By Chris Erickson
Why lawns and green roofs don’t go together.
New regulations (Local Laws 92 and 94) are going to see green roofs growing all over New York City. While the law is requiring them solely for their eco-friendly aspects, this is also an opportunity to create usable, vegetated rooftop amenity spaces for residents. After all, the thought is too much to resist: a yard in New York City! However, there is a classic feature of yards that does not go with green roofs: lawns. If you are tempted by the idea of creating a lush lawn on your rooftop we suggest you reconsider. We will discuss both some reasons why lawns are not good candidates for green roofs as well as some alternate design options.
The argument against lawns on roofs starts with green roof soils. They are sandy, even rocky, and have very low organic matter. Green roof soils are engineered this way to ensure they drain well, since holding too much water on a roof would cause weight issues. Some plants happily thrive in green roof soils but it is a terrible medium for turf grass, which prefers soil rich in organic matter and high in nutrients. Turf grass also wants to be constantly moist. If it is in soil that drains too well it will dry out and once that happens die-back follows.
Getting seed to germinate and root directly into this type of rocky soil medium is a true uphill climb. Instead sod is the go-to installation method. A layer of green roof soil is put down as the base and sod is rolled out over top. What this creates is grass growing in a very thin layer of hospitable soil (the sod mat) which is floating over very inhospitable soil (the engineered green roof soil). Despite the limited amount of available soil in the sod mat it is a real challenge to get turf grass to root into the green roof soil below. This limits the root mass which in turn limits the grass’s ability to weather dry spells and store nutrients. Good strong roots are essential to a durable lawn. To survive and look good the grass now must be watered and fertilized very regularly, babied even more than regular turf grass.
The other outcome of poor subsoil root penetration is that foot traffic, which is hard on turf in the best conditions, becomes even more damaging. One way to envision sod on top of green roof soil is like a rug on a rug mat. Too much movement on top and the rug will shift and bunch up. On turf grass this movement disrupts and damages roots. This can happen after too much foot traffic, but it could also result from anything high impact, such as running dogs, playing children, or furniture legs.
If you can keep pets off, minimize the impact from foot traffic, and maintain very regular irrigation and maintenance, you can grow a lawn on a rooftop. But even a healthy lawn will fail to fully capture the sustainability upside of green roofs. Their reliance on heavy irrigation reduces their utility as a sponge for absorbing rain water. As monocultures that do not flower and have almost no structure they also fail to provide any ecological benefit for local pollinators and wildlife. Plus, they lack the texture and color that give green roofs such an opportunity to visually soften and diversify the hard lines of the city.
Options Beyond the Lawn
A cool, soft green lawn is a wonderful thing to sit down on and it can be a disappointment to trade one for concrete or wooden pavers. But there are other options for softening space and creating inviting seating areas. At the residential 2nd-floor courtyard in Queens pictured below a lush green border separates this seating area from the surrounding patios and the dense foliage helps to cool the patio area.
Similarly, in the following example, an outdoor roof amenity space for patients and staff at a Manhattan hospital, the raised green roof bed frames an inviting bench that is softened by the overhanging foliage and brightened by the many flowering perennials.
When it comes to covering large spaces, traditional sedum green roofs offer the most sustainable upside, and while their low profile makes them more subtle they are truly beautiful. If an open roof is what is desired sedum roofs will help reduce roof temperature and provide a beautiful surrounding for your seating and amenity space, while still maintaining the wide-open feel of a roof.
Lastly, if your roof has sufficient structural capacity you can increase soil depth, add some supplemental irrigation, and grow a true meadow. At this rooftop on a Manhattan Community Center, installed by Highview Creations just this past year, a diverse and colorful plant community creates a feast for the senses and divides the space into two separate seating areas. In just it’s first growing season we documented numerous species of bees and butterflies, including monarchs.
The fact that lawns are not good candidates for rooftops need not be as limiting as it sounds. Options abound for turning rooftops into inviting oases and green infrastructure machines at the same time. There are endless plant communities that will thrive on roofs and figuring out how to tailor designs to specific needs and site conditions are a major part of the joy of bringing roofs to life.