Why You Should Pay Attention to Green Roof Soil

Ever tried to sit on a chair that’s missing a leg? Wobbly?

Well, the same is true if you try to design a green roof and forget to pay attention to all four pillars of green roof sustainability: retention, drainage, vegetation & soil. Forget one, and the system simply won’t function as intended; it starts to get wobbly and might fail completely.

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Green roof soil and the Purple-Roof quadrant diagram

Green roofs have not been known for their great soils. Green roof soils are often not even referred to as “soils” but as “soil media” or simply “media.” And this is a missing leg. Fix this leg, and we will be able to produce and design more stable and resilient roofs.

Below is the Purple-Roof quadrant diagram. Our quadrant diagram is there to act as a reminder never to forget to consider all aspects of a roof. This is especially important now as we live in a world with increasingly unpredictable weather and a changing climate. Roofs with missing legs will fail.

It is also critical that we adapt our green roof designs to local climates. It is plainly ridiculous to claim that a one-size-fits-all roof will function as well in temperate Baltimore with 114cm (45 inches) precipitation, spread across few and large storms, as in Cleveland with 99cm (39 inches) mean annual precipitation spread across many smaller rain events. Or why not compare with tropical Singapore with 234cm (92 inches) mean annual rainfall.

These are vastly different climates and this puts entirely different stresses on the roofs. We need to be more clever than this and develop green roofs that are adapted to the local climate.

Green roof soil – a missing leg

I believe that the soil compartment has been neglected for one simple reason and that is history. STEM has previously been separated into different subject matters, and cross-collaborations used to be rare. As green roofs traditionally have been an engineer’s game, few biologists have been involved in their development.

Excellent minds have developed these roofs to become the great volume reduction tools that they are. But we think the time has come to take this a step further; to create Green Roofs 2.0. The two main topics of focus from our perspective are green roof soils and detention. We call for cross-topic collaborations across the industry to achieve this.

We firmly believe that green roofs will become a standard stormwater management solution, and heat island and pollution mitigation tools for all major global cities over the coming decades.

So much opportunity! Let’s take a closer look at green roof soils!

Why green roof soil is key

Current green roof soil media is very porous and is not developed with plant health in mind. FLL has been the main driver in the US. FLL is written around concerns about building weight constraints, with virtually zero focus on plant happiness, performance, or soil life. FLL looks at soil as marbles of different sizes, we look at soil as life, of course still with the weight constraints in mind.

The past strategy has been to define certain engineering parameters, and if the soil media fulfills these, everyone is happy…mostly …except the plants.

If the soil is too porous, even the tough sedums will have a hard time surviving long periods of drought. Too little water is held by porous soil and the plants may not have time to refill their internal water storage if it drains too fast.

Green roof soil and nutrients

While it is true that you can sustain sedum growth on mineral wool alone, we shouldn’t forget that plants need more than just water. They need nutrients as well. Also, “sustained growth” is vastly different to “thriving”. We want the plants to thrive as the vegetation is the engine that drives retention.

So, what do plants need?

Plants need macronutrients such as nitrogen, phosphorus, and potassium, but also a range of micronutrients. In addition, the nutrients have to be supplied in the correct ratios.

A nutrient is referred to as the “limiting nutrient” if it is the one determining the growth rate of the plant. It’s like when you bake a cake and you have 200kg of flour but only a teaspoon of sugar. No matter how much extra flour you get, there just won’t be any more cake. The sugar is the limiting factor in this case.

A well-functioning soil includes many compartments where nutrients can be held such as a thriving micro and macrobiota with bacteria, mycorrhizal fungi, insects... These biotic compartments lead to nutrients being cycled more efficiently resulting in lower leaching.

Rain on a porous, but well-fertilized soil, as many green roofs are, will simply flush out the nutrients down the drain and you’ll end up paying for fertilization of the blue-green algae in your local water body instead. Not a dollar well spent, in my opinion.

We absolutely do not recommend using pure organic matter as green roof soil. However, we think that we can add the correct organic matter in the right percentage for the right climate.

Soil stoichiometry and volumetric losses

Organic matter is complex and can have vastly different properties. There are types of organic matter with nutrient ratios that promote microbial growth and respiration. This easily digestible matter leads to increased soil temperatures, can cook roots and results in significant soil volumetric losses over time, especially in hot and humid climates.

There are, however, climates where such organic matter would not have such a severe effect.

Other types of organic matter may have a different composition with large and complex carbohydrate molecules that break down very slowly. Adding this kind of organic matter would have a very different effect on the roof in a specific climate.

Organic matter mustn’t be packed with phosphorus or nitrogen unless you want exactly that. It all depends on your starting material.

My point is that nothing is black and white nor right or wrong. There are just mismatches of soil components and climates.

We need more research and data-driven approaches. We need more cross-collaboration projects. We need to step back and respect what has been done so far and then look forward towards a sustainable and profitable future for our entire industry.

This is just the beginning!

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Don’t hesitate to contact our experts if you have any questions or would like to discuss projects, look at some data. Also, don’t forget to check out the Purple-Roof Green Roof Modeler!

Reading tip:

Johannessen, Birgitte G., et al. "Investigating Substrate Amendments to Prevent Nutrient Leaching from Extensive Sedum Green Roofs." World Environmental and Water Resources Congress 2018. Reston, VA: American Society of Civil Engineers, 2018.