Monday, November 7, 2011

Eighth Floor: Green Roof

I had a chance to look over the 45,000-sq-foot (4,050-sq-meter) green roof that PECO Energy (the local utility) installed on its headquarters building in central Philadelphia last week.  Frankly, I thought that green roofs were mostly "tree hugger" hype, but this visit made me a believer.  The roof receives 1.5 million gallons (5.7 million liters) of precipitation during the year, only sheds 0.5 million gallons (1.9 million liters), and it sheds those half-million gallons over a protracted period that doesn't tax the city's stormwater system with excess runoff following heavy rainfall events.
The Cira Center, Philadelphia's most unusual skyscraper, a half-mile away
The roof has many other advantages.  It will last 2-3 times longer than a conventional roof, it is far cooler than a conventional flat tar roof, it absorbs air pollutants, and it actually provides some habitat for birds and insects; we watched honeybees gathering pollen.  The water management angle really captured my interest, though.
 Philadelphia Museum of Art across the green roof
The major part of the roof is planted with 12 species of orpines or stonecrops (Sedum spp.) (alas, no native species).  At this time of year, they are coloring-up beautifully.  The roof is three years old, and the designer knows and expects that some of the stonecrops will be better competitors than others, so the species diversity will decline over time, but right now they make an especially appealing mosaic.
The company also included deeper planting boxes along the edge of the roof where the building's structure could support heavier soil.  The boxes were planted with a mixture of attractive native perennials and native grasses.  Little bluestem grass (Schizachyrium scoparium) has "escaped" the confines of its boxes, and spread its seed into the main part of the roof.  It will be interesting to see if it persists over time in the shallow, droughty, low-density "soil" in which the Sedums are growing.


The Musical Gardener said...

I've seen writeups on these particular ventures, but it was good to hear your first hand account. I assume stonecrops would be very drought resistant, and probably pretty vigourous rooted as well. The building must be quite impressively designed to withstand the incredible weight that could come in a torrential downpour.

Scott said...

Gardener: The building needed a new roof anyway, so PECO Energy reinforced the existing structure to accommodate the added weight of the green roof and the water it would retain. I didn't get the impression from our tour guide that the cost of the roof reinforcement was cost-prohibitive; in fact, the company fully expects to recoup its investment many times over in the savings it will realize from not having to replace the roof so often (estimated life: 50 years), and the stormwater management taxes imposed by the city that the company will be able to avoid.

Irwin Floto said...

Wow! 45,000 square feet of greenery makes for a very huge roof garden. For city dwellers, seeing that much vegetation in one place is a welcome treat. This only goes to show that adding little patches of green in the urban landscape is possible, that is, with a little imagination.

Scott said...

Irwin: Alas, the only people who can enjoy this expanse of green are the workers on floor 8 and upward who can see the roof from their windows. The general public wouldn't have a clue from the ground. Fortunately, Philadelphia has a lot of ground-level public greenery to enjoy.

Corbin Linder said...

Seeing such beauty in a busy city is a refreshing sight. A green roof is definitely among the most wonderful innovations in the modern world. Also, this form of advocacy to reverse global warming has enabled contractors and building owners to join hand-in-hand in showing support to the environment.

Corbin Linder

Sammy Finch said...

Thanks for sharing this! I have seen a few green roof in Vancouver and I think they are a great idea!