Here is a great graphic showing how green roof construction mimics in a few inches what normal soil does in a couple feet. Plants want water, but not too much. They want something to dig their roots into that also provides nourishment.
Green roofs have to provide this but because it is a roof you can’t have tonnes of soil up there. So much soil would be very expensive to install and support.
Normal soil is deep so it holds lots of humidity for the plant to drink from. But because it is deep the excess water drains down. This way the plants get water but their roots don’t rot in standing water.
The green roof accomplishes the balance with the help of several layers. The growing medium, filter sheet and protection layer act as sponges to hold water. The drainage layer has little cups that fill up with water. This provides a lot of water in relatively little space and with very little weight.
At the same time when the drainage mat cups fill up the excess water overflows and easily passes underneath it away and down the roof drain. This means the roots don’t sit in water and rot.
The green roof medium is usually an aggregate for the roots to dig into so the plants don’t fly away, mixed with something very nutritious to feed the plants. This mix also has to be very light. The aggregate ideally holds water as well.
Some examples of aggregate are crushed brick, lava rock, shale, and shredded Styrofoam. Sometimes clay is mixed is as an added water retainer as well as binder. As nutrition compost or peat is added.
The growing medium duplicates in very little space and with little weight what normal soil does.
The end result may not be as perfect as natural soil, and this has to be remediated by using hardy plants that are ok with severe rooftop weather conditions and poor soil. Sedum is happy hanging on the sides of a cliff in the wild, as are many grasses and other small plants, so the options for a green roof are large.
As a New York green roof installer we use a mix of North American native sedum with some grasses and even small bushes. Even though the green roofs we install are typically less the 4″ deep, they look very lush and require minimal maintenance because the green roof replicate so well the natural soil conditions of the plants.
The trick with all green building, and this includes natural landscaping, is to replicate natural phenomena so that they work with little energy input from us. A green roof is a perfect example of this.
Image © 2011 by Erich Nagler