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Rooftop Gardening: Onward and Upward

By March 30, 2010November 22nd, 20116 Comments
by Val Khislavsky

Ever wonder how to create a green roof in the city? I was curious too, so I checked out a green roof workshop put on by Green Village Philadelphia. Here’s what I found out:

L: John Tull and his daughters on the roof; R: the rooftop garden and the city view beyond.
Photos: Diane Gentry.

Rooftop gardening has taken off all around the world. Take for example the recent BBC news story about the spread of Palestinian rooftop gardens in densely-populated city areas where arable land is hard to come by (click here to listen; story starts at 43:16). One of the residents and a new green roof owner was interviewed for the story and summed it up perfectly: “I haven’t land near my home,” she said, “but now I have land over my home.”

Although a world away, the story brought to mind a trip I took last fall to visit an intensive Philadelphia rooftop garden and to learn about its construction. (Intensive green roofs accommodate plants that require deeper soil than extensive green roofs, which are composed of low-lying plants, typically succulents like sedums, that can thrive in shallow soil.) Together with a good college friend, I attended a Green Roof Workshop put on by Green Village Philadelphia, a nonprofit organization committed to urban revitalization through sustainability.

L: Some of the rooftop tomato and pepper harvest; R: Echinacea adds summer color. Photos: Diane Gentry.

Hosting the workshop at her home was one of the founders of Green Village Philadelphia, Diane Gentry, who together with her husband John Tull installed an intensive Green Roof on their 100+ year old row house when they remodeled it. Participants arrived at Diane and John’s house, climbed up an unassuming staircase, and were transported to another world—a private rooftop sanctuary bursting with vegetables and brightly-colored annuals. There was even an enviable 7-ft tall papyrus in one corner! Immediately, we were all interested in learning more about how we could achieve this ourselves!

Constructing the green roof: L: Layer of conductive tin; R: Waterproof liner. Photos: Diane Gentry.

Turns out creating an intensive rooftop garden is, although a lot of hard work, attainable for homeowners. And there are plenty of reasons to transition to a green roof, especially in urban areas. Take for example the ability to grow vegetables and flowers right atop your home, the chance to create a green and accessible play space for children, and the capacity of green roofs to reduce winter heating and summer cooling costs. A green roof also increases a roof’s life span by protecting it from the sun exposure that breaks down traditional roofing materials. Instead, rooftop plants can take advantage of full sun to grow, filter pollutants and carbon dioxide out of the air, and can be watered by storm water while helping filter pollutants out of the water and reducing excess runoff.

L: Rolling out the geotextile fabric; M: Diane gets ready to pour out the expanded shale; R: The shale released. Photos: Diane Gentry and John Tull.

In order to make their green roof possible, Diane and John did all of the installation work themselves, although they did hire a structural engineer and a green roof consultant to advise on weight restrictions and materials. They began by removing old roofing materials and fortifying the roof structure with new, stronger beams. Once they rebuilt the bones of the roof, they began to set it up to accommodate plantings. A layer of tin, which conducts electricity and can be used to detect any water leakage, was laid down, followed by a waterproof pvc membrane, much like a swimming pool liner. Subsequent layers included two rounds of non-woven geotextile fabric topped with lightweight expanded shale (a manufacturing waste product) layer for drainage. Finally, lightweight compost (mushroom, in Diane and John’s case) was added to the top (planting) layer of shale and the lowest corner of the roof was designated for a drainage pipe and downspout. At the end of the installation process, Diane and John had actually taken away more weight by removing their old bitumen roof than they had put on with the addition of the rooftop garden.

Diane and John’s children enjoying their rooftop garden and its spoils. Photos: Diane Gentry.

Diane and John have encountered some challenges, including difficulty staking some plants against wind damage (due to limited soil depth) and the need to water often in hot summer weather, but overall their intensive green roof has been a great success and has provided their family with much-prized city green space and a great variety of produce and flowers to enjoy. Diane and John are enthusiastic proponents of green roofs and are eager to share their knowledge. Another Green Village Philadelphia Green Roof Workshop is slated for summer and fall of this year—check the GVP website for dates or subscribe to the newsletter to be kept in the loop. The workshop includes a visit to Diane and John’s rooftop garden, a course packet filled with instructions, photos, and resources, and lots of opportunities for Q&A and tomato tasting!

My college friend is still living in Philadelphia and I plan to head down to the City of Brotherly Love again this summer to visit. I hope to stop by Diane and John’s garden again for another dose of rooftop inspiration!

Rooftop Resources: resource page

Join the discussion 6 Comments

  • Bill Sumner says:

    Reminds of my days in Brooklyn although mine was no where this elaborate. Still it provided some nice tomatos and a few flowers.

  • Jana says:

    I am a little surprised they have less weight on their roof after the installation of their garden. How can that be? These facts need to get out there to get more people interested, no?
    Really, though, I am not surprised. I mean, gardens improve so many aspects of our lives!!

  • Diane Gentry says:

    Great story Val! In answer to the weight removed question, when we took off our old bitumen/tar roof it had about 7 layers (I think 2 or 3 is the legal limit) – we removed over 5 inches of old roofing material – two overloaded construction dumpsters!

    I can’t remember exactly the calculations (though I do have those dumpster receipts filed somewhere if someone really wanted to know the exact weight of the old roof), but we roughly put back about 1/2 (not including plants) of what we had removed. In addition, we had added structural beams inside our house to reduce the span of the roof by half. In retrospect it was no wonder our old roof was caving in!

    We love the green roof – I really recommend it if you don’t have yard space.

  • Val says:


    Thank you for the inspiration, and the beautiful photos! Let us know when you have the dates for the next GVP Green Roof Workshop!

  • Ibot says:

    What about Fungus? It seems you would grow fungus with all that water.. Great article!!

  • Val says:

    Proper drainage, use of a soil-less growing medium, and ample amounts of sunlight should be enough to discourage fungal growth, but I suppose you never know!