The Story of Purple-Roof

by Oscar Warmerdam on Thursday, December 6, 2018

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Why Purple?

Everyone else seemed to be doing green, or blue, and we wanted to be different because we are fundamentally different. We wanted to do purple.

Purple is a memorable color.

What made us start this journey?

Our team of Richard Hoek, Dick Bernauer and myself had been successfully supplying green roofs with Sempergreen Sedum mats for over a decade, and we had to learn the hard way what works, and what doesn’t.

The benefit of offering a performance warranty on our own product is that any failure is one that hits our own bottom line. This forced us to learn how to improve our product or simply go out of business. So, we improved it.

That has always been my deliberate intent: if you do not stand behind your product, you cannot be a leader. And many, many failures later we got pretty damn good at this game!

To solve problems, we had to visit hundreds of green (sometimes brown 😊) roofs. We became better growers with a plant assortment that was different than what was normally used in Europe and which we modified for the climate extremes of the US.

We also became experts in green roof soils.

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Soil biology – the forgotten key to a successful green roof

Green roof soil media was the biggest culprit that initially needed an improved spec. We started to understand and respect soil biology. These learnings resulted in us starting a green roof maintenance company that showed us how green roofs evolve over time and what attention and resources they require.

We started maintaining our competitors’ roofs as well, which taught us some valuable lessons: what to do, and what not to do. There is a lot one can learn from one’s best competitors.

Nevertheless, we started to feel that we could do more. Based on our long experience in the field and our utter fascination with the topic, we began research incorporating mineral wool for higher retention and better plant performance.

Three triggers accelerated our desire to become THE expert in green roofs:

  • Trigger 1:
    Let’s begin way back as the green roof industry started some 20 years ago by a group of plant-loving (Ed Snodgrass), engineering (Charlie Miller), and roofing experts (Matthew Carr). Over time these guys became my genuine friends. I admired their vision and determination to bring green roofs onto the US market, but it also seemed ‘complicated.’

    They charged what appeared to be a lot of money because they considered each project unique as individual solutions were designed for every project they undertook.


These high-cost projects and what appeared to be complicated solutions were an opportunity for the 2nd generation of green roof industry suppliers (us included) who dummied down green roofs and, more or less, promised simple one-size-fits-all mouse-trap solutions.

Without realizing it, these 2nd generation suppliers commoditized the industry and what happened next is not favorable. As the industry grew, many companies offered easy cookie-cutter solutions like Sedum mats and green roof trays that allowed anyone with two hands to install it.

This bypassed the landscape industry so we all could save money. We all told our audiences that green roofs do not require any but minimal maintenance.

Sure, in some northern European climates that might be the case, but the US is not Germany, and as a result the failure rate of green roofs sky-rocketed. In Chicago and Washington DC, the failure rate of green roofs is north of 50%!

What other industry would possibly thrive with these performance numbers?

  • Trigger 2:
    At one point a client said that he was frustrated that he frequently gets visited by all these green roof industry characters, and they all bad-mouthed each other!
    All of them claimed that the other systems were terrible, but if you choose mine you will be OK!!

    He said: “Oscar…who to believe???

  • Trigger 3: Companies offered the same solution in Washington DC, in Toronto, in Atlanta, as in Chicago. But how is this possible?! All these cities have different climates with different native plants specialized for these particular conditions. Yet, green roof companies claim that there is a SINGLE solution that works everywhere?

    That kind of information is easy to digest, but it is false, and it does not serve the industry well. Various companies had been making outlandish claims for years, but this became amplified, particularly in the Washington DC area, around 2012-2014. The inconsistency between how data was collected and presented to the design community was staggering.

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The testing facility – the creation of the STORMWATER LAB

So, my colleague Jordy De Boer (brilliant super handy ‘engineer’) and I set out to build a small testing facility. We increased the size and complexity three times which then blew up into a fully-fledged state-of-the-art $280.000 stormwater laboratory.

(That is what happens if you give two guys a dream, an ocean container, a welder and no budget).

We wanted to lay this retention argument to rest. We wanted to know what materials hold water, and what materials do not.

Water retention is what drives the industry, and plants need water to survive. Hence, it was only natural that this was the focus of our initial research. Often you see that research focus is placed on the mechanical aspect of retention, and less so on what the plants need.

As a result, you see green roofs with documentation that ‘proves’ they retain water, but the components that hold the water frequently do not support plant life as expected. Thus, we upped the ante on more rigorous research to test all of the most popular products in the industry. We wanted to determine which products met or didn't meet their claims, not to put others in a bad light, but to be able to point out the best solutions for the plants.

There is no industry association to do this, so we had to do it ourselves.

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Preparing our unique stormwater laboratory

As with most good research projects, we discovered new things along the way; we discovered that drainage layer retention cups are not that relevant after all. We were surprised to see that the retention offered by green roof trays is usually 50 % of what is promised as they have gaps that allow water to escape too easily. The situation is often further worsened by perforated trays or partial sidewalls as it allows the water to escape very easily.

The irony is that most of the answers in proper green roof design can be found in nature. Plastic solutions rarely meet or exceed the functionality of these natural systems, but an industry driven by manufacturing of hardgoods seems too often place the horse behind the wagon. This industry wants to sell plastics, and these well-intended miracle products usually do address a specific purpose, but rarely all that is needed.

