A green roof is a vegetative layer growing in a specially engineered soil on top of a drainage layer that spans the roof of a residential or commercial building. The types of plants included in the vegetative layer are chosen carefully and depend on several factors: climate, sun exposure, water requirements, and soil condition and depth. Soil depth is planned ahead of time and determines how heavy the vegetative layer can be. Shallow-rooting and drought tolerant plants (e.g., succulents, mosses, herbs, some grasses, etc.) are planted in soil five inches deep or less. These green roofs are called extensive green roofs by the International Green Roof Association (IGRA). The heavier plants (e.g., perennials, shrubs, trees, etc.) do better as soil depth increases, and are planted for semi-intensive and intensive green roofs. Plants and trees situated in planters are great too, but by definition don’t make a roof a green roof.
Green roofs are professionally designed to maximize the environmental benefits of the roof, which include climate change mitigation and adaptation. A traditional roof does little to deal with storm water, which is a concern as cities need to be resilient to natural disasters. Green roofs absorb storm water, reduce air and noise pollution, and also create an environment for animals and insects.
Perhaps the largest environmental benefit of green roofs comes from their ability to reflect more UV radiation back to the atmosphere than traditional dark roofs. Absorbing less UV radiation enables green roofs to reduce building energy use and associated carbon emissions, as well as reduce the building’s contribution to the heat island effect. In 2014, commercial and residential buildings represented 41% of total energy consumption in the U.S. so reducing energy use of buildings through green roofs sounds good to us.
Green roofs are more sustainable than traditional roofs so it’s no surprise that they are popping up on buildings all over the world. Although there is support for the installation of green roofs through the LEED certification system and other initiatives, feasibility greatly depends on where the building is located. Local building codes and other city ordinances can act as barriers and increase the cost of implementing a green roof. On the other hand, local policy can support the installation of green roofs through incorporating the concept into codes, development plans, and financial incentives. The environmental benefits of green roofs are extensive so we hope that their adoption continues in the future.