The addition of a green roof to a building or structure can be beneficial in many ways. In large buildings in can help with cooling and heating, storm water management, and can even be used for urban agriculture. In smaller buildings green roofs are very aesthetically pleasing and though it may not help with storm water management as much, it could help with water collection that could be used in different ways.
There are three different types of green roofs: intensive, extensive, and semi-extensive. In this project that Studio V is working on with the First African Evangelical Christian Church and Enough Pie and structure is going to be really small in scale so an extensive roof would be required. And extensive roof generally means that the substrate level of the roof would be less than 8 inches deep. The substrate level is going to be the heaviest level and is a serious component when deciding on how to build the structure where it can hold the weight. Even though there is a lot of different materials that can be used for the substrate (gravel weighs more than pumice for example) the average weight for a 4 cm (1.6 in) deep substrate will be around 40-60 kg/m₂ (8-12 lb/ft₂).
When deciding which types of plants can be successfully planted on this depth of substrate there are a lot of factors to consider. Some things may be are the plants drought resistant, can they withstand wind, and most importantly, how deep do their roots need to go. For a substrate between 2 and 4 inches deep it could support grasses, mosses and small bulbs. When it goes up to 4 to 8 inches deep it could support the same plants in addition to annuals, small shrubs and turf grasses.
Since Enough Pie and the First African Evangelical Christian Church would like for this to be a pollination garden I think that the green roof would need to be in the 4 to 8 in. depth range so it could properly support annuals and other pollinating plants.
Petrorhagia saxifraga (left) and Melica ciliata (right) are two plants that can successfully grow in a 4 to 8 inches deep substrate on an extensive green roof.
