fruits are less likely to absorb heavy metals, leafy vegetables with deep roots are a concern.  As the metals are non-biodegradable they can have adverse health affects including cognitive disruption, nervous, cardiovascular, kidney bone and liver diseases, along with cancer (Whittinghill 319).

Rooftop gardening and green roofs have the potential to negate these consequences, largely because the soil used in green roofs is chemically engineered and does not contain local contaminants, and because container gardening involves using potting soil as a medium (Whittinghill 319).  Because the mediums used are not from the ground outside the building, they are less likely to produce foods that would already be contaminated from urban industrial waste.    

While there are certainly concerns to think of in any "new" technology, it's pretty standard knowledge that gardeners have been gardening for a loooong time.  As such, they welcome the challenges with the benefits.  Because of their enduring love for gardening itself, food production can become a sustainable reality.  Mentioned in previous posts, but enforced here again so we don't forget, other benefits include reduced air and noise pollution, storm water retention, increased biodiversity and energy savings (Whittinghill 315).  

Unlinked Source:

Whittinghill, Leigh J. and D. Bradley Rowe.  "The Role of Green Roof Technology in Urban
     Agriculture."  Renewable Agriculture and Food Systems 27.3 (2011): 314-322.
     ProQuest.  Accessed April 5, 2014.