Living With Green Roofs

Green Roof Damage at Swarthmore College
Photo: Becky Robert

Green roofs are roofs that contain a layer of vegetation. The goal is to improve water retention and add an insulation layer to reduce water runoff and energy consumption. Green roofs are limited by weight. Water is heavy and a roof that retains water must be able to bear the weight. For some roofs, a thinner layer of media/soil for plant growth is planted with suitable shallow root plants instead of using a thicker soil that accommodates a wider diversity of plants.

Many varieties of sedum have shallow roots, tolerate rooftop conditions with thin media/soil layers and have few insect pests. However they are not pest free. In hot dusty weather, green roofs are subject to outbreaks of mites that not only damage the plants but can cause problems for building residents. A school in the UK suffered an outbreak of harvest mites which entered the building through the air circulation system. Reports of bites and allergic reactions prompted temporary closure of the school to fumigate.

A Swarthmore College green roof suffered damage from an infestation of white grubs feed on plant roots. Birds feeding on the grubs disturbed large numbers of plants that subsequently died.

Green roofs are a microclimate in delicate balance that requires inspection and maintenance. As problems, insects and other are documented, we can learn from experience on how to better manage our rooftops.