Once a roof’s loading capabilities are known, the specifics of the extensive system—which is composed of several layers of materials, each performing a different function—can be determined. The first of these is waterproofing, which is typically either an asphalt derivative, or made from nonasphalt material such as polyvinyl chloride (PVC) or thermoplastic polyolefin (TPO). It is important to understand which material is used because while plants can’t consume PVC and TPO, asphalt-derivative products can become food for plants and bacteria. If asphalt is used, then an engineered-fabric root barrier must be added to prevent the roof’s living organisms from feeding off of (and in so doing, degrading) the roof’s waterproofing.
Leak-detection systems are often incorporated into green roofs so that if a leak does crop up, it can be quickly located and fixed. The most common form of leak detection is electric field vector mapping, which uses a low-voltage current to create an electrical potential difference between the nonconductive waterproofing membrane and a conductive substrate. Water atop the membrane serves as a conductive medium. If there is a leak, the water passes through the medium and onto the substrate, creating a ground-fault connection, or vector. Technicians can detect these breaches with pinpoint accuracy.
The next layer installed after waterproofing is drainage. While part of a green roof’s primary function is the retention of stormwater, or at least the slowing of runoff, it must still fundamentally act like a roof, and that means shedding water. There are two types of systems used for this purpose: One, a drain mat, involves plastic or fiber channels that direct water filtering through the soil horizontally to drains in the roof. The other, granular drainage, is made from single-sieve aggregate, basically little stones that are all the same size (generally 1/4-inch in diameter). Each system has its benefits. Granular drainage has better horticultural performance because the plants’ roots can get down into it, however it does not move water off the roof as quickly as a drain mat because the grains form a kind of obstacle course; the mat provides direct paths for water to follow to the drains.
Above the drainage layer goes a separation fabric, a nonwoven geo-tech material that lets water pass through, but not soil or growing media. This layer does allow roots to pass through, but does not itself break up or degrade in the process. Its primary function is to keep the growing media from clogging the drainage.
Next comes the engineered growing medium, which is mostly stone and sand aggregate with very little organic matter. In fact, when installed, the surface looks more like a driveway than a garden. There are two main reasons for using this type of material. One, it does not compress like soil, and therefore allows water to drain through. Two, it doesn’t dry up, blow away, or decompose as organic soil would. Since the return on investment for a green roof is 20 or 30 years, it pays to use a material that will last. Sometimes irrigation systems are also integrated into the growing medium, either to get a roof through periods of drought or to help it along in its first year of growth.
The final, and most visible, layer of a green roof is the plants themselves. Many factors must be considered when choosing what to grow, including the local climate and the roof’s exposure to sun and wind. Those factors will determine what can actually live there. Then there are the client’s goals to consider. If they want to establish an ecology, plants must be chosen that have pollen for insects and birds and good structure for spiders. If they care about stormwater retention, then plants must be chosen that behave well for that purpose. If they care about the way it looks, then there are ornamental considerations. And then, perhaps most importantly, there’s the question of maintenance. A roof can be planted in a way that will require only a once-per-year checkup to make sure that the drains are clear and the flashing and penetrations are in good condition, but if clients care about maintaining a more complex ecology or a rigorous aesthetic, then they must be prepared to hire someone to weed, mow, and generally ensure the sculpted quality of their roof garden.
There are also different ways of delivering plants to the roof, each with its own level of installation cost. The cheapest is to start with seeds or sedum cuttings, with which you can cover a large area very quickly, though it is difficult to organize them in any precise pattern. The next option is to begin with small starter plants—1-inch-diameter-by-3-inch-deep plugs that include the plant with a bit of propagation medium. Those can be arranged in whatever pattern you wish, but must be planted one at a time, incurring a higher installation cost. The most expensive option is to start with mats and modules (or trays), which have been grown for a year in a nursery. The preinstallation growing time costs money, but the mats and modules are also more burdensome, and thus more costly, to transport and lift into place. On the other hand, putting in fully grown plants has the benefit of obviating the risk of wind scarification, which can plague freshly seeded soil. This method will also deliver an immediate vegetated roof, an important factor if the project is in the public eye, since in its first year a seeded roof may appear more like a dirt roof than a green roof.