Building a Future

 Rolf Cachat-Schilling, Horticulturist/Plant Records Coordinator

(Installment 2 of 3)

With all the benefits green roofs bring (see “Green Roofs, Safe Water, and Native Plants” LINK!)), it’s hard to understand why they weren’t more popular sooner. Actually, they were. Green roofs have a long history. Many plants are even named for their use almost exclusively as green roof plants, such as house leeks and Japanese roof iris. The American Orchid Society (A.O.S) began as a green roof club. American women on the Great Plains in the 1860s, mostly German, conducted contests called ”Das Schoenste Orchideenhaus,” in which women competed with displays of native buffalo orchids (now extinct due to the demise of bison wallows, which was their only habitat), various fringed orchids, and other choice native plants. From this arose regional contests, and eventually the A.O.S.

The recent decline of green roofs is actually a gap in the historical continuum of green roofing. Deep traditions of sod roofs, much elaborated upon with local gems, are common in the British Isles, Scandinavia, Japan, Northern China and Mongolia, the Middle East, and Native North America. Though tepees and wigwams are the common image of First Nations housing, sod roof, semi-subterranean houses predominated through the northern half of the continent, adobe predominated in the southern half.

Bringing the green roof into the modern era bears some explanation. There are two basic types of green roofs: The main type of green roof is the do-it-yourself retrofit to an existing conventional roof. The other kind of green roof  is added to existing roofs, and those designed into the building plan at the start. High profile green roof projects tend to involve municipal spaces on a grand scale and usually use high-tech materials, which provide lightweight composite frames and aggregate planting media. These sorts of materials are long-lasting and function well, but are also expensive and often use high-energy input materials (reducing the sustainability of the project).

Tips for the Do-It-Yourselfer

When building any green roof, some points must be considered. There is no use in placing an expensive, high-tech roof on a temporary structure of low value. Similarly, it makes no sense to place an ad-hoc roof on the main pitch of your valuable home. If the structure has value, consult a professional roofer before you begin. After that, there are several elements that should be included in any roof built atop an existing roof:

• Waterproof barrier must be placed between green roof drainage and the existing roof.
• An air gap between the two roofs is needed to retard “sweating”and excessive, constant moisture.
• The total weight of the green roof (wet and planted) plus potential snow load must not exceed the load per square foot the existing roof can bear. Most municipalities have codes covering minimum acceptable weight loads per square foot. This will give you a baseline.
• Wider spans and weaker structures mean thinner soil, and less plant material for your green roof.
• The green roof must have sufficient drainage away from the underlying roof.
• Selected plants must endure extremes of heat, sun, wind, and cold. Alpine and coastal plants are great candidates for green roofs. Also, shallow-rooted plants work best.

Building Our Roof Begins

The green roof project at Garden in the Woods started with a look at our ”sideyard”—a junk space where we store used construction materials, hoping to use it the future. We thought: “Could we use some of this stuff in the construction of our roof garden?” “Yes,” was the resounding response.

Disposal of these items is a cost, both in terms of money and in terms of habitat loss for landfill space. By reusing the materials already manufactured and shipped, we not only reduce dumping, but also save energy by not initiating the production and shipping of more materials. Already the ecological benefits are huge. There were piles of: 2”x 4” ’s, 2”x 10” ’s, old nursery benches with galvanized grating for drainage, corrugated steel sheets, and weed barrier fabric.

My initial design seemed simple enough: Add a green roof to an existing peaked roof garden shed. This roof would be used to demonstrate new native plant material for green roofs, gather data on their performance, and exhibit green roofs as a do-it-yourself project.

The roof area was approximately 6’ x 12’ per side and the nursery benches were about 3’ wide each. We would cut the benches down to 6’, including their grated bottoms, and use these as plant trays, four across each pitch of our roof. After we placed these on the roof, we would slap a wooden frame around it to hold the trays together. On the sides, I planned to use “L” brackets to keep the whole roof from sliding. It seemed logical as well that the weight of the trays and the contact area would keep it in place. All this would sit on the corrugated sheets to provide an air gap between the trays and the original tab shingle roof, as well as acting as water barrier and drainage channels. I thought myself clever for finding simple means.

Well, things seldom work out exactly as envisioned. The benches were varied in width. Planting on the ground and hauling trays up to the roof sounded difficult without a cherry-picker. The had no spreaders or collar ties to keep the walls from bowing and far too few wall studs. We began  by installing spreaders and studs so that the shed resembled something like a code-worthy structure.

Everyone in the Horticulture Department pitched in on labor and enthusiasm. By good fortune, one of our Native Plant Horticulture Interns, Anna Breeze Mulligan, arrived equipped with great carpentry skills. Hort Volunteer Dick Anderson contributed both carpentry skills and a gift for design. Hort Volunteers Peter Brehm and Jim Schilling-Cachat, plus Native Plant Horticulture Intern Andrew Palinski, Director of the Botanic Garden Scott LaFleur and Horticulturist/Facilities Coordinator Kristin DeSouza all brought ideas and plenty of elbow grease to the task.

The Work Continues

To compensate for some extra inches in width, we removed the side walls of some of our trays. Fastening the trays, the frame, and our corrugated water barrier/air vent became something of a bailiwick. Dick Anderson suggested a 2’ x 10’ wrap-around frame, attached to the gable ends and leaving the tray grates long so they could be bent over the peak of the roof. This way, the two sides anchor one another. We quickly realized that the weight of the trays would hold our corrugated barrier in place. Our frame was designed to leave an eave, where the grating extends beyond our barrier, allowing excess rainwater to drip away from the roof. The eave created something of a weight concern against the frame. Dick Anderson came up with a clever cantilevered support by cutting holes in the side fascia boards, through which support boards ran from the base of the eave into the shed, and along the inside roofline. Once fastened, this pushed the outward and downward weight back up against the weight of the roof itself.

After much hard work, a bunch of laughs, and some tough moments, we turned around to discover that we were almost finished. Dick Anderson commented, “This project was meant to be. We didn’t need to buy any materials and the shed had everything we needed to create a sturdy and very attractive green roof.”  

The Finished Product

I selected planted native plants of the Northeastern coast and upland ridges for the north roof face and native plants of the Southeastern coast and upland ridges for the south face. These plant were chosen for their tolerance to wind, snow, sun, and thin soils. Once planting was complete, it was satisfying to see how lovely it looks and how much interest and delight it holds for our visitors. Our first green roof is thriving and blooming profusely. Over time, it will yield a wealth of enjoyment and research on new native plants for green roofs.

Stay tuned for the final installment in our three-part roof garden series, a focus on native plant selections for green roofs.