Despite the up-front cost – from $5 to $35 per square foot – green roofs provide multiple benefits that offset their expense within a few years. The U.S. Environmental Protection Agency estimates a five-year average to recoup costs.
Covering building tops with soil, sod, and gardens provides a pleasant place to spend time, particularly in urban environments; processes CO2 in the atmosphere; and produces oxygen, provides food, and helps insulate buildings so they don't use as much energy to heat or cool. The plants also help filter the air as the breeze moves through them, pulling out particulates that can cause respiratory illness.
"Green roofs intercept the solar radiation that would strike dark roof surfaces and be converted into heat, thereby improving energy conservation," said Hitesh Mehta, a landscape architect who has designed many sustainable resorts around the world. He recently released a guide to the world's most eco-friendly lodges, Authentic Ecolodges (Collins Design, 2010). "Because green roofs reduce the surface temperature of a roof by minimizing heat-absorbing surfaces, a green roof helps to reduce energy costs inside the building as well. Like urban forests and reflective roofing surfaces, they absorb and/or deflect solar radiation so that it does not produce heat. The urban heat island effect increases the use of more electricity for air conditioners, and it increases the rate at which chemical processes generate pollutants such as ground-level ozone. It also exacerbates heat-related illnesses."
Intensive green roofs have deep soil that can support trees, shrubs, and lots of visitors.