Myth #1: Green roofs are heavy and damage buildings
Many structures are now designed by default with roofs equipped to handle 25-35 lbs per square foot of dead load. This load is equivalent to the certified weight of a fully saturated extensive green roof. Intensive green roofs may require additional structural support, but in a building still in the design stages, that’s a minimal expense.
For retrofitted green roofs, the key is to consult a qualified professional to advise you about what kind of load your green roof would carry and what structural reinforcement is necessary. As a rule of thumb, existing roofs with concrete structural systems generally need the least reinforcement, while roofs with steel decks may require the most. No two pre-existing roofs can be assumed to be the same. Owners and building managers who skip this step to save money are the ones who are most likely to suffer from structural damage.
Myth #2: All green roofs leak, eventually
Any kind of roof will leak if poorly designed, installed, or maintained. All roofs, including the green variety, include a waterproofing membrane -- that’s what keeps any roof from leaking. The important difference between conventional and green roofs is the placement of the waterproof membrane. On conventional roofs, the waterproofing membrane is on top, leaving it vulnerable to thermal expansion and contraction, sun exposure, and hail or similar damage. Conversely, on a green roof, the waterproofing membrane is covered by multiple layers of the roof system itself, including several inches of plants and substrate which provide natural protection from weathering.
Bottom line: properly designed and installed green roofs leak LESS than conventional roofs.
Myth #3: It’s expensive and difficult to water a green roof
While the vegetation on almost any green roof will need to be watered when it’s still getting established, many extensive green roofs won’t need additional irrigation beyond that. The key is to choose native plants that are well adapted to the climate, which may include a natural drought tolerance. In climates where there are long summers with relatively rare (but intense) rainfall, it is possible to design a roof with a thicker layer of growing medium that’s formulated to hold plenty of water. The drainage layer can also be designed with deep cells that can store water (when it does fall) which can be drawn from during dry periods.
If it comes down to it, though, irrigating a green roof is no more difficult or expensive in the long run than any other planted area.
Myth #4: Nobody will warranty a green roof installation; once it’s in, you’re on your own
Professional green roof manufacturers, suppliers, and installers usually do offer warranties, typically between 10 and 20 years. They’re similar to warranties on other building products in a home.
Commercial green roof installers often include a maintenance contract for the first year as part of the package - in addition to checking the performance of the roof, workers may make regular visits to ensure the new plants are getting established, including weeding, replacing plants that aren’t thriving, etc. In many cases, those maintenance contracts may even be offered for multiple years, for long term ease-of-mind.
Myth #5: A green roof is a great project for DIYers
Green roofs are not a good choice for a DIY project. In retrofitting a green roof onto an existing structure, a structural engineer needs to ensure the load exposures can be supported, especially when the growing medium is saturated. An experienced professional needs to design the layout and integration of each component in the assembly and reliable, professional installers are necessary to ensure that all conditions of any warranties are met.
Myth #6: Green roofs are just a passing trend.
Various forms of vegetated roofs have been around for thousands of years, including the Hanging Gardens of Babylon, dating to 500 B.C. Sod roofs have been popular in Scandinavia since the Middle Ages; the first modern green roof in the US was installed on the Rockefeller Center in 1931, and in 2009, Toronto was the first North American City to pass a green roof law. (Interesting fact: The Rockefeller Center’s green roof still has the same waterproofing membrane that was originally installed.)
The fact is, green roofs have been around for thousands of years. It’s the technology that’s changed in recent decades, which permits lighter and thinner roof profiles. Research has repeatedly demonstrated that green roofs save energy by providing cooler ambient air, improve stormwater management, and improve air quality. In addition, green roofs provide additional protection to vulnerable roofing elements like the waterproof membrane, which can double or triple the roof’s lifespan.
Myth #7: I can’t install a green roof if I want to use solar.
Solar energy panels and green roofs work well together, in almost a symbiotic relationship. Solar cells lose efficiency when the surrounding temperature gets too high, which basically defines a conventional rooftop in high summer. Fortunately, the natural effects of a green roof (evaporation, evapotranspiration) keep the ambient air temperature significantly cooler, which can increase photovoltaic efficiency as much as 15%.
