Green Roofs for Successful Stormwater Management in Boston

by Oscar Warmerdam on Tuesday, August 13, 2019

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Green roofs in Boston

Purple-Roof at the Climate Resilience Summit in Boston, Part III

Boston is a city deeply affected by water-related issues and is in dire need of innovation and functional solutions to both new and old problems. Green infrastructure is one such solution that also brings with it numerous secondary benefits such as mitigation of heat island effects, or pollution, higher biodiversity, and longer roof membrane lives.

Still, no one at the Climate Resilience Summit in Boston mentioned vegetated/living roofs as a stormwater management solution!

How can it be that we have failed so utterly in our communication as an industry?!

Part I and Part II in this series:
Boston – a Drowning City?
& Digital Twin Cities – the Magic Mirror of Stormwater Management

What stormwater solutions are available?

There is a range of stormwater management and tidal protective solutions available. Some of these solutions have been implemented for buildings in Boston. Some developers place their ceiling on the 1st floor at 5.5m (18ft) tall so they can raise the floor level a few times, up to 2.7m (9ft). Others put inground mechanical barricades around their buildings that at a push of a button make a fortress out of a building similar to what they do in Singapore.

One uplifting part is what the city of Boston intends to do. The Boston mayor and his team want to use the natural landscape such as dunes and dikes, wetlands, oyster banks, to be the main solution to the rising sea level. This was a truly inspiring initiative. However, the water threat isn’t only coming from the ocean, but also rain-induced floods. These events, in combination with the changing sea level, are what makes up the grand problem.

The green roof industry and stormwater management

At the summit, we heard a range of truly great solutions. Most were large with 10-100 million-dollar budgets. Albeit, not a single time in three days did anyone mention vegetated roofs as a solution!

What a terrible job have we done as an industry, as companies that sell them, that we are now 15-20 years old and we as suppliers completely missed the boat!

No one cares about green roofs. No one talks about green roofs. Actually, no one talked about the roofs at all! How can this be?! In an urban setting, the roof surface is 20-50% of the city. This is the lowest hanging fruit. Roofs offer open space without much alternative use. This space could be used for stormwater management.

Green roofs completely ignored as stormwater management tools!

At this moment we stood up in the room. I declared myself as “Oscar Warmerdam from Purple-Roof, and our company manages stormwater on your roof.” Silence. I asked everyone why they are not managing water on roofs? Everyone knows that the best way to fight stormwater is not at the bottom of the stream, but at upstream.

If you manage, in this case temporarily hold, water at the top and release it slowly, that you don’t have to deal with it at the bottom because rapid moving downward water always bottlenecks towards the bottom. We said: “if we hold that water on the roof until low tide, there is plenty of receiving-capacity to release it into. We just need 6-12 hours of delay”.

So how did we do this? “Well,” we continued. “We offer a range of solutions that are primarily detention based because the solution needs to work EVERY time.

Most storms come in clumps, which we identify as 2nd-day storms. Blue-Roof technology can hold water on the roof, albeit without a volume reduction. The water is merely delayed. All of it will eventually go down the drain after the storm, but it will flow slowly and at a later time.

Retention plus Detention green roofs

Purple-Roofs can do the same things, we can delay water outflow, but Purple-Roof has the additional benefit of retention, which means that evapotranspiration can bring about a volume reduction up to 70% of annual rain.

That means Purple can be a lot more valuable than Blue, but there is a time and place for both. Check out our article on the various detention-based roofs that are available on the market and how important it is to have every solution on available on the market: Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water.

  • Blue-Roofs consist of a restricted orifice on a naked or gravel-covered roof, or a roof covered with pavers with water underneath.
  • There are also intensive Blue-Green roofs with smart technology that has 10-30cm (4-12”) storage capacity combined with pre-programmed release rates and water wicking capacity to reduce irrigation needs and further improve the annual evapotranspiration rates.
  • Purple-Roof is in the middle with what you know as an extensive green roof with 2.5-10cm (1-4”) detention capacity.

With 20-40% of the footprint of a city covered with roofs, this is indeed low hanging fruit. I also suggested that all residential homes would need detention-oriented rain gardens and/or rain barrels that slowly release the water over time (over 6-12 hours). After some silence, there was instant positive feedback: “This is easy, we can deploy this right away!!!” “Can you do retrofits???” To me, this was shocking. Why did no one think of the roof?

We had all stormwater departments in the room at all levels, from design, permitting, planning, sewage, transportation, climate resilience, and hands-on city workers that clean up the mess after the storm.

As an industry, we have wasted 15 years.

Recognition needs to go to Charlie Miller because he did see this 20 years ago but articulated this within a non-engineering green roof industry, one that was obsessed with color and commoditizing the opportunity, not with stormwater management.

But, as I am ever the optimist, I see opportunity...

How do we cover the Boston buildings with living roofs?

So what to do now? How do get green roofs on the agenda at every single resilience meeting globally? Yes, we should set the bar that high because that's what this technology deserves. Green/living roofs are a fantastic tool to increase the wellbeing and quality of life of our urban population as well as a critical factor for efficient stormwater management in crowded urban places. Note that the new green roof generation is able to both retain & detain stormwater which satisfies project stormwater goals.

We like to think of this process as a three-step-rocket

Step 1 is to get noticed

Let's stick our heads out there and talk green roofs. We do this the best in unison, together, as an industry. Let's make an impact!

Step 2 is to have real solutions with real engineering

More and more data is currently being produced supporting new ideas and innovation. The data gives proof to old wisdom which can then be easily communicated outside our business community. In the end, numbers talk.
The data might also kill some old assumptions, but we should see this as something positive as it enables innovation and progress which is needed for long-term sustainable business growth.

Step 3 is to analyze the impact on a city-wide scale, neighborhood scale, and on a granular scale

This last step is extremely costly, but companies like Deltares, which is the largest non-profit civil engineering laboratory in the world, has such technology fully functioning & ready for use.

Deltares Modeling Software

Deltares advise cities across the world with this software as it can zoom out to the city scale, zoom in to the neighborhood-wide, or even zero in on a single site/building. Interestingly, green roofs have not been included in their software modeling.

Consider this, the largest stormwater engineering laboratory in the world, and they have never suggested traditional green roofs. Their software considers bioswales, sand filters, tanks, soil infiltration, slope/topography, any other solution but not green roofs. The reason for this is that they knew green roofs couldn’t detain, but even retention had been a hurdle. Deltares simply had not been able to find reliable green roof retention data anywhere in the world, and they have largely neglected the green/living roof option for that reason.

The future is green!

We strongly believe that the future belongs to the green roof industry. We also strongly believe in collaborations and that there is plenty of space for many types of green/living roof solutions on the market. We believe that we will soon have more green roofs in Boston!

If you have any questions or comments, please, contact us at info[at] or through our contact form. We would be happy to have a chat!

Go directly to Purple-Roof Green Roof Modeler

Purple-Roof is a NON-PROPRIETARY specification for the optimal green roof. The Sponge Roof offers excellent retention whereas Purple-Roof adds detention on top of retention making this an excellent stormwater management. By intercepting stormwater on the roof, you can get rid of expensive tanks and cisterns and free up urban developmental space.