What grows on a garden roof?
If you’re like me, you’ve probably been hearing a lot about garden (or ‘vegetative’) roofs recently. They’ve been getting a fair amount of attention, and deservedly so. It’s not a new technology, that’s for sure – it wasn’t even new at the time of Little House on the Prairie – but it’s being applied in new ways. So just what does it take to make a garden roof – is it as simple as putting a few trays of plants and some sod up on top of your building?
Garden roofing (and I’m generally talking about low-slope or commercial garden roofing in this posting) is broken down into two major categories, extensive and intensive, defined essentially by how deep the growing medium is and how much upkeep is required. Intensive roofing is more park-like, and can even accommodate small trees (you can think of it as ‘labor-intensive’ along with having a more intense group of plants growing on it); while extensive garden roofing is shallower, and generally requires less maintenance, often just a little weeding or reseeding.
Vegetative roofs often have major advantages. A growing plant takes in water and releases it throughout the day (this is known as transevaporation), helping keep the roof cool. And as the weather cycles from wet to dry, a garden roof can have a cushioning or damping effect, regulating moisture and temperature. The growing medium, which might seem like it’s a fancy word for dirt, is really a light, water-retaining compost mixture optimized for the application. It’s being asked to do a lot; be light-weight, allow water to drain through it, but also retain some water and be suitable for plant life. Combining the plants together with the growing medium, you get a system that can keep the roof cooler, help insulate the building, and reduce storm water runoff.
Seems like a simple, practical technology, and it is. However, there are a lot of misconceptions about garden roofing: that a vegetated roof won’t last, or at least won’t last as long as a conventional roof; that they’re guaranteed to leak; or are essentially nice-to-do’s. However, when you really look at it, a properly installed garden roof is no more likely to leak than any other roof – in fact, since the system starts with an ultra robust membrane roof system and the vegetative cover layer protects the membrane from UV and freeze-thaw cycles, the roof system can last longer. And data shows that in many cases garden roofs are cooler than conventional roofing, even cooler than reflective white roofs. However, it is true that it can be hard to define a payback from a garden roof, the benefits of may sometimes accrue more to the community at large rather than to the individual building owner, particularly when thinking of the urban heat island effect. However, there is one primary benefit of a vegetative roof that can be quantified, and does potentially accrue to the building owner – storm water retention. When a downpour occurs, antiquated stormwater systems often become overwhelmed. A roof that retains and then slowly releases water, allowing the system to recover, is desirable. An extensive garden roof such as GAF’s GardenScapes, with 4 inches of growing medium, can effectively retain stormwater, releasing it gradually. In some municipalities, this means savings on sewage charges and sometimes even allows for a larger building on the same piece of property, optimizing land usage.
So garden roofs can provide significant benefits. They don’t even take that much upkeep. An irrigation system is typically put in place when the system is installed, to help get the plants started. Properly grown plants from local nurseries, once rooted, are selected to not need much (or any) irrigation, but it’s still important to provide an irrigation system, in case of an extended drought. Depending on local codes, gray water or collected rainwater can usually be used, if/when irrigation is needed.
Also, a vegetated roof is one of the best ways for a building project to obtain LEED credits. There are up to six categories (and even more points, depending on the project) available in LEED 2009 related to vegetated roofs. This makes a garden roof one of the single best measures a Project Team can include in the design. For example, the Project Team can earn credits for Protect and Restore Habitat, Stormwater Quantity Control, Stormwater Quality Control, Heat Island Effect, Protect and Restore Habitat, and Maximize Open Space.
So, to answer the initial question, just what grows up there? Well, if you think about what grows on nature’s roofs – plants that grow on cliffs, with small amounts of soil, and through all kinds of environmental conditions – those are generally the types that work well on roofs. Most often these are types of sedum or stonecrop, a low-growing, hardy plant. Other locally adapted plants and herbs can work as well – one of the most interesting things about a garden roofing installation is that your roofing system manufacturer will generally work with a nursery that is close to you, and select the optimal plants for your climate and needs. As you can see in these pictures, plantings grow quickly and fill in, and they don’t have to be the color green – depending on where you live, many different colors are possible.
The GAF team will be at USGBC’s Greenbuild show in Chicago this year, featuring our GardenScapes Roofing (along with other Sustainable products), in one of the cities that has been on the leading edge of adopting vegetative roofing. We hope to see you there!