Minimize Green Roof Irrigation

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I got to my car this morning and observed a bone-dry circle underneath the roses in my garden caused by a small rain event. This phenomenon is important to consider when designing green roofs as it has a direct effect on rainfall and thus irrigation practices.

Green Roof Diagnostics laboratory has tested over 800 different green roof profiles over the last 2 years, and many of them showcase that green roof soil, in particular, is a funny product that is not always very good at managing water, or better said it doesn’t compensate for uneven rainfall distribution very well.

Water distribution on a green roof and why it’s important when considering irrigation needs

Let me ask you a question, when it rains 2.5cm (1”) across a green roof is the water absorbed evenly across the roof plane, or is it distributed unevenly?

The answer would be “evenly” but that is only true if the soil were perfectly consistent and placed in the exact same profile thickness and was compressed to a uniform density. Assuming that was all done to perfection.

But life isn’t perfect and the devil is in the details. Many green roofs are made with trays, whereby the trays themselves are perforated or have partial soil connections. Such trays often need supplemental irrigation because water finds the road of least resistance, and that is usually downward through the cracks that are in between the trays.

Laboratory tests in 2017 showed this phenomenon and its shocking to see that 30-50% of a large storm flows out in between the gaps of the trays before the soil is saturated. In warmer climates like the Mid-Atlantic, these systems can thrive only with regular or supplemental irrigation.

Regular built-in-place green roofs made with exceptionally porous soils also allow water to escape too easily and people forget that in the summer, soils contract and create large gaps for water to enter. In a natural garden these cracks are great because it allows a LOT of the summer storm to enter quickly deep into the soils (available to wick up later) but on a tray green roof or your typical old fashion 4” (10cm) green roof that is a problem, because the water is not stored in the aquifer, it simply rushes to the drain.

Now add plants to it. You may ask yourself, why does it matter, but to us, it is a big deal because it fundamentally affects the performance of a traditional old-fashioned green roof, in a not-so-good way. The way green roof soil interacts with rain dramatically is altered once you add plants to the roof.

Can we solve this? Absolutely!

Deep dish or thin crust?

Traditional green roofs are ‘Thin Crust’ and nothing like a ‘Deep Dish’ garden or farm scenario. On farms and in your backyard there is a 120-240cm (4-8ft) deep soil profile that is most often clay-like or loamy and easily absorbs water, but also channels water through tunnels and cracks in the soil towards deep reservoirs underneath.

How rainwater travels through the soil and implications for irrigation on green roofs

No one ever talks about the uneven distribution of rainfall on a garden plot or a farm because of the capillary capacity of that soil combined with deep reservoirs levels out the majority of the moisture differences that may take place in a soil profile.

Water always enters the soil in a V-shape funnel and water migrates down (gravitational force) and the clay or loamy soils pull it sideways (capillary action). The deeper the V-shape can sink, the wider it becomes at the top and the V-shapes start to overlap making the upper 10-20cm (4-8”) levels of the soil evenly moist (depicted in the drawing below).

Why does water enter the soil unevenly on a green roof? The presence of plants, uneven soil surface, friction in the surface, organic debris, moss or surface algae growth, bird nests, different soil densities, and even the slope of the roof will determine where the water flows to. Gaps between plastic green roof trays make this even worse.

In the summer, the surface can even be hydrophobic (rejecting water) and water droplets slide to lower areas on the soil surface, where it sits until it gets absorbed.

In nature, it matters less where the water falls, or if the water landed on the soil in an uneven distribution pattern because slope and surface friction in the landscape combined with the capillary capacity of the deep soil horizons are able to redistribute the water from the saturated areas to the drier areas.

The majority of the water that falls on your garden or on your farm gets absorbed by the soil, perhaps about 20% of the annual rain budget sheet flows above ground and ends up flowing to a lower area of the landscape where it once again can percolate into the soil. Very little of the water is wasted.

