Image Above: Green Roof at the Gerber Memorial Hospital Emergency Department, a C2AE project

In a watershed— an area of land that’s water drains off into a common outlet —urbanization increases the amount of total impervious area (TIA), effectively altering the land’s natural hydrology.  This modification of land use from vegetated and forest land prohibits infiltration of rainwater into the ground, producing stormwater runoff.  Stormwater runoff has become a major concern for developing communities as municipalities are forced to deal with the social, economic, and environmental consequences that stormwater runoff creates. 

Green roofing, or vegetated roofing, is a relatively new technology to North America that entails growing plants in a media layer on top of a waterproof membrane.  Many people see green roofs as a part of the “green” movement associated with today’s building and construction industry.  What they don’t know is that green roofs date back to the ancient civilizations that lined the banks of the Tigris and Euphrates river valleys.  These ancient civilizations used “green roofs” as protection from the natural environment and for added insulation in their homes.  Their green roofs typically used indigenous materials to construct a waterproof membrane, on which they placed the surrounding natural vegetation.  Following the garden roofs of ancient times, the Italians constructed villas with extensive terracing and roof gardens in the 1500s.  Green roofs also have roots in Scandinavia that date back to the 16th century.  There, a waterproof membrane would be constructed out of birch bark, and sod would be removed from the building footprint and placed on top.  These roofs were constructed in small towns and rural areas because they were cheap to build using local materials and had adequate insulating properties.  The building method during this period was typically timber frame construction, consisting of heavy structural members that could support the additional weight of the green roof.  The modern green roof originated at the turn of the 20th century in Germany, where vegetation was installed on roofs to mitigate the damaging effects of solar radiation on the roof structure.  The contemporary design is primarily used to alleviate environmental problems associated with high density urban development.

The greatest benefit green roofs offer is widely recognized to be stormwater retention in urban areas.  Green roofs mitigate impervious surfaces created on building rooftops, reducing volumes of stormwater runoff, delaying peak flows, and improving stormwater runoff quality.  This process is critically important in urban areas where there is limited space for traditional  best management practices (BMPs) such as detention ponds.  Water retention is a function of many design factors including substrate depth, media composition, plant selection, and roof slope as well as weather factors such as rainfall intensity and duration.

Another benefit of green roofs is their ability to increase the lifespan of roofing membranes.  The growing media and vegetation layer in a green roof system protect the underlayment from solar exposure and radiation.  The green roof system also regulates the temperature of the membrane surface.  The reduction in extreme temperature variations help to extend roof membrane lives by minimizing the expansion and contraction of the membrane material, which can lead to its degradation.  An increased lifespan helps recover up-front costs associated with green roofing and makes the lifecycle cost of green roofs smaller than the lifecycle cost of traditional roofing materials. 

Yet another advantage of green roofs is their ability to conserve energy and reduce the urban heat island effect.  Green roofs provide shade and an extra insulation layer that help to make buildings more energy efficient.  Media depth, shade from plant material, and transpiration can reduce solar energy gain by up to 90% compared to non-shaded buildings.  On top of reducing surface temperatures on the roof membrane, green roofs also reduce the ambient air temperatures surrounding the building.  Air temperatures above the building have been shown to be 30°C (86°F) lower when vegetated compared to a conventional roof, resulting in up to 15% annual energy consumption savings. 

Green roofs have the ability to create wildlife habitats in urban areas where vegetation and natural habitats have been destroyed due to building development.  Green roofs that can offer more developed landscape structure and ecology will attract more bird and insect species.  Green roofs can also improve air quality by filtering air borne pollutants as well as reducing ambient air temperatures.  Vegetation removes pollution by taking up gaseous pollutants through their stomates, intercepting particulate matter with their leaves, and breaking down organic compounds in their plant tissues and soil. 

Finally, green roofs provide social benefits through aesthetic and therapeutic appeal.  Intensive green roofs are often designed as rooftop gardens where building occupants have access to additional social space.  Rooftop gardens can increase property values for building owners and in turn allow them to charge more for rent.  Viewing green plants and nature can have beneficial health effects on humans,  reducing stress, lowering blood pressure, releasing muscle tension, and increasing positive feelings.  This has led to many hospitals installing green roofs to provide their patients with another level of therapeutic care by having patient rooms overlook a rooftop garden. 

Green roofs are typically classified as either intensive or extensive.  The difference between these two types of green roofs is the depth of the soil media, plant selection, accessibility, and maintenance.  Intensive green roofs are typically classified as having greater than 15.2 cm (6 in) of media.  This allows for a wide range of plants to be used that are traditionally found in landscapes on the ground including trees, shrubs, and herbaceous perennials.  These roofs are often designed to be rooftop gardens that function as a social space for users of the building.  The decision to install an intensive green roof is determined by the structural capacity of the roof.  Because these roofs are typically accessible to the public and mimic a park-like setting, they are limited primarily to flat roofs.  Intensive green roofs also have higher associated maintenance costs because of the maintenance required for more developed landscape ecosystems and accessibility concerns.  


