Research indicates that green roofs are great at reducing runoff volume, but are not so great at reducing nutrient concentrations. Their relatively low nutrient removal, however, is not a major drawback, since most nutrients generated by rooftops comes via atmospheric deposition alone. Green roofs are an attractive LID option in urban areas of the Bay watershed, given their runoff reduction benefit (as well as their roof longevity, green building, energy efficiency and architectural benefits.
Several great books on green roofs have been published in the last year, and green roofs continue to command attention and interest among stormwater aficionados. While many green roof demonstration projects have recently been installed, they are still not widely used in the Bay watershed.
One reason for the low use of green roofs is the absence of a detailed design specification in the Bay States. This gap has closed when CSN released a new green roof specification in June, 2009 that is specifically adapted for the unique climatic conditions of the Bay watershed. The new spec focuses on extensive green roofs that are primarily intended to treat stormwater runoff, and consolidates guidance and experience from East Coast applications. Some of the useful features of the specification include:
- Green roof sizing equations to maximize runoff reduction
- General material specifications for the seven functional elements of a green roof
- Recommended plant species that can tolerate the unique climate of the Bay watershed and the stressful conditions encountered on exposed roof surfaces
- List of nurseries in the Bay States that specialize in growing green roof plants
- Detailed construction sequence and inspection checklist
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