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November 2015
guest feature article

Designing Green Roofs for Stormwater Management
Technological Advances Offer Low-Weight Options


By Karen Liu, PhD
Bonar Xeroflor

Publisher's Note:  We saw a version of this article first published in Construction Canada on April 10, 2015 and  thought it would be a great beginner or refresher overview for greenroofs plus an intro to policies here in North America.

Images courtesy Xeroflor Canada.

Low-weight option green roof.

Green roofs have been consistently experiencing double-digit growth over the past decade.  An annual market survey conducted by Green Roofs for Healthy Cities showed that the North American green roof industry grew by 10% in 2013 over 2012, with a total of 519,151 m2 (6.4 million sf) on 950 projects installed in 2012 [1].

Green roofs offer multiple environmental, ecological and economic benefits to the urban areas.  Many municipalities in North America are encouraging their implementation through bylaws and incentive programs.  There are currently 33 cities in North America that have dedicated policies, incentives or guidelines to promote green roof implementation and they are working – seven out of the ten North American metro regions with the highest green roof area installed in 2013 have supportive policies or programs. [2]

Policies come in many forms.  Some programs offer financial incentives directly through grants and rebates (New York City NY) or indirectly through reduction in stormwater fees and property taxes (Washington DC).  Others involve accelerated building permit (Chicago IL) or low interest loans (Cincinnati ON).  Some programs give density bonus for additional floor space based green roof coverage (Portland OR).  While most North American policies generally involve incentives that encourage green roof implementation, there are also those that penalize for non-compliance (Toronto ON).

Stormwater Policies:

Prescriptive vs objective-based
Many of the green roof policies and programs focus on stormwater management.  Current green roof policies tend to be prescriptive-based that mandate a minimum depth and/or composition of green roof growing medium.  For example, Portland OR’s Density Bonus Program and Nashville TN’s Green Roof Credit Program require a minimum depth of 100 mm (4").  Others are objective-based that require the green roof to achieve a specific stormwater performance.  For example, New York City’s Green Infrastructure Grant program requires the green roof to manage at least a 25 mm (1") rainfall event, which also applies to Washington DC’s RiverSmart Rooftops program.

While both prescriptive and objective-based policies are meant to promote green roof implementation and mitigate stormwater in the urban areas, sometimes the good intention can become a disincentive or even hinder green roof implementation.  It is important that we understand how green roofs work to mitigate stormwater runoff.  The policy makers can then implement requirements that ensure effectiveness of their green roof policies while the architects and landscape architects can design green roofs to maximize the stormwater management potential.

How Green Roofs Mitigate Stormwater

Green roofs are generally categorized as intensive or extensive systems based on weight.  The principle components include: vegetation or the living component; growing medium; a filter/retention layer; a drainage layer; and, a root barrier. [Figure 1].

Figure 1.

During a rainfall event, rain is intercepted by the plants before it reaches the growing medium on a green roof.  Much of the incident rain is infiltrated, absorbed and adsorbed by the growing medium and plants.  Excess rain travels through the growing medium, exits the filter layer into the drainage layer, flows along the roof membrane to the roof drains.  The stored water is either used by the plants or returned to the atmosphere.

Although a green roof cannot fully mimic natural catchment because of its limited soil profile and diversity of vegetation, nevertheless, it can minimize surface runoff which contributes positively to stormwater management in both quantity and quality.

The stormwater management potential of a green roof depends on its water storage capacity as well as the rainfall pattern where it is located.  There is no one-size-fits-all green roof solution, the system buildup must be designed and optimized for the local climate.  The storage capacity of a green roof comes from various components in the assembly.

Vegetation:  Plants take up water from the roots and release it to the atmosphere from their leaves.  They provide runoff mitigation by removing water from the growing medium and releasing back to the atmosphere thus "recharging” the green roof system’s water storage capacity for the next rain event.

While most plants take up and release water during daytime, succulent plants such as sedums can store water within the tissues such as leaves, stems and roots and release it in the cooler time in the night, making them more heat and drought tolerant – characteristics that are particularly suited for the survival on rooftops.

Growing Medium:  Typical green roof media are higher in mineral aggregates and lower in organic matters (<25%) compared to regular garden soils to maintain soil structure and therefore performance over the long term.  Water is stored in the cavities of porous mineral particles (e.g. lava, expanded clay), small capillary pores between particles and the organic matter fraction.  Water stored in the growing medium is taken up by plants or returned to the atmosphere via evaporation.

