Are All Green Roofs the Same?
An important difference between conventional roofs and green roofs, aside from the functional layers, is the location of the waterproofing membrane. Green roofs are “inverted” or “upside down,” meaning that the waterproofing layer (and insulation layer) is located beneath most of the roofing components rather than on top. This position is important, not only because it allows the plants to absorb and help control water flow across the roof, keeping them alive, but the membrane is buried under several inches of growing medium and drainage material. These layers prevent penetration of UV rays to the membrane, which degrades rapidly when exposed to the sun. Since breakdown of the waterproofing membrane is one of the major causes of roof leaks, this enhanced protection allows green roofs to last 2 to 3 times longer than a conventional roof.
What’s Different About Extensive Green Roofs
Extensive green roofs are popular when your focus is primarily on environmental benefits and energy savings. Extensive setups can be quite attractive, but they’ll likely never be as dramatic as an intensive rooftop garden or mini arboretum. They are shallower and lighter than their showier cousins and are designed to be rugged and require much less maintenance. This means that it’s less likely that additional structural support will be necessary, an important consideration if you’re transitioning from a conventional roof.
Extensive roofs may also be known as “thin profile” and “performance” green roofs. Since they’re thinner and generally simpler systems, they’re faster and less expensive to set up. Extensive roofs are also well suited to use of modular trays or other systems that promote quick planting establishment.
The planting medium in an extensive setup may range from as little as 2” to no more than 6”. This shallow profile helps discourage weeds from establishing and works well with shallow-rooted and drought-resistant plants like sedums, small grasses and other small, low-growing types. Generally, plants that are adapted to extreme climates may be good choices since they may be able to tolerate the environment without an integrated irrigation system. For extensive roofs, the focus should be on hardy perennials that self-generate, spread rapidly, and have minimal nutrient requirements.
Another interesting difference in an extensive green roof is their adaptability to use on sloped roofs up to 30°. In this kind of installation, there would need to be additional support to keep the growing medium or modular trays in place, as well as some securing of underlying layers. Keep in mind that rain will flow off a sloped roof more quickly than a flat one, so it may not have an opportunity to retain as much water.
What’s Different About Intensive Green Roofs
Intensive green roofs are used for a rooftop park or recreation area, a rooftop cafe, or a beautiful, peaceful meadow-like expanse with a path and a few benches. If you want a variety of carefully tended blooming plants, shrubbery, or even trees, then an intensive system is right for you. Intensive roofs are almost limitless when it comes to plant choice, and they’re most comparable to a conventional garden, park, or arboretum.
Building owners may install an intensive green roof with the intention of saving energy, but they’ll almost always have a secondary goal of offering an attractive view or space for tenants or the general public to access and enjoy. This kind of use naturally requires substantial maintenance, including weeding, planting, and irrigation, just as it would for any typical garden.
Intensive roof systems are heavier and require more structural support than their lighter alternatives, and the additional weight of foot traffic should be considered as well. Irrigation is almost always necessary, since the plant choice will be more varied and is not likely to be devoted entirely to drought-resistant plants. Remember, too, that keeping the rooftop environment comfortable for your guests requires moisture - for evaporation and transpiration - so xeriscaping with wide expanses of gravel will pretty much render your efforts useless. If your climate permits, investigate the possibility of capturing rainwater from the roof or elsewhere to re-use on your green roof. If you’re concerned about being able to maintain your green roof during a drought period, talk to your local and state governments. They may have planned water use exceptions in place for this kind of installation because of its positive effect on heat islands, reduced energy consumption, air pollution, and water quality.
What’s Different about Rooftop Farms?
Urban gardens and rooftop farms can be considered a subset of intensive green roofs. Their structural and maintenance requirements will be essentially the same, and they’ll require supplemental irrigation, but their mission focuses on growing food instead of strictly aesthetic appeal.
Rooftop farms are part of the urban agriculture movement, which underpins a push for sustainable communities, food security, and improved nutrition for city dwellers. Rooftop farms may use an intensive green roof setup by planting directly into the growing medium, but they may also use planter boxes or raised beds to make access easier for caretakers with limited mobility. This wouldn’t change much in the layered setup of an intensive green roof - excess water would still need to drain, the roof deck would still need to be protected, and the growing medium would still need to be lightweight.
Many urban gardens and similar activities are run by volunteer organizations or co-ops. Some businesses or building owners may opt to allow an organization to operate a rooftop garden as part of their effort to engage and support the community.
Our next article will focus on the structural layers of a green roof.
