Learn About the Top 8 Green Roofs Myths

by Angie Mims on Friday, July 24, 2020

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What is a green roof?

Simply put, it is a roof with a layer of vegetation on top of it. You may also hear them referred to as living roofs, vegetated roofs, eco-roofs, or any combination of the prior.
Vegetated roofs are diverse in design, but there is a general distinction between the two main classifications.

Intensive green roofs are regularly used on commercial buildings and are referred to as “rooftop gardens”. These larger buildings offer extra outdoor prefer amenity space where people can lunch, relax and sunbathe and think of this as usable outdoor space with perennials, shrubs, and small trees, but, on top of a roof. This requires a soil depth of typically request more coverage and incorporation of a variety of sizes and types of plants. This type of vegetated roof can accommodate larger plants, such as trees, due to its planting medium depth, which is typically 15 cm (6 in) or more.

Extensive green roofs are also known as “thin profile” and “performance” green roofs. These roofs are simpler and less costly to install due to the commonly used pre-grown mats of vegetation or another method known as “plugging”, where one uses small plants called plugs (size of a salt and pepper shaker) and plant them in a triangular pattern of holes (approximately 4-6 inches or 10-15 cm apart).

The extensive green roofs planting medium ranges from 5 to 15 cm (2 to 6 inches) deep and favor drought-resistant plants such as Sedum, short grasses, or other plants with shallow fibrous roots. The specifically engineered design of these roofs withstands harsh conditions yet minimizes maintenance as the shallow profile during times of drought eliminate unwanted weeds, since most plants cannot survive in this shallow and relatively dry profile. There are many misconceptions and general assumptions around green roofs and we hope to address a few of them in this article.

Myth Nr. 1: Green roofs are expensive

There are many factors that go into the cost of a vegetated roof. First, one must think of what purpose the roof will serve. A couple of examples of common functionality include stormwater management, LEED, enhancing aesthetics, noise abatment, reduction of energy usage, and mitigating pollution.

Second, one must decide which type of vegetated roof best serves the chosen purpose: intensive or extensive. Ultimately, intensive roofs are more expensive, but they serve an amenity space function and this may be well worth the investment. Remember, intensive roofs closely resemble a traditional garden, therefore, they will require similar upkeep. Incorporating larger sizes and more diverse plant palettes add weight to the structure and require more care which adds to the cost. These plantings also require additional irrigation.

Extensive green roof expenses are kept lower due to the opposing reason. So, are green roofs expensive? That depends on your payback strategy. Are you eliminating a stormwater tank and you can park 4-5 more cars, can you increase the rent because the extra green space is desirable to tenants, are you considering doubling or even tripling the lifespan of the roof membrane? These are things to consider that can offset the cost partially or completely.

Myth Nr. 2: Green roof maintenance requires a lot of irrigation and is costly

A period of vegetated roof maintenance is usually included in the installation cost. The frequency of maintenance weighs heavily on the type of roof you install. For an extensive living roof, one can expect to pay $0.75-2.00 per sq. ft ($8-22 per m²) each year, after all, they are a real garden. On the lower end of the spectrum are extensive green roofs where Darwinian "survival of the fittest" does most of the weeding for you, and this means lower maintenance costs of around $0.25-0.50 per sq. ft (2.50-6.00 per m² ) annually for extensive green roof maintenance.

Irrigation is completely dependent on local climate and plant variation. For example, extensive sedum roofs are very drought tolerant and rarely need irrigation to survive during standard Northern and Central European or North and Central US summers. However, if the aim of the roof is to cool the building through the evapotranspiration process, drip irrigation systems can be installed that lead the water straight to the plant roofs for inexpensive and very water-efficient delivery. In general, extensive roofs are only irrigated if they are highly visible and part of the aesthetics, or when a certain amount of water needs to be evaporated.
Furthermore, it should be noted that nothing stops you from re-using rainwater for irrigation, especially if you live in a region with very long, hot days intermingled with heavy sporadic rain.

