Different Types of Green Roofs and Green Roof Plants

different types of green roofs

Exploring the different types of green roofs might help you discover a slice of heaven in a brick-infested city center. Green roofs are an urban alternative and provide a natural habitat for flora and fauna. Pleasing to the eye and soul, they are also energy efficient and economically beneficial.

There are three different classifications for green roofs: extensive, intensive, and semi-intensive.

different types of green roofs - Chicago City Hall
Image: CC 3.0, Tony the Tiger, via Wikimedia

Why Install a Green Roof?

A well-tended green roof can bring natural beauty to one’s life in a big city. The Environmental Protection Agency (EPA) describes a green roof as a “vegetative layer grown on a rooftop.” The benefits of the many different types of green roofs are undeniable. Green roofs are tools for reinventing the urban environment. They provide an oasis of beauty and a restful place to gather thoughts and rejuvenate.

So, why invest in a green roof? First and foremost is the undeniable aesthetic effect. Like that tattered paperback novel we have read a thousand times, a roof garden can become an escape. Creating a utopia in the midst of a concrete jungle can be an aid to maintaining sanity in an insane world. Flora can be as simple as grass, a colorful array of flowers, small trees, or even a full-scale vegetable garden.

different types of green roofs
Image: CC 2.0, by Design For Health, via Flickr

Another benefit is that more plant life means better overall air quality. Green roof plants eliminate harmful air particles and produce oxygen. They become the new urban habitat for birds and insects, which increases the biodiversity. They help reduce Urban Heat Island Effect (UHI).

Additional benefits from green roofs

UHI is a phenomenon that causes it to be warmer in an urban environment. Pavement, rooftops, and other urban structures cause this unnatural rise in temperature. Because the heat produced under UHI is more intense, green roofs can be a useful combative tool for reducing UHI. Green roofs also last longer than conventional rooftops while they act as a natural shield against extreme heat.

…green roofs use a process called ‘evapotranspiration‘ to keep their buildings cooler, by using heat from the air to evaporate the water in the rooftop plants, fibers and mesh. In the winter, buildings also stay warmer, as the green roof works like an insulating blanket, prolonging the life of the roof, and driving heating costs down.

In addition to the apparent aesthetic and environmental effect, vegetated roofs provide economic benefits. A green roof will also help the surrounding area by decreasing the flow of storm water being absorbed into the ground. They also serve to increase the property value of the building. Installing any of the different types of green roofs provides many benefits with few detractors.

Different Types of Green Roofs

Once you have decided that you want a green roof, the next step is to determine how extravagant you want it to be. Modern green roof technology offers endless varieties of vegetated roof covers. Many guarantee high functionality levels for 50 years or longer. There are three different classifications for green roofs; extensive, intensive, and semi-intensive. From here on, it all comes down to how much effort and expense you want to expend in keeping your roof in excellent condition.

Extensive green roofs

Extensive green roofs need less maintenance and are the most popular selection. Designed for environmental solutions, they are best suited to roofs with little load bearing capacity, low-sloped residential roofs, and retrofits. The mineral substrate layer contains little nutrients. It is suitable for humble, less demanding low-growing plants, such as desert grasses, sedums, and herbs.

The best feature of an extensive green roof is that they require very low maintenance. Once the installation is complete, they do not need watering after approximately one year. While they do not require an irrigation system, annual fertilization (in fall or early spring) is recommended to ensure proper growth.

different types of green roofs
Image: CC 2.0, by thingermejig, via Wikimedia

Extensive green roofs significantly reduce water run-off. Another benefit to installing an extensive design option is its relatively low cost compared to the other two types. Any green roof company should be able to give you an accurate estimated cost per foot, including the cost of labor, materials, and any extras you might need.

Intensive green roofs

Intensive green roofs require dedication to maintain. They are fully landscaped park-like gardens. The growing medium of those vegetative roofs is 8 – 12 inches (compared to 3 – 6 of the extensive ones). These gardens require regular maintenance. Permanent watering systems and fertilization programs should be planned into the initial design and installation.

In addition to plants, intensive green roofs may have small trees, benches, walkways, and even playgrounds. They might also include small ponds or other decorative features. However, people may pay more for the initial set-up as well as higher maintenance costs.

different types of green roofs
Image: CC 4.0, by KVDP, via Wikimedia Commons

Before opting for this type of system, it is good to keep in mind that regular roof access should be provided. Paved walkways, walls, and benches can provide an oasis from a bustling office environment during breaks. Allowing employees this temporary respite can assist in increasing productivity, which creates another economic benefit.

Semi-intensive green roofs

Semi-intensive green roofs have elements of both extensive and intensive systems. This green roof system offers the best of both worlds: the environmental benefits of the extensive roof system and the diverse garden of the intensive one. And it falls within the scope of having a manageable maintenance budget. The growing medium is 5 – 8 inches. Because of the additional weight and root barriers the initial cost outlay is higher in comparison to the extensive roof system.

different types of green roofs
Rooftop farming at the Food Roof Farm in Downtown St. Louis, MO
Image by Oztafi US via Wikimedia Commons / CC BY-SA 4.0

Semi-intensive green roofs provide the added bonus of allowing a more extensive variety of plants that are suitable. The level of maintenance required is higher when compared to the extensive roofs systems. These plants will need pruning, regular watering, and fertilization.

In planning for your green roof project keep in mind the different types of green roofs, the building structure, the beginning and extended maintenance budgeting requirements and the desired end result. These factors will assist in choosing between an Extensive, Intensive, or Semi-Intensive green roof plan.

What Plants to Choose for a Green Roof

Choosing the right plants is possibly the most essential thing about having a beautiful green roof. Determining which plants will work best will depend on which of the different types of green roofs are installed. Due to the varying depths of growing medium, some plants will thrive better in one design but will wither and die in another.

Succulents and evergreen ground coverage

With an extensive roof system, using a desert-style grass is allows the plants plenty of room to breathe. A variety of Sedum and Wildflower plants are also a good choice. Succulents such as those in the Sedum family are low growing evergreen plants that are extremely low maintenance.

The low growing and spreading habits of succulents, high drought tolerance, and contrasting foliage colors make them ideal for green roofs. Evergreen vegetation flourishes in harsh environments. Additionally, people find that they are very affordable when covering large areas. Succulent plants are well-adapted to rooftop gardens because of their ability to store excess water. Some of the most popular choices are Sedum, Euphorbia, Sempervivum, and Delosperma.

Different types of green roofs
Image: CC0, by amazonite, via Pixabay

Flowering plants: adding bright, beautiful color to your green roof

Climate and micro-climate have a significant impact on selecting plants for extensive roof systems. Taking the characteristics of a plant into account when you are planning a green roof garden landscape will guide your choices. The ideal species include long-lived plants and those that replenish annually through natural seeding. Plants well-suited for semi-intensive green roofs can include succulents, ornamental grasses, herbaceous perennials, and small shrubs.

different types of green roofs
Image: CC 4.0, by Conservation Design Forum, via Wikimedia

Intensive green roof systems, being park-like gardens, often feature trees as dominant elements. Trees with sparse canopies, flexible stems and high tolerance to heat are the best choice for areas of high wind exposure. Plants for semi-intensive green roofs can include succulents, ornamental grasses, herbaceous perennials, and small shrubs.

With such a wide variety of plant selections available we cannot explore them adequately here. The Chicago Botanical Gardens has produced a fantastic, full-length report on plant types and suitability for the different types of green roofs. This comprehensive report is a great tool to use while in the research and planning stages of any green roof project.


Featured image: CC 2.0, by Arlington County, via Flickr


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