Up on a (Green) Roof

November, 2007

In a sunny corner of Manoa, Leyla Cabugos, a master’s candidate in botany at the University of Hawaii, is conducting one of only two research projects that are studying green-roof applications in the Islands. Currently, she is carefully monitoring the growth of five different native Hawaiian ground covers and sedges in 28 planter boxes, 576 square feet in all. She is growing each species individually and in different combinations to determine the most effective mix for a green roof.

Vegetation – Cabugos selected a variety of native Hawaiian ground covers and sedges that have a high tolerance to drought, high wind, shallow soil and salt spray – general conditions characteristic of roofs in the drier parts of Hawaii. The plants include ilima, akulikuli, aki aki, pohinahina and Carx wahunesis. Pohinahina, ilima and akulikuli are plants often used in lei-making.
Growing medium – The project’s growing medium is a mixture of coconut fiber, Big Island cinder and locally produced compost. The right combination is crucial, because the medium must be able to meet the nutritional and drainage needs of the plants without adding unnecessary weight to the roof.

The benefits of a green roof can vary widely depending on the environmental conditions of a particular area and the architectural characteristic of each building. However, according to greenroofs.org, a grass roof with 3.9 inches of growing medium will reduce a building’s cooling needs by 25 percent. A roof with 6 inches of growing medium will reduce heat gains by 95 percent. Green roofs, which can last twice as long as conventional roofs, also insulate buildings from sound, reducing it by as much as 60 decibels.

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David K. Choo