In the green roofing world we recognize two types of green roofs: those built on new construction, and those built atop existing buildings. The latter is known as a “retrofit.” When considering a retrofit, a structural engineer must confirm that the building is strong enough to support a green roof, and a waterproofing representative must evaluate the roof’s existing waterproofing membrane. If all the stars align, then the retrofit is a go!
So can a retrofit green roof be retrofit with a rooftop farm? I had never considered this scenario until several weeks ago, when a repeat client of my firm, Roofmeadow, approached us with the question. We designed and built a 13,000 (0.3 acre) green roof for this client in 2007, on top of his factory in NJ. The 3″-thick green roof continues to manage stormwater and perform beautifully, but now the client is looking for more. He wants to grow food on his roof in order to sell to local restaurants and markets.
Luckily, this particular green roof will be easy to retrofit. Green roof systems contain various types of drainage layers, depending on the particular needs of each project. Some drainage layers are made of granular material, while others consist of synthetic sheeting (i.e. plastic peg sheets, egg crates, or tangled filaments). One drainage layer on the market is made out of foam scraps that are salvaged from car seat manufacturing, and bonded together to form a thick mat. In addition to managing the flow of stormwater and retaining water for plants, this multi-purpose layer can protect the waterproofing membrane from shovels and hoes when used in a rooftop farm buildup. Luckily, the green roof in question contains this type of drainage layer.
When retrofitting a green roof such as this with a row farm, the strategy would involve peeling back the top layers of the system to expose the foam drainage layer. Additional green roof media would be brought to the site and mixed with the existing media to create a deeper system. While the existing media contains a very low organic content (4%-6%), the new media would contain more organics to appease the veggies-to-be. Next would come grading, laying out irrigation lines, and planting, and then vwalah – rooftop farm.
Not every retrofit retrofit would be this simple. Nevertheless, there is always a way to figure out a solution, so go ahead and dig in.