And as with all solutions, it needs to be applied within the FULL context of a specific green roof for it to succeed. Sometimes it is a great solution in project A, but not in project B.

Plastic manufacturers are experts in plastic, nothing wrong with that. Their products often excel at a SINGULAR TASK.

Plant nurseries (like us) care about plants, and these plants depend on the complete SYSTEM that is underneath them. They cannot perform on a poorly designed roof. Plants cannot grow on poor soil that more closely resembles a gravel driveway, and they cannot grow on drainage layers that don’t drain or drain too well.

Likewise, plants cannot grow on trays that lose 50% of the annual rainfall. Less than 4” (10 cm) profiles can be successful, but not in all locations.

Plants cannot grow long-term without some level of minimum maintenance. The list goes on...

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Soil biology and other missing links

Who else in our industry watches out for these little guys!? Who speaks out nationally for the sake of the plants, and by extension the green roof buyers who want sustainable and durable roofs?

Who makes sure that all green roof components fit together like the musicians of an orchestra? All parts in sync, so the whole produces a fantastic musical piece. Unforgettable, durable, timeless.

Whatever solution is applied, plant happiness or plant performance is the music. This music should be our main focus as a green roof, first and foremost, needs to be green, easy to maintain and live for 50+ years.

If retention is the reason for buying the green roof, retention needed to be a priority but applied in harmony with the vegetation, not in isolation of the vegetation. If aesthetics and design are critical aspects for your roof, then chose plants that survive in a shallow profile (few do).

Regardless if you design a green roof for beauty or “just” stormwater management, the roof needs to fulfill those three aspects: stay green, be fully vegetated at all times to protect the soil quality, be easy to maintain, and survive for decades.

A green roof is a harsh environment. It has a super shallow ‘unnatural’ 2-24” (5-61 cm) soil base made up with coarse ‘unnatural’ soil medium that sits on top of a plastic drainage layer. This is, of course, a huge contrast with what we find in nature. To be frank: it doesn’t exist in nature at all.

Eventually, we started to look at a green roof as a ‘complete living system’ and started to see soil as a critical component supporting plant life.

This realization completely changes the dynamics of the green roof game.

If you emphasize soil quality, and thereby soil biology, the green roof (the music) starts to perform almost effortlessly.

We discovered how to maximize retention with products that not only retain water for stormwater management purposes but also do so for the benefit of the plant!!

We discovered how a resilient green roof relies not on one or two pillars (such as retention). If a green roof should perform well long-term, it needs to be perfectly balanced. Thus, we began to fashion recipes for how to achieve this. Through repeated trials and coordination with other experts, we started to stake out the way.

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Trials and errors and hopes for the future

A typical Sempergreen presentation has 120 slides, five are about our company, and the remaining 115 are about what worked, what didn’t, and the failures and mistakes we made or noticed elsewhere in the industry over the last 15 years.

What a careful reader might have observed is that we started to re-introduce some of the complexity expressed by our industry ‘grandfathers’ that we once tried to eliminate.

Only a fool cannot admit his mistakes.

The data we drew from the stormwater laboratory combined with the knowledge we have acquired over the decades is what drives us forward.

Our lab and approach are unique, more specialized, broader and deeper than any other resource out there.

We began evangelizing the benefits of this approach by revealing these "recipes" to various architects, civil engineers, and others in the industry.These’ recipes’ were not selling a proprietary system; we simply said: hopefully you will buy our products, but regardless if you do, please design the green roof that is built on the experience and lessons of our failures and you will have the best damn green roof imaginable.

Designers had an overwhelmingly positive response but demanded a complete specification.

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Our non-proprietary specification called PURPLE-ROOF

Having a green roof assembly specification was somewhat antithetical to the goal.We wanted to convey an approach, some recipes, and let the market sort out who had the most clever approach to marketing, packaging, delivering a product that met those specs.

Our first approach needed some work. Designers didn’t like it that we told them that we could only provide them specifications for sedums mats, but they needed to contact a soil company for soil, and then contact a hard goods company for a drainage layer.

One designer was flat out angry:
“First you tell me what I need to do, I like it, and then you tell me to figure out the details for myself! How do I know the end result is what you intended to achieve? This is not helping me, Oscar!!”.

So, Purple-Roof was born.

It’s a non-proprietary spec. As every ‘thing’ it needed to have a name, so we started to refer to it as “Purple.” To be clear: we do not sell a Purple-Roof Assembly. Purple-Roof is a recipe, built on experience with a seal of approval.If you build it according to Purple principles, your green roof will succeed, effortlessly.

To make procurement easy, there are a broad range of informal partners who provide Purple-Roof-compliant assemblies, but anyone who sources materials from anywhere can potentially get their green roof certified as a Purple Roof - if it meets the specs.

Purple-Roof is driven by science and unmatched field-experience and certainly not introduced because we think we are smarter than the rest of the industry. We do however claim to know better how all these products work together then the rest. This is because of our extensive research and decades of experience.

Please, don’t hesitate to contact us if you have any questions or would lke to discuss research with us!