Purple-Roof Living Roof Biology Explained

When developing the Purple-Roof concept, we paid careful attention to the entire green roof profile ranging from plants to drainage, from engineering to biology. All our decisions and recommendations are based on solid research provided by Green Roof Diagnostics

Read more about the Purple-Roof classification of green roof criteria

What is Green Roof Biology?

We like to imagine the green roof system in four parts: drainage, retention, soil, and vegetation. The different green roof compartments work together to form a well-functioning unit: a truly resilient and lush green roof that requires minimal maintenance.

A vegetated roof should always be designed based on your local climate. We have developed a green roof retention modeler to help you do this. Let us know if your city has not yet been added to the modeler!'

We talk a lot about Detention on this site, but that’s not everything we do. After all, remove the biological parts of the roof: the plants, and the soil, and the roof it’s not a green roof anymore.

Also, green roof biology is critical for roof functionality. All live roof parts must work in unity and should be carefully considered and tested. One example: green roof plants are the primary driver of evapotranspiration and this, in turn, determines green roof water retention capacity. But if the soil drains too well and doesn’t hold sufficient water for the plants, there will be no water to evapotranspire and the plants will die...

What is Green Roof Soil?

Green roof soil has often been an overlooked compartment, this despite being the very foundation for plant survival. The soil is where plants retrieve nutrients and water and where the roots anchor themselves to achieve stability.

Green roof soil is often referred to as “green roof media” to distinguish it from real soils. The green roof soil media is engineered to follow certain guidelines, mostly the FLL guidelines. The FLL guidelines have helped to standardize the green roof soil industry but focus merely on weight and flow-through rates with very low organic components. The biological possibilities have not been fully explored. We have exciting times ahead!

Green Roof Soil Biodiversity

Why do we care about soil fungi, bacteria, nematodes, and other organisms? It’s because they do a fantastic job of getting nutrients to the plants and ensuring that the soil has the right structure. The best thing is that they do this for free without any extra costs or expensive products!

A biodiverse soil community also means that nutrients are held up in multiple compartments instead of just being loosely associated with the soil. This results in lower nutrient leaching and better recycling of nutrients.

These guys help you save on nutrient applications and ensures that your local lake is not flooded with nutrients that trigger toxic algal blooms.. This is how natural soils function, so we know it works. It has been tried and tested by nature across eons.

A green roof is an engineered ecosystem, but we should aim at working with nature and not against it. Our goal is to drive green roof soil research forward to ensure biologically diverse and stable soil communities and efficiently recycled nutrients.

What About Green Roof Plants?

Select the right plants for the right climate, but also be aware that a vegetated roof is an extreme environment. Many local plants might have a hard time surviving in the extreme green roof environment without extensive irrigation.

A good solution for extensive green roofs is to cover the soil with a diverse sedum mat to protect and cover the soil and then interplant perennials for diversity or aesthetics. This ensures short-term fast coverage and long-term plant health through increased soil ecosystem stability.

Sedum mats contain a mix of around 15-20 different types of sedums with various characteristics. Sedum plants are very drought tolerant and will keep the roof alive even during dry summer months. We often like to think of them as "the camels of the plant world."

You can read more about sedums and their fantastic adaptations in our article: Green Roof Plants - Masters of Survival

If you are interested in finding out more about how to select sedums for green roofs, take a look at this article: Guide: Sedum Green Roof Plant Selection

If you have any questions, don’t hesitate to contact us! Info[at]purple-roof.com or through our CONTACT FORM

Further Reading: green roof soil

We have published a range of blog posts on the topic of green roof biology. You can find a few of these below. These blog posts also include links to peer-reviewed academic articles for further reading.