Green, blue-green, blue-gray, purple… yes, the green roof industry is colorful. Almost as colorful as the foliage on our roofs. However, this diversity is important if we are to cater for the diversity of building structures, climates, local regulations, and economic realities of our clients. It is crucial if green roofs are to become a reliable and attractive option for stormwater regulation, as well as tools for heat island and pollution mitigation in our future cities.
Why one-size-fits-all doesn’t cut it anymore
Long gone are the days of the one-size-fits-all mentality. Green roofs have the potential to be major players in our future cities as stormwater management-, heat island reduction-, and pollution mitigation tools.
As an industry, we have huge opportunities ahead of us, but in order to capture these opportunities, we need diverse products.
A one-size-fits-all is great for a subset of projects, mediocre for many, and might be a terrible solution for the rest. The ones failing may tarnish the reputation of the entire industry and reduce the acceptance levels of green roofs as stormwater management tools. This may hamper the growth of our entire sector.
Green roof history
Architecture and landscape architecture have really been pushing the envelope on green design the past several years. 20 years ago, green roofs were quite a novelty. 10 years ago, the options were mostly restricted to extensive green roofs, intensive green roofs (or planters), and a few types of green walls.
Now we are seeing a huge demand for diversity. There are projects that blur the lines between green roofs and green walls; skyscrapers that are planted like forests. And we see demands for green in places where it was unfathomable even a decade ago.
Demands for design-diversity are rising but so are the demands on green infrastructure performance such as stormwater management as well as urban heat islands and pollution-reductions. The healthy response to these demands is diversification within each sector of the green infrastructure industry.
We think we are just at the beginning of this diversification.
Let’s take a quick look at what’s currently on the market!
Do you want green, blue-green, or purple?
That all depends… there are a few options out there!
When to go for a green roof
Sometimes, a client wants a green roof for visual purposes. It should look pretty, and their main objective is PR or branding. They may also gain from the cooling benefits of the roof due to evapotranspiration. This cooling may result in significant cost reductions in building energy use.
The client may also want a stormwater volume reduction tool. Remember that a standard green roof is not a true stormwater management tool as it only retains about 50% of the average annual rain volume which corresponds to about 80% of the rain events. However, the volume reduction it provides still gives cities huge benefits as less water must be cleaned in the sewage treatment plants. Sometimes, just like in Washington DC or NYC, the city requires or incentivizes the implementation of green roofs.
Also, we should not forget other positive aspects of green roofs such as increased biodiversity and the positive effects on mental health that have been observed as we are bringing more nature into our urban environments.
A green roof is a great thing!
Detention-based green roofs
A detention-based green roof offers something standard green roofs do not, and that is true stormwater management. Retention is still provided, but the roof also stores water temporarily which allows the roof to deal with the remaining 50% of the rain volume that is not captured by retention
A detention-based roof provides a clear ROI as instead of storing water in bioretention tanks or cisterns, the water is temporarily held on top of the roof allowing for better use of developmental space such as enabling revenue from extra parking spaces or other business.
Detention-based green roofs reduce peak runoff and are thus putting less stress on the urban sewage system allowing it to drain and prevent combined sewer overflows.
Detention-based green roofs add this aspect on top of retention.
There are two main groups of Detention Based Green Roofs:
When to go for a blue-green roof
A blue-green roof can be a great solution if the roof is completely flat and the structural requirements allow for the storage of a lot of water on the roof. It is a great stormwater management tool. However, a blue-green roof can be more expensive to install and maintain than the lighter weight Purple-Roof. But a blue-green roof is also a great solution for the right project.
When to go for a Purple-Roof+
Purple-Roof+ is also a detention-based green roof but tends to be less costly to install and maintain than a blue-green roof.
Purple-Roof+ can be installed also on sloped roofs, which makes this detention-based solution a great alternative for roofs that cannot be equipped with a blue-green roof.
How to control the whole building
For some buildings, the optimal solution may be a mix of different types of roofs. A green roof may be suitable for some parts of the building, blue-green for others and purple for the rest. Even blue-gray solutions may be the best option if it is known that vegetation will have a hard time surviving in certain locations such as under vents, in wind uplift areas with pavers, or underneath HVAC equipment on the penthouse deck.
In the end, what matters is the transformation from a black, non-vegetated roof to a green roof. How this happens is irrelevant as long as the solution fits the situation and the client.
A stable, resilient roof that meets the requirements of the civil engineer, the architect, landscape architect, and regulators is what will give our entire industry a good reputation. If we achieve that, we will all gain from it.
Don't hesitate to contact us at Purple-Roof if you have any questions, comments, or would like to discuss projects or collaborations!
Purple-Roof is a NON-PROPRIETARY specification for the optimal green roof. Purple-Roof offers excellent retention and 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.