Data-Driven Green Roof Research

by Dr. Anna Zakrisson on Monday, November 18, 2019

Go straight to Purple-Roof Green Roof Modeler

Purple-Roof DETAILS

The modeler is based on data from Green Roof Diagnostics.

Independent green roof research: Green Roof Diagnostics

Green Roof Diagnostics is a consortium driven effort that funds independent R&D that aims to help the green roof industry properly analyze, quantify, model, and test individual green roof components and various combinations thereof.

In 2016 GRD build the basic laboratory in Culpeper, Virginia, USA, in order to fully understand how multiple layers of green roof components affect the overall retention capacity of a green roof. Green Roof Diagnostics wanted to move away from single layer lab testing and look at fully assembled situations only.

Then in 2017, GRD recognized that traditional vegetated roofs are not causing a substantial time delay (detention) once the green roof was saturated from a prior storm and has focused on Detention based R&D since.

The green roof industry is in dire need of reliable retention and detention data to gain trust and provide policymakers with more than just hunches.

Green Roof Diagnostics Industry Relationships

What is GRD’s relationship with Sempergreen and other companies? Though the concept for GRD came out of Sempergreen USA, GRD operates as a completely independent entity.
This is because Oscar Warmerdam (CEO Sempergreen, USA) realized very soon after beginning green roof research that the work of GRD had potential far beyond the reach of a single company and single product line, and that this work would be better performed by an independent company with rigorous research oversight.

There are many organizational methods to try to tackle important research: the most common being university-funded projects, and industry associations being another.

GRD is taking a different approach by attempting to bridge the divide between academia and industry. GRD’s industry partners know that not only do they benefit by investing in new technologies, but the industry benefits as a whole. GRD’s academic partners know that not only does GRD’s work meet academic standards, but the outcomes of that work are deployed onto real projects that start accumulating real benefits quickly. GRD believes this is a needed path forward, as our world is in dire need of creative solutions.

Green Roof Diagnostics funding transparency

Green Roof Diagnostics is financially supported by the captains of the green roof industry: Sempergreen (SG USA, and SG Europe), Knauf, American Hydrotech, Green Roof Specialty Products, Uniseal (Asia), and Next Level Stormwater Management (Canada). All are worldwide companies and/or industry leaders and determined to offer high integrity solutions.

Green Roof Diagnostics is supported by an advisory board to ensure high quality and reliable research:

Charlie Miller, Founder of Roofmeadow, Dr. Elizabeth Fassman-Beck, Professor at Stevens Institute of Technology, Toine Vergroesen MSc., senior researcher hydrology at Deltares, Dr. David Sample, Professor and Biological Systems Engineer at Virginia Polytechnic Institute and State University, Peter Maybach, Soil Consultant at Brookside Laboratories, Scott Jeffers (stormwater engineer) and Virginia Stovin, Sheffield University (UK).

Vegetated roof retention data

GRD is now able to provide an extensive retention data set that is able to answer many of the irking questions surrounding the industry for the past decades.

Maximum retention is determined by the ET (evapotranspiration) rate of a local climate.

GRD has developed a hydrological model to determine the amount of water that leaves a green roof in the form of vapor, and how much leaves as excess outflow. This should be the basis of any green roof design to maximize resource use.

However, despite the retention dataset and modeler being able to answer many of the questions that were relevant on a city scale, it did not bring us the expected enthusiasm from civil engineers to appreciate our newly established assembled retention-based data.

The engineers we spoke to kept on saying that they are less interested in how much we retain; they were more interested in the water we did not retain; the 50% of the water that cannot be held by the green roofs.

Civil Engineer Joshua Robinson and Charlie Miller pointed out the issue for us: Retention-based solutions are not valuable during 2nd days storms. A 2nd-day storm event is where the retention capacity of the living roof has been fully saturated already on Day 1, so the rain volume of Day 2 passes through the system like water through a wet bath sponge; The vegetated roof acts like a sponge on day one but as a pipeline on day two.


Retained water that leaves as water vapor. Retained water never becomes runoff.

Detention deals with delaying the outflow of the runoff.

Green Roof Diagnostics data used for Mexico City airport project

While working on this problem, we came across a project at Mexico City Airport project where it rains for five months almost continuously and often 10-20 days in a row.

Despite this, retention-oriented solutions were still specified by a firm despite hard data showing that retention-based solutions were not sufficient for stormwater management, especially in places like Mexico City.

This is when we set out on a quest to find a solution. The ideal green roof would be able to retain as much as possible and detain any of the excesses it couldn’t retain. We wanted to find a way to deal with 100% of the water and not just a part of it when they were dry or partially dry.

GRD tested 71 different layers and assemblies to try and slow down and detain the outflow of a fully saturated green roof system while testing profiles wet-to-wet, i.e. 2nd-day storms.

Finally, in the late fall of 2017, we found a product that indeed achieved this. We call it the Detention Layer (GRSP). This product is 5mm in height, and has 280 vertical threads per cm2 (1800 threads per sq. inch) that create lateral friction underneath the green roof during large storm events, which in turn creates a rise in water table in the green roof profile as it fills up all the macropores of the profile with temporary water storage volume.

To add extra storage space, GRD researched the use of a layer that holds water in the vertical tubes while preventing sideways flow. This later received the name "honeycomb layer" due to its resemblance to giant bee honeycomb cells.

These are the products that became the core of the Purple-Roof™ detention concept.

However, we must clearly state that GRD is an independent research entity, clearly separated from Green Roof Specialty Products (Purple-Roof).

Green roof modeling

We have gathered a treasure trove of green roof data, and now, the exciting journey continues; finding the patterns and understanding the systems. GRD is currently focusing on modeling and parametrizing this data so that we can predict green roof behaviors and climate adapts green roofs for optimal performance in different locations around the globe.

Current projects also include testing, predicting, and modeling the temporary macropore storage capacity of green roof profiles, as well as the flow rate behavior at different path lengths and different drain edge openings so that the combined effort can be calibrated and modified to reach site-specific detention goals.

But this is only the beginning… and we are continuously collecting data from our rain laboratory and outdoor platforms to create the world’s most comprehensive green roof data set.

Green Roof Diagnostics and testing protocols

GRD specializes in innovative testing protocols not yet adopted by ASTM or others. Many of the ASTM protocols used for green roofs were not strictly intended for these purposes. We are in the process of finding existing or developing new, more suited protocols for green roofs to bring more consistency to the industry.

Our current focus is on developing protocols to estimate retention performance and estimate the detention storage capacity

Green Roof Diagnostics, water quality & plant health

Water quality is also an important part of our research plan, with focus on phosphorus and nitrogen runoff loads.

We are also carrying out evapotranspiration research for crop coefficients of mixed sedums.

If you are interested in our data or have questions or collaborative suggestions, please let us know!

GRD is currently expanding the monitoring activities across three continents.

info[at], or through the GRD contact form.

Contact person: Brad Garner