Roof top gardens, eco roofs, vegetative roofs, living roofs, green roofs, however they are named they refer to the same phenomena; buildings with plants on the roof. As defined by the nonprofit, Green Roofs for Healthy Cities  “A green roof system is an extension of the existing roof which involves a high quality water proofing and root repellant system, a drainage system, filter cloth, a lightweight growing medium and plants.”

Green roofs are growing, in numbers that is. They are becoming a common component of architecture in the urban and rural landscape. Perhaps not as new as we might think, earthen and planted roofs have been a part natural building practices as long as man has built structures from mud. Sod homes of early settlers in the US, were a common occurrence in the prairie states. They were practical, insulative and the mud most readily available.                                                                       SE Portland- Ecoroofs Everywhere

So why the interest in green roofs in our city centers from Chicago to Seattle and now a progressive incentive program in Portland? As local landscape architect, blogger and designer of several large projects, Jason King of Greenworks, PC, states, “The many reasons to install a vegetated roof are that you are taking something that is purely a functional part of a building and embedding it with a range of multiple benefits – storm water management, habitat creation, insulation and cooling, aesthetics, urban heat island reduction - all while increasing the longevity of the roof and providing green space in our cities.”

The published data is clear. In dense urban areas a phenomenon called heat island effect occurs. According to the EPA  the term "heat island" describes built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90°F (50°C) warmer. Installing eco roofs contributes to the reduction of heat island effect while reducing green house gas emissions. Studies have estimated that if all the roofs in a major city were "greened," urban temperatures could be reduced by as much as 7 degrees Celsius.

Another easily measured benefit of green roofs is the reduction of storm water runoff. Impermeable surfaces like rooftops discharge untreated water into our watersheds. Often the runoff contains pollutants such as mechanical wastes and oil, sediments that clog waterways, bacteria, pesticides and other chemicals. Eco roofs capture as much as 50% of the runoff as compared to a standard asphalt roof. This means with larger commercial buildings there is often a considerable cost savings in infrastructure and mechanical construction to capture and treat storm water runoff.

Eco roofs can save in heating and cooling costs. They have been found to dramatically improve a roof’s insulation value. A study conducted by Environment Canada found a 26% reduction in summer cooling needs and a 26% reduction in winter heat losses when a green roof is used. Green roofs also increase the life expectancy of a roof.

Still not convinced installing a green roof in Portland is cost effective in times of tighter budgets? Currently the City of Portland has initiated an incentive program that will further entice a homeowner, builder or developer that installing an eco roof is a good investment. Homeowners and building owners are eligible for a maximum 35% discount on their municipal storm water fee for qualifying eco roofs. This initiative was developed to increase sustainable stormwater management practices, control non-native, invasive plants, and protect sensitive natural areas. In addition, Portland has created an incentive fund program for up to $5 per square foot of an ecoroof project. Installation costs for eco roofs in Portland range from $5 to $20 per square foot.The deadline to submit applications for projects to be considered in the current $5 per sq. foot incentive is Dec, 1st, 2009. There are other requirements a project must meet in order to qualify. Those specifics can be viewed on the city’s web site.

If you’d like to explore green roofs further the City is hosting a 2-day technical seminar on Nov. 3rd and 4th. There are many examples of green roofs in Portland. Take a photographic tour of some examples on the City of Portland’s web site or visit some of the many individuals and companies promoting and installing green roofs. One local green roof installation company has the ambitious 5-year goal of Ecoroofs Everywhere and what better way to turn thousands of acres of asphalt into green oases.