We built the green roof! 

I helped some with the installation, as per my budget deal with DC Greenworks, the nonprofit helping build my roof.  But my front porch roof is too small to hold more than two people working at a time.  The experienced people needed the space to coordinate the layering of the protective materials, use circular saws to cut edging, etc.  

So I hovered by the window taking pictures and asking questions.  After the preparatory work was done, I was able to help spread the growing medium (a low-organic soil and rock mix), and then helped plant the small sprigs of plants that will grow to cover the roof.

View Photo Gallery: A Petworth homeowner puts in a vegetated roof on her rowhouse.

Some questions and problems arose during the process, however.

First was an issue in installing the 4 1 / 2-inch-high metal edging.  The edging is screwed together on all four sides to form a box, which then holds in the growing medium, similar to a raised garden bed.  The edging itself is L-shaped, so 30 percent of it is held down by the weight of the materials and the growing medium. Normally on a flat roof, this combination holds the box in place without needing to screw the edging to the roof (which could cause potential leaks). 

The edge of my roof apparently dips in one place, however, so when the edging was first placed down, it kept sliding too close to the gutter.  To correct that issue, DC Greenworks’ Erik Vollmerhausen made the box smaller and stouter — keeping it from the dip in the roof edge by two inches, and, once filled with growing medium, it stopped sliding.  But I am to keep an eye on it; if need be, DC Greenworks will come back and install braces.

I also learned two unexpected things.  First, I had the option of adding mulch.  I had been told that the percentage of organic material is supposed to be kept low in a green roof in order to keep the roof light and free from rot.  But Andrew Benenati, the project manager, explained that an initial mulching helps suppress weeds while the plants get established.

Also, the irrigation turned out not to be soaker hoses, like I expected, but a sprinkler system, which Andrew said provides better coverage of the entire roof than a soaker hose. A battery-based timer will ensure that the tiny new plants can be watered even if I am away, which is critical during this hot summer.  In the future, irrigation is meant to be used only during long droughts. 

Finally, before the rock and soil growing medium was added, I remember thinking how soft and comfortable my roof looked with the plastic drainage sheets and three layers of fabric (protection, root barrier and filter). I later wondered, though, how long these soft-looking fabrics could last, especially the filter fabric against the edging where it is somewhat exposed to the elements. 

I had a hard time believing these fabrics could last 30 years or more, the length of time DC Greenworks and reports suggest green roofs can last (they provide a one-year warranty on the green roof system.) 

However, online research revealed that filter fabric is a “geotextile,” i.e., usually made out of polyester or polypropylene, and thus is non-biodegradable.  Articles in various journals show that geotextiles have lasted in the field for a minimum of five to 10 years when exposed, and 35 to 40 years when buried, with the potential to last more than 100 years.  One factor that may affect buried fabric, however, is soil acidity, which I’ll be monitoring as part of my maintenance agreement. 

So all is good; DC Greenworks put in a lot of work for a small roof.  I’m still nervous about this new type of roof, especially considering the cost.  (Overall, I spent about $4,000 for the green roof part, including the structural review, initial consultation and build. Then I received a $750 District rebate.) Maintenance will be important, and I’ll describe the needed steps, as well as neighbor reactions, in the next, and final, post.

Overall, however, the installation was an interesting experience, and I now have a green roof that should provide a beautiful sight outside my bedroom window for years, and that will help keep stormwater from District streets.  I have already noticed a side benefit — instead of the clatter of rain on an asphalt roof outside my window, I hear pleasant silence, as water hits the mulch and plants.

Read more about Annette L. Olson’s green roof installation

Annette L. Olson is a Petworth homeowner who will share her experience of installing a green roof on her row house.