I’m waiting to hear about the permits to start the green roof, so I have been reading some more about green roofs in Washington. My 150-square-foot roof, which feels big to me, will be added to the hundreds of thousands of square feet of green roofs built each year in the District. Last year, the Washington area led the nation, installing 800,000 square feet of green roofs (doubling the amount in 2010), and for the first time surpassed Chicago, which is known for its green roof initiatives.
The District now has more than 1.2 million square feet of green roofs, according to the Green Roofs for Healthy Cities survey. Andrew Benenati, a project manager for the nonprofit helping build my green roof, DC Greenworks, believes 2012 probably will surpass 2011.
Commercial roofs form the bulk of green roofs because they generate the most return in terms of insulation and offsetting District stormwater and impervious surface area fees. Residential green roofs are increasing due to their ability to insulate against weather and noise, as well as for aesthetic benefits. My green roof is just one of many in the city, albeit one of the smallest.
The District gives permits for three types of roofs. Like many green roofs, mine is the simple “extensive” design, which is usually approved quickly, according to DC Greenworks. The roof has a shallow bed of growing medium (three to four inches deep), consisting mostly of expanded shale and slate and only 30 percent organic material. A higher percentage would make the roof heavier and potentially lead to rot.
I’ve been warned to keep dead leaves off the roof in order to keep the percentage of organic material low. The plants typically used are shallow-rooted, drought-tolerant species that can survive a low-organic environment.
The two other green roof types permitted in the District, depending on the design, are “semi-intensive” and “intensive” systems, which contain deeper (and more organic) growing mediums that support plants with deeper roots. In the intensive system, vegetables, bushes and even trees can be grown. Obviously these types are too heavy and not suitable for a small front porch roof like mine.
In all three types, the green roof components (the plants, growing medium, drainage system, irrigation, root barrier and insulation), can either be laid down in layers across the roof, or they can come pre-installed in small modular trays that hook together.
The benefit of the former is that you can easily include or exclude different components, such as irrigation, to customize to a user’s need. A modular system, on the other hand, allows one to easily take up different sections to look for leaks or to replace dead plants (EPA Report). Also, modules usually come pre-planted, and thus the new roof has complete plant coverage sooner. A modular system is heavier and more expensive, which ruled it out for me.
A final note on what the District allows: I’m not in a historical district, but even in those districts, green roof permits are not uncommon, according to Andrew. The main requirements are that the plants are low in height and that they are set back from the roof edge so they are less visible from the street.
Resources on green roof design and function can be found on the District Department of the Environment, DC Greenworks and Green Roofs for Healthy Cities.
Now back to waiting on the permit.
Annette L. Olson is a Petworth homeowner who will share her experience of installing a green roof on her row house.