How to Start a Green Roof Garden

published May 21, 2008

Garden-lovers without backyards, go vertical!

Such is the cry of a Park Slope resident who recently showed a crowd of New Yorkers (including yours truly) how to start a green roof garden. Since starting a roof garden is a bit more complicated than planting on the ground, he took us step by step through the process, including how to set it up, what to grow, and how to maintain it.

A green roof generally means a carpeting of plants on a roof, which improves the energy performance of buildings, and reduces storm water runoff. Though many urban dwellers have flowerpots or container gardens on their roofs, these aren’t providing the energy savings that green roofs do.

So Jeff Hens set out to create a vegetable garden that would provide some of those benefits. While his garden far from carpets his roof, it does sit right against the roof membrane, helping to insulate his house, while providing fresh food for his family.

If you’d like to start a green roof garden of your own, here are some things that you might want to take into consideration:

Safety First and foremost, you want your garden to be safe. This isn’t just about protecting those on the roof from falling off. Since roofs are generally windy places, you’ll also want to protect passerbys from falling tools. So install fencing and securely tie down all equipment. Consult your local department of buildings for any necessary permits and regulations about fence height.

Weight Wet soil is heavy. So too is a foot of wet snow on top of that soil. So you’ll want to figure out the sustainable weight load of your roof (consult a structural engineer), as too much load in the wrong places can cause structural damage. In most cases, you’ll need to use a planting medium instead of soil.

For his wood roof, Jeff used a mix of 15% stalite permatil, 15% vermiculite, 25% compost, and 45% stalite, (available through gardening centers.) Since the nutrients have been consumed, the compost gets renewed at the start of every season, but otherwise he estimates he will not need to replace this. When it snows, he makes sure to push the snow off the beds, and hasn’t had a problem with weightload yet.

Containers You’ll need a container that is lightweight, but allows drainage. For maximum energy benefits, it should sit flat against the roof. Jeff uses a modular planting system designed for green roofs made by Green Grid. So that the plant roots don’t burrow into his roof membrane, he lines the recycled-plastic trays with a fabric weed block. He’s also seen roof gardens created in wading pools, feed sacks, and even recycled tires.

Watering In the hottest months, you’ll need to be watering daily. Jeff solved the problem by running a garden hose up the side of his building, and attaching it to drip lines and a timer system. This allows him to go away for several days at a time without worrying about his plants dying.

Plants Finally, what can you grow up there? Part of it depends on your available weight load. Jeff grows most of his vegetables in an 8″ deep tray, which he figures to weigh 24 lbs/sq foot when damp, and 28 lbs/sq foot when soaking wet. Even still, not everything does as well as it would on the ground. Here’s the list of what he’s gotten to work, and a few that failed:

Worked Worked with Some Caveats Didn’t Work
Bush Beans
Pole Beans
Snow Peas
Bok Choy
Cherry Tomatoes
Mustard Greens
(some wilt problem)

Ornamental Squash
(some wilt problem)

(stayed small)


He’s concluded that his planting medium is too gravely and perhaps not deep enough for root vegetables. But the breadth of his successful vegetables is impressive. We’d love to have that many things ripe for the picking on our roof, wouldn’t you?

Images: JHBklyn