Lifecycle Costs
Just like a conventional roof, expenses will continue. Regular maintenance is necessary for all roofs to ensure they are in good structural condition and are fulfilling their function adequately (no leaks, effective insulation, etc.) After a green roof is installed and established, the ongoing maintenance may be relatively similar, or it could be substantially more complex. Again, those extremes are dependent on the goals and choices made during design and construction.
All roofs will have a certain amount of “growing in” time when the plants will require regular attention: weeding, watering, and replanting for damaged or underperforming specimens. In the case of an extensive roof, once the plants are fully established (up to 2 years usually), and cover the entire growing area, the amount of necessary maintenance should drop substantially. Depending on the options you’ve chosen, it could be as little as twice a year. Many green roof installers will include a contract for regular maintenance during the growing-in period and may offer agreements for quarterly or semi-annual upkeep thereafter. Opting for this kind of service will increase your up-front costs, but it could substantially reduce worry on the part of the building owner. On the other hand, since intensive roof systems are pretty much a rooftop garden, the maintenance needs for an intensive roof will remain largely the same over the years.
Regular maintenance for both extensive and intensive roofs might include replenishment of growth medium, fertilization, irrigation, weed control and replanting when necessary. Intensive green roofs will almost certainly require more maintenance, to include maintaining aesthetic appeal with regular trimming, rotating seasonal flowers, repairing and maintaining hard elements like benches, walkways, and other items necessary to the intended function of the space.
The costs for these maintenance activities will vary according to factors like project size, proximity of maintenance services to the project site, and the desired complexity of maintenance activities.
Irrigation is often considered a point of friction when green roofs are being discussed, but it should be a nuanced discussion. After all, the need for irrigation is completely dependent on the local climate and the varieties of plants selected. For example, sedum roofs are popular for low-maintenance extensive roofs in North and Central US states. A sun-loving succulent like sedum is very drought tolerant and very rarely needs irrigation to survive.
However, keep in mind the overall purpose of the roof. If one of the principal goals is to cool the building by keeping the roof and surrounding air cool, it may be extremely effective to install a drip irrigation system that distributes water directly to the plant roots. This maintains the roof at a peak level of attractiveness, conserves water very efficiently, and allows the plants to continue their cooling activities at maximum level. An attractive option for a system like this would be to install a rain barrel system that can catch and store water from periodic rain events for use during a dry spell.
Of course, for an intensive green roof, you’ll almost certainly need to install an irrigation system since demands for a closely managed environment will be wholly different from the “survival of the fittest” approach in an extensive roof.
Lifecycle Savings & Payback
Savings in cooling costs is probably the primary goal for green roofs in today’s environment. As urban environments get hotter from sprawling heat islands, business owners are becoming more and more interested in lowering their energy bills while still maintaining an environment that keeps their employees comfortable and productive. Let’s look at some considerations here:
Energy Savings
While we often see dramatic pictures of green roofs topping 40 story buildings in highly developed downtown areas, the truth is that green roofs provide the most energy savings in buildings with fewer floors. After all, the roof size is static, and in a typical high rise the roof will span about 20,000 sq ft. When dividing the energy cost savings among 20 floors, you simply won’t obtain the same impressive numbers that you’d achieve in a building with the same footprint that’s only 3 stories tall. That’s not to say green roofs aren’t worthwhile on high rises - on the contrary! Additional benefits, including the mitigation of urban heat islands and improved stormwater management are especially effective in those locations.
Extended Longevity
The longevity of green roofs is one of the most convincing arguments for choosing them over conventional roofs. Probably the most damaging elements to an average roof are rapid, intense temperature fluctuations from daytime to nighttime temperature swings and exposure to UV rays for sensitive materials like the waterproof roofing membrane. Green roofs are naturally protected from those damaging effects because of the way the roof is structured. Temperature fluctuations are largely eliminated through the same mechanism that keeps the roofs cool during the intense summer heat, namely evaporation and evapotranspiration.
UV-sensitive waterproofing membranes are protected from the sun’s rays by virtue of being buried beneath multiple layers of material. In fact, green roofs are often called “upside down” roofs because the insulating and waterproofing layers are not placed on top of the roofing material, but rather beneath it. In a typical build, the roof decking is covered directly by the waterproofing membrane, after which insulation, filtration and drainage layers, planting medium and the plants themselves are placed. In this setup, the membrane is very well protected from UV radiation. This significant level of protection for the most vulnerable elements of a conventional roof means that the lifespan of a well-maintained green roof is typically between 2 and 3 times greater than a conventional roof.
In a quick calculation, you might even compare the cost of a single installation of a green roof to two or three installations of a conventional roof. In that kind of comparison, a green roof will almost always come out on top.
