18 Jun Green Roof Insallation at the Wister Center-Part 2
Next the crane arm went up and down many times delivering pallets loaded with buckets of coarse (2-3”) crushed quarry stone (Read Part 1 here.) This larger stone is used all around the edges of the roof and around any features that protrude above the flat surface of the roof.
Here is where our hardy installation crew got a workout hauling heavy buckets of rock to all corners of the roof. Swarthmore College gardener Bill Costello made many trips up and down the roof’s access ladder helping to load the pallets in the parking lot down below and then unload them on the roof.
This larger stone is used in places where the plants have a harder time growing, for example under the edges of the pitched roof where the water drains. It is also used along the edges of the roof to prevent material from being blown off in this windier zone.
The majority of the roof is covered with our final layer of material, the growing media. The essential attributes of green roof growing media are: it is lightweight and drains well. It also must contain some compost to provide nutrients to the plants. On the Wister Center roof, as on our previous green roofs, we used a growing media called rooflite that is blended in nearby Chester County and delivered to us in large sacks weighing hundreds of pounds. Two of these sacks were lifted by crane and suspended over the roof so that they could be slit open and pushed around to pour the material roughly where we wanted it. The media was then raked around the roof to an even 2-3” depth.
Two weeks later, when work schedules allowed and rain was in the forecast, the stars of the show, the plants, were put to work. We were able to dig several dozen clumps of various species of sedum from our David Kemp green roof as well as harvest a variety of sedum cuttings. This plant material was immediately carried up the ladder to the Wister Center roof and planted or, in the case of the cuttings, evenly strewn over the growing media and watered in.
Now it is up to the plants to do their job of establishing themselves and filling in the bare spaces to create the colorful mosaic of a vegetated roof. We continue to be awed and inspired, as well as educated, by these green roof installations on campus. In addition to the significant ecological services they provide, green roofs are ever-changing kaleidoscopes of life.