Myth Nr. 3: Green roofs are heavy and can exceed the building’s allowed structural load

Weight load is already accounted for in new construction roofs. Many structures are designed with roofs equipped to handle 120-195 kg/m2 (25- 35 lbs. per sq. ft) of dead load which is the certified weight of a green roof that is considered fully saturated) which is the range of most extensive vegetated roofs, and even for friction-based detention roofs such as the Purple-Roof Concept. Intensive roofs are often 200-400 kg/m2 (40-80 lbs. per sq. ft).

An evaluation by a professional can provide you with information about which type of loads the roof structures would need to support your vegetative roof. The cost of increasing the roof load capacity is minimal if designed into the structure from the beginning, often adding 2-3% increased cost in the structural design of that section.

Myth Nr. 4: Green roofs leak

Roofs, traditional or green, will leak if improperly installed, designed, or poorly maintained. Green roofs cover the waterproofing membrane, which protects it from the natural wear and tear what is called expansion and contraction of the membrane due to temperature swings, and it will protect the membrane from the sun which doubles or triples the life span, and of course, it will protect it from hail damage. Properly installed, green roofs leak LESS.

A recent study published by Soprema unequivocally proved that membrane protected underneath a green roof has the exact same characteristics as brand new material, even after being on the roof for many years. Some manufactures prefer a root barrier to stop roots from breaching the membrane. The installation of green roofs should be handled by experienced professionals to ensure these preventative features are correctly installed.

Myth Nr. 5: You can't get a warranty for a green roof

Warranties that last for 10-20 years are usually offered by green roof manufacturers, suppliers, and installers. Warranties on green roofs are comparable to warranties on other home projects because they cover material, waterproofing, irrigation, labor, etc. for the lifespan of the green roof.

Myth Nr. 6: Anyone can install a green roof

At this point, we know that green roofs are far more than just dirt and plants on a roof. If someone were to take this on as a DIY project, it could turn into a very expensive mistake. As previously explained, green roofs require a structural engineer to verify load exposures, it requires professionals to design the correct component assembly, and it requires professionals that can safely and properly install it on top of a roof.

During our 15 years we have seen (and made) plenty of mistakes, some were easy to fix, and some needed a complete replacement. Ask for their list of prior projects and make sure they are experienced. It is best to contact a professional for your green roof needs.

As a first step, however, take a look at our online retention modeler where you can start investigating what type of green roof would be the most suitable for your climate:

Myth Nr. 7: Green roofs are poorly researched

The world’s most famous green roof is the Hanging Gardens of Babylon, which dates to 500 B.C. Since then, there have been many technological advancements that have helped preserve their longevity. The start of modern living roof technology began in the early seventies in Germany.

This was the first move in advanced irrigation and protection for rooftop gardens. The first green roof in New York was built on the Rockefeller Centre in the mid-1930s. It still has the same waterproofing membranes that were originally installed. In the last couple of years, New York has pushed green roofs to better the environment and reduce the effects of rain on the city’s infrastructures. So did Toronto, Chicago, Portland, Seattle, San Francisco, and the list goes on.

Over the years green roofs have increased in popularity. This recognition has piqued people’s interest in preserving the environment. Research has proven that green roofs save energy by providing insulation and cooler ambient air, green roofs provide protection of the roofing membrane which increases its lifespan, green roofs reduce stormwater runoff that would often go into overflowing municipal systems, and green roofs improve air quality by absorbing pollutants and radiation.

Myth Nr. 8: You cannot combine solar panels and green roofs

Biosolar is the combination of solar panels and green roofs working in the same system. How does this work? Green roofs reduce the temperature surrounding the solar panels this causes photovoltaic cells to work more efficiently. This combination of a vegetated roof plus photovoltaic cells can increase the photovoltaic efficiency by 2-15% compared with a photovoltaic roof with no vegetation. And the green roof acts as ballast for the solar array. It’s a win-win.

Don’t hesitate to contact us at Sempergreen or Purple-Roof if you have any questions!

Additional Resources

Manage Stormwater with Green Infrastructure

Green Roofs Mitigate Air Pollution

Urban Development: From Gray-to-Green Roofs