Green roof water distribution

In essence, the foliar canopy of a plant is truly a very efficient solar panel array that tries to harvest sunlight from all directions utilizing every square inch possible. The leaves are angled towards the sun at approximately 45 to 70° so that both direct and indirect light can be harvested.

Just like solar panels any water that falls on it, slides towards the lower end where it falls off the edge, and in this case, it will land on the leaf below which also transfers the water drop to the next, and then the next and then the next and then the next until it hits the ground on the outer parameter of the plant. That means that most, or all the water that falls on top of the plants actually gets diverted to the parameter edge of the plant. Not to worry, this is also where the majority of the plants’ roots tips that drink the water are (see image below where the soil ball (brown) is roughly the same as the foliage canopy (green).

Because water enters the soil in a funnel shape that is determined by cracks in the soil, slope, surface frictions, density differences, and uneven soil dimples, the deeper the soil profile (like in nature) the more the V-shaped funnels will overlap and even out the water content in the soil in the 1st 60-90cm (2-3 ft) of soil. See below in the example of a 120cm (48”) deep profile.

In a green roof, there is only 10cm (4”) so the V-shaped funnels cannot drop very far, and worsened by the fact that green roof soil behaves more like a low capillary sandy soil than a high capillary clay soil there is very little sideways movement of water, most of the water flows out into the drainage layer as marked with the W-shaped layer in the drawing below, and as a result of gravitational forces, the top layer of the soil has very uneven water content: dry spots and soaking wet spots, with a much lower overall water balance.

A green roof system - an example

A 30cm (12”) diameter circle has about 112 in² of surface area, so if a 2.5cm (1”) rain event lands on the plants, 1840cm³ (112 in³) of water is diverted towards the edge of the plant where there is a 2.5-5cm (1-2”) band of soil-surface to collect the water. The circumference of this circle is approximately 90cm (37”), so now let’s imagine a "water band" of 3.8cm (1.5”) that wraps around that circle, so now we have about 387cm² (60 in²) that can receive the water. In a nutshell, the outer perimeter of the Sedum plant receives twice as much water as the surface area of the plants would’ve received if the plant didn’t exist now and it over 50% disappears into the drain. To worsen it, the soil has what it’s called antecedent water content, which is water that is held in suspension from the prior rain events), so now an even higher portion of the re-allocated water will leave the green roof. This is super important to understand when you design a green roof.

Let’s try to illustrate this in a much-simplified format. The complexity of soil and water movement within is so vast, it would take 100 more pages to illustrate this with 100% accuracy so we are taking a few big short cuts if you don’t mind.

We have solved this water inequality problem for green roofs

We wanted to make the green roof into a better sponge roof, and as we looked for natural permanent remedies that can compensate for the shortcomings of traditional green roof soil then needled mineral wool was the answer. It mimics the lateral redistribution capacity of clay soils so that any excess water that ‘falls’ through the green roof soil is caught and wicked sideways to drier areas in the soil profile.

Above you see how water diverted by the plants will leak out of the green roof profile without fully saturating it. Below you see that when you add the yellow Needled Mineral Wool that the water is being pulled sideways from saturated areas, to the non-saturated areas underneath the plants. Water storage means resilience, and increases plant happiness.

While the Needled Mineral wool saturates, it can share the water upwards, through a process called wicking, with the green roof soil throughout the profile. (see below).

And in our final example, the water is now occupying the majority of the soil profile, water has now wicked upwards underneath the plants and as a result, we have leached the minimum possible in a thin profile green roof.

In our experience, all green roofs designed going forward need this, or benefit from it. Mathematically it adds about10% to the annual retention capacity of the soil which is a very big deal if you are a garden plant or a city sewage plant manager.

We hope you learned something new, enjoy!

If you are interested in learning how green roof retention is traditionally measured and why this method can be problematic, check out the video below!

Purple-Roof is a non-proprietary green roof concept that is also a stormwater tool equivalent to stormwater tanks and cisterns!
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