The topic of rooftop farming recently came up on the AUA Google Group. AUA member Breanne Heath had some great advice for the novice farmers. Will permission, we would like to reprint that advice here.
First, you want to get a structural assessment to figure out how much load your roof can handle. That will determine your design and substrate depth, and may also guide a decision whether or not to reinforce the roof beams.
All of our beds are on the sides of the building, rather than spanning the entire roof. This was because we wanted our beds to be as deep as possible, so we concentrated the load on the two exterior walls. This was also to accommodate all of the skylights. Our planting depth is about 12″, and our growing area is approximately 525 square feet.
The green roof insulates and prevents water runoff quite well. This winter, only the top 2″ of the beds ever froze. We think the rest of growing medium absorbed heat escaping from the apartment below. Since we have very little water runoff, we have an agreement with our neighbor to collect his rainwater. We really wanted to avoid using city water for irrigation. We recently won a judge’s choice award for the design of the watering system. You can see complete instructions (and also a picture of the roof) here.
The rainwater is pumped via a well pump to an irrigation system on the rooftop. This was necessary because we couldn’t generate enough pressure to reach the roof with a regular garden hose. Each bed is individually irrigated with a drip line that can be programmed individually.
For the drainage layer, root barrier, and filter fabric, we chose Henry brand products. The walls of the planters are built of cedar and the south side of each box is raised about an inch to allow drainage. The roof has a 2% grade and drains very well. Also to note is that nothing is actually attached on the roof.
Our growing medium is a mixture of perlite, compost, and peat. I know coir is supposed to be more sustainable, but we couldn’t find large quantities locally. The compost and peat is readily available at Home Depot, and we found horticultural grade perlite at Silbrico Corp., in Hodgkins, Illinois for a good price. We have leftover materials if you would like to check them out. We supplement everything with organic amendments and homemade vermicompost.
Originally, we only used perlite and compost for the growing medium. We found that it dried out much too quickly, and experienced a lot of moisture/nutrient loss. The peat helps with this a lot, while keeping everything lightweight.
This winter we had two hoophouses on top of the planters. We made ours from plastic sheeting and leftover conduit. This winter we grew a ton of carrots, kale, and other hardy greens. We will also be using them to get a head start on spring planting.
It does get very windy on the roof, but we have not had any plant or trellis damage so far.
So far I have seen many food-producing roofs in Chicago. They are all very different and this is just one way of going about it.
It might also be worth nothing that we did the entire project, including reinforcing the roof, all labor and materials, and installing the irrigation system, for around $10,000. I think this might be one of the less-expensive food-producing rooftops in the city. All of our plants are started by seed and grown under re-used fluorescent shop lights except the hops, kiwi, strawberries, and grapes.
AUA member Bill Morrissett chimed in as well, and added several helpful links for novice rooftop farmers:
(Photo Courtesy of Sprouts in the Sidewalk.)