Water Retention Layer:  Absorptive materials such as synthetic fleece and hygroscopic mats store water in the space between fibres.  They are highly effective in storing water compared to growing medium on a per-unit-weight basis.  At the same time, they are also permeable so they do not create water-logged substrate or promote root rot.

Drainage:  In addition to drainage function, some geo-composites consist of a 3D drainage core bonded to a water retention fleece that stores water and releases it to the growing medium.  Some drainage panels are molded with “cups” to act as water reservoirs to provide both drainage and water retention capabilities.  Open pore aggregates, such as expanded clay, can also provide both drainage and water storage but they add considerable load on the roof.

Root Barrier:  Is a flexible sheet of polyethylene that prevents plant roots from damaging the roof membrane.

How effective is prescriptive-based green roof policy that mandates a minimum growing medium depth?  All growing media are not made equal.  The water storage capacity of the growing medium depends on many factors such as composition, particle size distribution and organic contents.  Porous mineral such as lava and expanded clays can hold considerable water in their pores.  Different particle size grading changes the capillary pores and thus the water holding capacity.  Adjusting the silt, clay and organic contents can affect the water retention in the growing medium, and so can addition of water absorbent additives.

Figure 2 shows the water retention capacity of several typical growing media designed for extensive green roof systems, normalized to 25 mm (1
") thickness for ease of comparison [3].  Each bar represents the thickness (volume) that is composed of the dry component (red) and the stored water (blue).  The water retention capacity varies from 40-65% by volume.

Figure 2.

When an incentive policy mandates a green roof to have a minimum of 100 mm (4") growing medium, it will achieve a water retention capacity of 65 mm (2.6") when GM#1 is used but only 40 mm (1.6") if GM#2 is employed.  While both growing media will meet the prescriptive requirements of this green roof policy, GM#1 can retain 25% more water on a volume basis compared to GM#2.  While it is easy to specify a minimum growing medium depth, this prescriptive policy does not necessarily achieve specific stormwater performance.

In addition, soil is heavy when wet.  Typical green roof growing medium weighs 24–34 kg/m2 (5-7 psf) per 25 mm (1
") depth when fully saturated.  Prescriptive green roof incentive programs in North America generally require a minimum growing medium depth of 75–100 mm (3–4"), which translates to an additional loading of 72-136 kg/m2 (15-28 psf).  This extra weight often prevents lightweight construction such as factories and warehouses from adopting green roofs.  Unfortunately, these buildings often have large footprints, which exert considerable burden to the municipality’s stormwater infrastructure.

Water retention fleeces and hygroscopic mats offer lightweight alternatives to growing media to achieve water retention in green roof system.  Figure 3 compares the saturated weights of typical growing media and water retention materials normalized to 25 mm (1
") for ease of comparison [3].  Each bar represents the saturated weight that is composed of the dry component (red) and the stored water (blue).

Figure 3.

The saturated weight of the water retention layers are on average about 25% lighter than typical growing media.  In addition, only 9.8 kg/m2 (2 psf) out of the saturated weight of 38.8 kg/m2 (7.9 psf) or 25% by weight, for the lava/pumice/dolomite growing medium is water.  In comparison, a high fraction of the saturated weight for the water retention layers comes from the stored water as evident from the high blue fraction compared to the small red fraction in each bar.  For example, out of the 24 kg/m2 (4.9 psf) saturated weight of the mineral wool, 23.4 kg/m2 (4.8 psf) or 98% of which is water.  It is clear that the water-retention-to-weight ratio for the water retention materials is significantly higher than growing media.  Replacing all or part of the growing medium in a green roof with water retention materials such as fleece and hygroscopic mats can achieve equal or better water storage capacity while keeping the system weight low.

A prescriptive-based policy mandating a minimum growing medium depth can impose too much loading on the roof structure and prevent buildings with limited structural capacity such as factories and warehouses and the existing building stock to adopt green roof.  However, an objective-based policy that requires specific stormwater performance such as water retention capacity enables these buildings to adopt green roof systems based on lightweight water retention layers instead of growing media and contribute positively to the overall stormwater management goal in the municipality (see Sherway Gardens’ case study below).

While molded plastic drainage boards with reservoirs can also store water, the total water storage is usually low, at around 5 kg/m2 (1 psf) for each 25 mm (1
") depth compared to 10 kg/m2 (2 psf) for typical growing media, because it must remain relatively open to perform its primary function to divert excess water off the roof.  Therefore, drainage layers with built-in water reservoirs usually play a smaller role in the water storage capacity of a green roof system.


