Finding the right depth for green roof soil media is so critical in hot and dry climates for when the rains come the roots must be in place where they can take full advantage of any and all rainfall.
This design maxim is why we use shallow soil media depths of 100mm or less, usually ranging in the 50mm thicknesses.
Green Roof plant roots will spread horizontally once they reach an impenetrable root barrier or the bottom of the green roof system. If your green roof system is deep, then many of your green roof plant roots will travel downward, vertically. If your green roof system is shallow, then the roots will spread out horizontally.
Most plants in nature have a horizontal root architecture. I see large oak trees sometimes blown over and their entire root mass is no more than 60cm or so deep (1-2').
Here in Jacksonville, Florida we have been experiencing a severe drought this year and for several years past. Our weather service has classified the local vicinity as being under a severe drought. I understand from speaking to others across the south, some areas, such as Houston has not received any significant rain in four months.
Yet for the last two days we've had strong intermittent thunderstorms and rain.
The flower pots were filled with water and buckets overflowed. However this morning when I was working in the garden I noticed the soil below 20mm or so (an inch approximately) was completely dry! I had expected this, having seen rain time and time again when it comes, barely penetrate the ground surface layer.
Shallow, extensive green roof systems encourage horizontal root architecture. When the rains do come here, I need the green roof plants installed on our nature irrigated green roofs to be ready to adsorb as much of the rainfall as possible, wasting little if any. They cannot exist deep down, hidden in the dry.
With a deeper green roof soil media, even if the media is well drained, I still see almost complete dryness just below the surface after a typical thundershower here.
Shallow green roof soil media can promote horizontal root architecture, and in turn capture more rainfall for use in the plant's photosynthetic processes.
Additionally, shallow root architecture can facilitate enhanced nitrogen and nutrient uptake. Nutrient uptake is important for several reasons, including of course plant nutritional requirements but also the removal of available nutrients from stormwater. Shallow root architecture cleans stormwater and allows for optimal plant growth.
The root architecture model, SimRoot developed by Penn State is shown in the below video clip to illustrate how nitrogen is taken up in the upper margins of the plant root architecture system. Note how the root hairs take up the most nitrogen and water.
I've heard many arguments for deeper soil media systems. Yet they are heavy, expensive and require so much more water wasted. Rather than build heavy stormwater ponds on top of buildings, put them under parking lots for a much more reasonable cost.
There are many benefits to shallow green roof systems. Water efficiency and nutrient uptake are just two.