The Evolution of Green Roofs at Swarthmore

Editor’s note: This article was initially published in The Daily Gazette, Swarthmore’s online, daily newspaper founded in Fall 1996. As of Fall 2018, the DG has merged with The Phoenix. See the about page to read more about the DG.

Swarthmore’s first green roof, built in the fall of 2002, was one of the first in the Philadelphia area. It and the other green roofs, on Alice Paul and David Kemp, are still recognized as distinctive: in 2011 the Green Roof and Wall Conference will be in Philadelphia, and Swarthmore’s roofs, known locally as the “best maintained”, will be featured on the program’s tour.

In 2002, The Swarthmore College Engineering Department had planned to build a storage shed easily accessible from its basement shop of Papazian hall and, as the roof of the building would be almost at ground level, the architect recommended the addition of a green roof. The shed, small, flat roofed, and un-heated, made for an easily implemented and successful green roof.

The conception of the two larger green roofs on the Alice Paul and David Kemp residence halls began later that year, before the dorms had even been built. Jeff Jabco, Director of Grounds and Horticulture Coordinator, taught a seminar on water issues in conjunction with the engineering department. Students in the seminar decided they wanted to concentrate on green roofs.

Photo by Se Eun Gong.

As Jabco explained, “Reducing storm water runoff is a major benefit of green roofs. The vegetation absorbs the rain water and releases it slowly. On a normal roof, water collects and flows into the drainage system all at once, overwhelming it. It also helps to mitigate the effects of acid rain and rain pollution by absorbing harmful chemicals that would normally be transferred into the runoff.”

Students monitored the effectiveness of the Papazian’s roof for water drainage and decided it would be beneficial to implement this new technology on the soon-to-be built dorms. Administrators, at Jabco’s invitation, attended presentations of student’s findings and suggestions. After further research by the college, it was decided to implement green roofs on the dorms.

Though there was excitement for the project, funding remained an issue, as Alice Paul’s construction had already been budgeted for. The building was to be built to LEED specifications, an international rating system for green building, but those standards did not include a green roof. The process to receive LEED certification is costly, but the decision was reached to re-allocate the money budgeted for the certification process into building a green roof.

Jabco said, “We decided it would be best to do something more than the specifications, rather than receive a certificate.”

The green roof on Alice Paul is 5100 square feet, much larger than that of the Papazian shed, and required a more complex planning process. Green roofs were still such a new idea in the United States in 2004, the time of construction, that consultants from Germany were hired to help plan.

Lars Rasmussen, a Swarthmore College gardener involved with the green roof project, says that the two main factors considered when building a green roof are making sure that the roof is sealed, so that no moisture can leak through, and that the roof can hold the extra weight of soil and plants.

To account for weight concerns, a special type of soil is used on the Swarthmore green roofs. A combination of heated shale with compost and clay provide a light soil base. The soil depth determines the types of plants that can be grown on the roofs. The depth of the soil on Alice Paul and David Kemp, built after Alice Paul and also having a large green roof, are fairly shallow: about 3.5 inches and two inches of dirt respectively.

Photo by Se Eun Gong.

Any plants selected must also be able to withstand heavy rains and periods of drought. Sedums are the plant of choice on Swarthmore’s green roofs. There are many different varieties; a few of the most utilized are album ‘murale’ and spurium ‘roseum’.

In its choice of sedums Swarthmore consulted the green roof expert Ed Snodgrass, and the roof of David Kemp is now featured on the cover of his new book. Aside from sedums, the roofs also have some raised beds with deeper soil to grow grasses, and Rasmussen is experimenting with small cactuses.

After laying down the dirt and selecting the appropriate plants, the planting was accomplished fairly quickly. For the lower roof on David Kemp plugs were used; the traditional trays of plants form a nursery.

For the upper David Kemp roof and all of Alice Paul, a process known as ‘cutting’ was used. Rasmussen described the process: “You take cuttings, little pieces or leaves of mature sedums, and in the fall throw them out onto the soil. You leave out and they shrivel up. It looks like they are dying, but actually all the plant’s energy is going toward putting down root systems. In the spring sedums begin to sprout.”

The maintenance for the green roofs is fairly minimal. In the beginning it is important to weed, but as the plants become established, that is not as important a consideration. Cutting back is practiced to curb the growth of over-active plants, and fertilizing is important as the soil is a synthetic combination of materials and not very nourishing. Water is needed less regularly, though: Rasmussen said, “I only have to water the roofs if it has been two weeks without rain.”

The green roofs function as a mechanism to reduce storm runoff, lessen rain pollution, save energy by limiting temperature fluctuations in buildings, and extend the roof life by protecting it from rain and sun damage. They were never intended to be a place for students to congregate, but for a lucky few in Alice Paul and David Kemp, they provide an aesthetically pleasing view.

Photo by Se Eun Gong.

Through the Swarthmore Arboretum tours of the roofs are also available once a month, and private tours are arranged for classes and garden clubs. There is also a series of videos on the arboretum’s website of the construction process.

Jabco is pleased with the expanded educational program surrounding the existing green roofs, but hopes to do more. “The pump house on Cunningham Field needs a new roof, and I think it would make a great Engineering project for students to re-fit it with a green roof. It would be an accessible example of a sloped green roof.”

Another plan for the future is a green roof on the Lang Performing Arts Center. The building has a large, flat roof which will need to be replaced in the next several years. Jabco has created a proposal for a green roof on the Lang building, but was denied a state grant. The college would have to fund the project, and with current concerns over the budget it might be some time before the project is realized.

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