Green roofs are recognized by many municipalities as part of their stormwater management strategy.  Bylaws and incentive programs have been effective in encouraging green roof implementation in North America.  While prescriptive-based policies that require a minimum growing medium depth are easier to deploy and manage, objective-based policies that mandate specific stormwater performance are likely to be more effective in achieving stormwater management goals and maximizing the benefits of green roofs.

Advances in green roof technology have provided designers with many options to increase water storage capacity of green roofs while keeping the system weight low.  These offer opportunities in greening lightweight structures that would otherwise not possible with traditional growing-medium-based green roof systems.

See the short aerial video which features three green roofs that use lightweight options, including at Sherway Gardens.


1. 2013 Annual Green Roof Industry Survey, Green Roof for Healthy Cities, April 2014
2. “Green Policy–Why Green Roof and Wall Policy is Important,” Living Architecture Monitor, Volume 16 / Issue 4 / Winter 2014-15, p12-13
3. FLL Green Roofing Guidelines-Guidelines for the Planning, Construction and Maintenance of Green Roofing, FLL, 2008

Karen Liu, PhD

Case Study

Sherway Gardens’ Green Roof
Lightweight system offers high “water-retention-to-weight” ratio

Sherway Gardens Shopping Centre Expansion Green Roof.

The new XF301 Sedum Standard green roof on the Sherway Gardens’ Shopping Centre expansion at 9,500 m2 (102,000 sf) is the largest vegetated roof on a single, free standing commercial structure in the Great Toronto Area.  One of the main reasons behind the green roof retrofit is that the owner wants to reduce stormwater runoff from the roof.
This green roof system comprises of a root barrier, a drainage mat, two layers of recycled polymeric water retention fleeces and a pre-cultivated sedum mat [Figure 4].  At a total profile of 65 mm (2.6
"), the system weighs 59 kg/m2 (12 psf) at saturation with a maximum water storage of 36 l/m2 (1.4").  The sedum mixture was specially selected for their heat/drought tolerance and all-year-round visual interests.  The green roof is not irrigated.

Figure 4.  Cross section of the vegetated roof system on the expansion of Toronto’s Sherway Gardens Shopping Centre.

The Toronto Green Roof Construction Standard 2014 edition states: “For a successful green roof, the design and selection of growing media, irrigation systems, and plantings must be carried out as a system.”  In particular, the City of Toronto By-law mandates the following:

In order to support plant survivability:

(1) When structurally possible, the growing media shall be at a minimum 100 mm, or

(2) The Applicant shall provide a report confirming that the engineered system as designed provides plant survivability comparable to that of an un-irrigated system with growing media at minimum 100 mm.

While the system does not contain a minimum of 100 mm (4
") growing medium required in (1), it meets the City of Toronto’s requirement as per (2).  Typical green roof growing media have a water retention capacity of 8-10 l/m2 (0.3"-0.4") for every 25 mm (1") depth.  Therefore, 100 mm (4") of growing medium has a water retention capacity of 32-40 l/m2 (1.2"-1.5") which is similar to the 36 l/m2 (1.4") water storage capacity in the lightweight pre-grown mat system selected for this project.  However, while 100 mm (4") of green roof growing media weighs 100–140 kg/m2 (20-29 psf), this lightweight system weighs only 60 kg/m2 (12 psf) offering significant weight saving for the building structure.

Sherway Gardens Shopping Centre Expansion Green Roof.

© Xeroflor Canada Inc. 2015

Publisher's Notes: The Sherway Gardens Shopping Centre Expansion was featured as's Project of the Week of November 16, 2015.  See the Project Profile in The International Greenroof & Greenwall Projects Database.

We were very happy to have Dr. Karen Liu participate in our recent Greenroofs & Walls of the World™ Virtual Summit 2015 ~ Connecting the Planet + Living Architecture as part of the "From Passive House to the Cold North—How Vegetative Envelope Components Impact Buildings" Panel, along with Dr. Bob Cameron, Dr. Allen Lee, and Chris Wark.  Read about my Sky Gardens Blog post about it here.  Watch for our release of all of the 2015 Virtual Summit videos in early 2016.

Karen Liu, PhD, Bonar Xeroflor

Karen Liu, PhD, is the product manager for Bonar Xeroflor GmbH. She has been conducting green roof research since 2000 when she was a research officer at the National Research Council in Ottawa and led the green roof research team at British Columbia Institute of Technology in Vancouver. Liu established the first North American field facility dedicated to green roof research and conducted several field studies across Canada to study the climate sensitivity of the technology. She now focuses her work on product development at Xeroflor. Liu has been awarded CSC’s F. Ross Browne Award for her writing in Construction Canada.

Contact Karen Liu, PhD:

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