Rooftop gardens, also known as “green” or “vegetative” roofs, are becoming a trend for commercial buildings in urban areas. These gardens offer building owners several benefits. Vegetative roofs can reduce a building’s energy consumption, provide sound insulation, reduce storm water runoff, help lessen the urban heat island effect, and filter pollutants. A vegetative roof can also help extend the life of the underlying commercial roofing system by providing protection against nature’s elements.
If the idea of shrubbery on your roof appeals to you, there are things to keep in mind before planting, perhaps the most notable being weight. A properly engineered, watered 6-inch-deep vegetative roof system (typical for “extensive” systems – more about that term, below), can weigh up to 40 pounds per square foot. Is the building structure robust enough to support that weight?
But weight isn’t the only factor. Is the integrity of the waterproofing roof membrane underneath the prospective green system sufficient for the long haul? Obviously, finding leak problems after the veggie roof is in place is considerably more problematic than beforehand. Repair crews will have to remove the garden components to locate the problem and repair it, adding time and expense to the project.
The upshot: garden roof systems themselves are expensive. It’s a relatively small incremental cost to replace the roofing membrane at the same time to minimize your potential maintenance headaches down the road.
You will also need to decide what type of rooftop garden is best for you. An extensive vegetative roof uses soil that’s not very deep –six inches or less – and plants are usually limited to low-growing, hardy, ground cover varieties, collectively known as “sedum.” These types of systems require less maintenance and are considered lightweight.
Some green roofs are also designed to be used as recreational space for building occupants. An intensive rooftop garden typically includes a landscape-like design, and is a walkable, aesthetic, even recreational space. The soil is deeper than six inches to allow for many different types of plants, even trees, and because of this it will weigh more on your rooftop. It will also require more upkeep and “landscaping” maintenance, as the variety and size range of plants used is much greater.
Another consideration is the warranty for the underlying roof membrane. Rooftop garden systems are a type of “overburden” and overburden warranties, as they are known, are written to provide roof system coverage when materials are installed over the membrane, including vegetative roofs. Typically, roof manufacturers will require a certain type or thickness of membrane to issue an overburden warranty, and installing a veggie roof on top of a non-approved membrane may void its warranty.
Your vegetative system and the roof system beneath are two different technologies that are interdependent. Your garden roof installer and roofing contractor should work cooperatively to ensure both components – roofing membrane and garden system – are installed properly and functioning together.