Intensive Green Roof

Extensive green roofs are classified as having less than 15.2cm (6 in) of growing media.  These roofs often serve a specific function, such as stormwater retention, and are not designed to be accessible by the public or sometimes even visible at all.  This type of green roof is lightweight and is often used on existing buildings that can only handle the reduced weight of an extensive green roof system.  The shallow media depth restricts what plants grow on the roof.  These plants include herbs, grasses, mosses, and drought tolerant succulents such as Sedum.  Extensive green roofs are low maintenance and often do not require irrigation unless there is prolonged drought. 


Extensive Green Roof

There are numerous aspects of architecture and engineering that go into the design of a green roof system.  Green roof design truly requires a multi-disciplinary approach in order achieve the ultimate project success.  As a landscape architect, when it comes to designing a green roof, I am passionate about creating new rooftop space, coming up with a creative planting design, selecting proper growing media and irrigation to ensure a sustainable rooftop ecosystem, and site related planning issues like stormwater and open space calculations.  However, that is just the tip of the iceberg in terms of design considerations.  I must rely on other disciplines within our firm to cover all aspects. 

Inter-Functional Disciplines 

At C2AE, the collaboration among our landscape architects, architects, structural engineers, civil engineers, and mechanical engineers allows us to maximize the investments of owners by ensuring that green roof benefits are fully realized.  We start our green roof design process by understanding the goals and objectives of our client.  Why do they desire a green roof?  What benefits are important to them to maximize?  A clear understanding of the client’s vision will inform the rest of our design decisions and ultimately what team members will need to be involved and in what capacity. 

Every green roof (extensive or intensive) will create additional live and dead loads on the roof structure.   If the building already exists, the structural engineer will analyze the existing roof and determine what structural limitations may exist.  These limitations will inform the design process for selection of growing media, plants, pavers, etc.   For new constructions, there is more freedom to make the structure as strong as needed to suit the design. 

The architect selects the appropriate waterproof membrane for the roof.  Every green roof must function as a normal roof, keeping water out of the building, so there will always be an underlying waterproof membrane with some sort of protection layer from the green roof system.  I work with the architect and structural engineer to create pleasing views from inside the building onto the green roof and implement different access points to the rooftop.  Often, we bring the mechanical engineer into the conversation to determine an R-value (the degree of insulating power) for the green roof.  It is important for this collaboration to happen because of the additional energy efficiency realized by the green roof, which can impact the design and sizing of the building mechanical systems.

Finally, I collaborate with our civil engineer to strategize on the overall stormwater management for the site.  The most widely recognized benefit of a green roof is its ability to provide stormwater management.  The green roof can function as a stormwater BMP and alleviate some of the need for traditional stormwater systems (grey infrastructure) on the ground level.  We also discuss site zoning issues and how the green roof can impact overall site open space calculations and implications for building size, parking lot size, etc. 

Architecture and Infrastructure Congruity 

A green roof is a unique element of a building project because it can have dramatic impacts on both the building and the site.  A green roof is both a building system and an infrastructure system.  It is a prime example of how architecture and infrastructure come together to create a successful, holistic design.  The green roof must honor the structural capabilities of the building while supporting the stormwater infrastructure of the site.  When the complex, multi-disciplinary aspects of green roof design are all considered and congruity between architecture and infrastructure is emphasized, the maximum value of a green roof project will be realized.

Aggressive Collaboration 

Green roofs are a relatively new concept to most building owners, municipalities, contractors, and even some design professionals.  Most have an idea of what a green roof is, but do not understand all of the details, potential benefits, design considerations, operation, and maintenance that goes into them.   By collaborating very early in the design process with owners,  we can explain how a green roof could help them achieve their goals and ultimately help them make the appropriate decisions moving forward.  We carry this aggressive collaboration into the planning realm when we work with local planning departments to help them understand how we can use green roofs to meet their local stormwater and open space ordinance requirements.  Finally, we collaborate with our contractors to make sure they understand how the green roof system needs to be installed for optimal performance.  Whether it’s a roofing contractor or a landscape contractor doing the work, many do not have experience in green roof installations and we make sure the intent of our design and specifications translate onto the roof.

By knowing a bit more about what green roofs are and what goes into the process of implementing one, building owners can take an active step toward reducing their environmental impact and have more efficient properties. C2AE has the skills and understanding to help.



Erik Cronk, PLA, LEED AP, is a landscape architect at C2AE specializing in green roof design. Erik, a graduate of Michigan State University’s green roof research program, is frequently a featured speaker at conferences regarding green roof design and has won awards for his work.