pdf | print

Marcia Lee plans to take her green project to the next stage by diverting roof drainage into a rain barrel. Photo: Judith Capen.

I learned most of what I know about green roofs from our office’s successful green roof project at the District’s Reeves Building. I have data about weight, experience with the systems, and have kept an eye on an actual green roof for over a year. (You can see a picture of our green roof, although unattributed, at the Building Museum Green Communities exhibit.)*

I’ve been encouraging a neighbor to install a green roof on her carriage house. One recent evening I walked over to measure roof framing. I pulled out a calculator and the 1986 edition of Wood Structural Design Data published by the National Forest Products Association and totaled up a design load for the roof:

• dead load (the weight of the construction itself, figured at 10 pounds per square foot, psf) plus the weight of a modest modular green roof (13.5 psf: 2 pounds of plants;
• 10.4 pounds for 2 inches of saturated soil;
• 1.05 pounds for water retention and drainage mat, saturated;
  and 0.035 pounds of root barrier).

Then I added building code-mandated 25 psf of snow load to the 23.5 pounds of green roof and building construction. In case the numbers and complicated sentence structure have gotten to you, the total projected weight of snow on top of a green roof along with the weight of the roof itself totaled forty-eight and a half pounds. And that is a very modest green roof, only two inches deep.

Wood Structural Design Data gave me the capacity of the historic 2 x 8s averaging two feet on center spanning almost 19 feet. This where I must advise everyone, including myself, not to try this at home. I took my last class in structural engineering sometime just after the last ice age and remain puzzled as to why architects take so many of these classes. Maybe it’s part of a plot by liability insurance companies to tempt us to do things we shouldn’t. Anyway, I use simple arithmetic and Wood Structural Design Data for a rough idea of the ballpark we might be in before calling an engineer.

The table I consulted suggested that the existing framing was good for about 12 psf. That means WITHOUT a green roof, the roof structure could theoretically support about a third of the calculated load of construction and snow. I’m thinking my neighbor may not be terribly interested in a green roof on her carriage house with its historic roof framing unless she is willing to beef up the structure.

This brings me to the difference between reality and calculations. In January 1922 the roof of the Knickerbocker Theater, in “exclusive” Mt. Pleasant, collapsed under snow from the storm subsequently named for the theater collapse. The Washington area got 28+ inches of snow that day, yet my neighbor’s (theoretically) wildly inadequate 19th c. carriage house roof survived.

Even though structural engineering requirements are conservative, providing safety factors of two, any roof considered for greening should nonetheless both be checked for structural adequacy and strengthened, if necessary, to meet building code standards. The snow load number from the code isn’t exactly negotiable.

Is a Green Roof for You?
As I was calculating my neighbor’s roof loads, my editor emailed me about a Capitol Hill green roof, so I went to see Marcia Lee, retired Federal Government employee who spent much of her career abroad: Morocco, Paris, Italy, Japan, and India. She bought her first Capitol Hill house in 1987 and her current house in the two hundred block of Eighth Street NE in 1999. Marcia built her garage, now partially covered with a green roof, in 2005. This summer her contractor, who was installing a big green roof, offered her some surplus green modules. So, Ms. Lee who has long been interested in sustainability (she has linoleum floor tiles in her kitchen) said, “Sure!”

Bottom line for us -- homeowners, renters, with or without garages and carriage houses – is that plants are good for us, and for the world. And socialistic, in a good way. If you live at 1600 Pennsylvania Avenue or on a commodious estate, green roof suitable or not, take a page from Michelle Obama and Queen Elizabeth II, both of whom have planted vegetable gardens. (Lawn is pretty much the worst way environmentally, to have plants.) Hill residents, whose lots are smaller than the American average house size (a Capitol Hill lot of 18 x 80 feet is 1,440 square feet, smaller than 2004’s average American house size of 2,349 square feet), can still help in sustainability, personal quality of life, and private and public comfort by planting trees, vegetables and herbs ( in pots if not in yards), and maybe green roofs.

If you are thinking about a green roof, here’s what you need to do:

1. Have a professional calculate the adequacy of your roof structure.
2. Figure out, preferably with someone who has green roof experience, what green roof will work for you: intensive, modular, or extensive and be sure to include a root barrier and a water retention and drainage mat.
   
3. Assess your existing roof and factor its condition and age into the equation. If you want to go the whole way, brace yourself for the expense of a premium roof (usually one with a fully adhered membrane).
4. See if you can get the District subsidy to help pay for the green roof. http://ddoe.dc.gov/greenroofs The subsidy is $5 per square foot, up to a total of $20,000. Owners of residential or commercial buildings may apply for the subsidy for any green roof project of 4,000 square feet or less. The project must cover at least 50 percent of the building and funding is on a first-come, first-served basis.