Rooftop farms and gardens.

We had a great time at New Green City in Union Square last week. The day was so gorgeous and we were happy to see a lot of interest in green roofs. In particular we noticed interest in rooftop farming, which was to be expected given the crowd and the clear vogue for rooftop farms in sustainability/food justice/urbanist fields at the moment. So here is a little post about rooftop farming that can hopefully answer some common questions on the subject.

Eagle Street Farm in Greenpoint, Brooklyn.

Rooftop farms require more soil than regular, extensive green roofs. You need over six inches of soil to grow anything productively, so the part of the roof where you are gardening will have to to be intensive. This weighs more and costs more than an extensive roof, which has less than four inches of soil. I know you really want a cost estimate but this depends so heavily on the type of roof, location and size of the building that it is better to just get a quote. It could be somewhere between $25 and $60 per square foot.

Brooklyn Grange Farm in Long Island City, Queens.

The first thing on your agenda should be to find out the load-bearing capabilities of your roof. If there was construction done on your roof or a structural analysis performed in the last five years, you may already have this information. If not, contact a licensed structural engineer. In New York City, you can check the Department of Buildings database to see what documents are on file for your building. This is one of those tools provided by the city, like the NYCityMap that I think everyone should know about because I use them all the time. Maybe it's just me. Anyway, your roof will need to safely carry upwards of 60-100 pounds per square foot.

Frieda Lim's sub-irrigated planters at Slippery Slope Farm in Gowanus, Brooklyn.

If you've got that, you'll need to find a green roof company to do the design and installation. This is the easiest part! Alive Structures is here for you.

We think the best system for green roof gardening is a combination intensive/extensive green roof, where most of the roof has a low soil depth and the gardening plot or plots have higher depths. If you can't install a green roof on your building for whatever reason, other options are planters, sub-irrigated or regular, or hydroponics. For information on how to build sub-irrigated planters (SIPs) yourself, see Inside Urban Green. Keep in mind planters can also weigh quite a bit, so a structural analysis is really necessary for safety whatever you do.

Gotham Greens' hydroponic greenhouses in Greenpoint, Brooklyn.

Some other potential issues to consider when growing vegetables on a roof are nutrients and wind. The soils that are used for green roofs are engineered to be lighter than regular soil. They have a high amount of shale and are not designed for their nutrient content. Unfortunately, this means your veggies will be less nutritious. Spinach can't be high in iron if there isn't any iron in the soil. You should be prepared to spread a lot of compost or compost tea around your plants, so start saving those coffee grinds. Last, rooftops are windy. Any plant that grows off the ground, like tomatos, will need extra staking and care and if your roof is extra windy due to the location, you may lose soil over time.