Urban Development: From Gray-to-Green Roofs

by Oscar Warmerdam on Thursday, December 5, 2019 updated Tuesday, April 26, 2022

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Why do developers and civils resort to outdated gray infrastructures such as pipes and tanks?

Green infrastructure comes with so many direct and secondary benefits. Nonetheless, the transition from gray-to-green is moving at a very slow pace, especially when it comes to green roofs.
We started to ask the different personas involved in construction projects to try to understand where the issue keeps hanging.

What is hindering this critical transition?

We started by asking developers how they deal with stormwater on their projects. In almost all cases they, per automation, mention gray infrastructures such as stormwater pipes and storage tanks. More than often, they told us how they plan to use naked, non-vegetated roofs.
Why? We asked. In almost all cases they replied, “It’s what the civil recommends”.

So why is the civil only recommending gray infrastructure?

We went to the civils and we asked them the same question: “Why gray?” We wanted to know how come they recommend naked roofs, pipes, and tanks to the developers when there are cost-effective green options available?
The civils generally answered: “Well, that is what the client asked for”.

Why are green roofs not more interesting for the civil engineer? Why are green roofs not a tool she would consider?

In order to change things, it was important to understand more. Our task was to change the civil engineer’s default from “grey” to “green”!

Green roofs should not be seen as a “novelty item”

Green roofs should not merely be a novelty item only installed if the client requests it or because the city tells them that they have to install one. Green roofs should be something necessary that brings value to the single project beyond policy or benefits for the city.

Vegetated roofs provide many secondary benefits to our urban environment: reduction of heat island effects, reduction of the volume of sewage that needs to be treated by the city’s sewage treatment plants, increased urban biodiversity, pollution mitigation, and much more. It is a technology that will make our current and future urban environment enjoyable and healthy.

Why is this incredible tool not an obvious choice for civil engineers?
What did we miss?

Green roofs - a reliable stormwater tool?

For several years, we had completely missed to take in what the civil engineers tried to tell us: that traditional green roofs cannot be used for stormwater management from their perspective: from the project-perspective.

To summarize the issue: A dry vegetated roof absorbs a lot of rain, but when the roof is soaked, it simply cannot absorb any more. It sits there like a wet sponge. One drop in, is one drop out.

This isn’t good enough for a civil engineer as she cannot propose a stormwater solution that only works some of the time. The stormwater tool must work every time or the property is flooded during the first major rain event.

This is where the catch is.

We needed a green roof that could fulfill these requirements.

Defining the problem and designing a new type of green roof

In 2016 Green Roof Diagnostics built a stormwater laboratory that measured green roof performance from a civil engineer’s perspective.

As expected, the traditional green roof did not perform well for stormwater purposes: when the green roof sponge was full, it simply didn’t work as a stormwater management tool.

This is where we saw an opportunity and a few years later, we had found a solution.

A detention-based green roof is almost like placing a bioretention cell on top of a roof. This de-synchronizes the storm peak from the outflow peak and hence make it a real stormwater management tool that the civil engineer could use on the project scale.

How does a friction-based detention roof work?

The outflow speed of a saturated traditional green roof is typically maximized by the drainage layer, and the job of this antiquated layer is to expedite the water off the roof. Green Roof Diagnostics changed this completely by testing and identifying layers that can slow down the outflow below the green roof profile such that the assembly now stores the maximum amount of water in the macropore space of the profile.
It turns out it was possible to enhance this macropore space even further by putting a 2.5-5cm (1-2”) layer in between the detention layer and the mineral wool in order to increase the temporary stormwater storage. Since this layer looks a bit like the honeycombs of bees, it got the name “detention honeycomb”.

These modifications ensured that it was possible to hold and release the stormwater over 6-12 hours at low and predictable flow rates, past the peak of the storm.

How detention creates a green roof ROI

Imagine how fast the transition from gray-to-green will go if green roofs are supported and suggested by every civil because they see clear project-based benefits and other players see a clear return of investment.
A detention green roof, whether a Purple-Roof friction detention roof or a smart blue-green roof like the excellent solution from Metropolder Company, creates a return of investment for green roofs.

These detention roofs ensure that the building owner can abandon outdated and space-consuming gray infrastructure and instead manage stormwater on the roof.

One less tank means more space available for development and this generates revenue that pays for the roof in urban settings.

It is also good to note that there are many different types of detention roof solutions on the market all with different benefits and constraints. The unique selling points for the Purple-Roof concept version is that is inexpensive to install and can be used on sloped roofs.

The future is green!

Top-down approaches such as policy changes combined with bottom-up approaches like creating a clear ROI for green roofs promise fast greening of our urban areas.
We are on a mission to help to create sponge cities for a resilient future using data-driven design.

Please, don't hesitate to contact us if you have any ideas, comments, or want to collaborate!

Also, check out the Purple-Roof Green Roof Retention Modeler! With only a few clicks you can find the optimal setup for a green roof in your climatic conditions! Based on data from Green Roof Diagnostics. Let us know if you cannot find your city in the modeler by contacting us either via our CONTACT FORM or through info[at]purple-roof.com.

Read also:

Mineral Wool on Green Roofs - Compaction and Toxicity?
Why Purple-Roof Was Founded