Seen mostly in commercial applications now, “green” roofing systems have the potential to revolutionize residential roofing. Cities like Seattle and Chicago are starting to specify the systems, which create rooftop vegetation, on city-and government-owned buildings.
Regular roofs allow about 95% of water to run off, says Cary Robertson, product manager for Henry Roofing Systems. The company's green roofing system retains up to 70% of rainwater in its lightweight layers. “These systems not only retain water in the growing medium,” Robertson explains, “they also have a water storage component in what we call a drainage water retention mat. These have small cups that act as water storage reservoirs. The growing medium retains a certain amount of rainwater. Once it gets saturated, the water penetrates into the retention mat, which stores the excess water and uses it to hydrate the soils above it during drier periods.”
Cities are interested in this technology not only because it reuses water that would otherwise have to be treated but also because it reduces the urban heat island effect, which is caused when black roofs absorb heat and light. The vegetation that is grown on the system absorbs less solar heat than a traditional roof and therefore radiates less heat into the environment. It also provides additional insulation for the building and can reduce heating and cooling costs.
Robertson thinks that green roofs will become more appealing to the residential market in the future. The cities that use the system are trying to set the trend, he says, and as people see more applications and become more aware of the benefits, they'll want to incorporate the idea into their own homes. “It's a little expensive for most homes, but higher-end custom homes could take advantage of it.”
A homeowner in Seattle did just that, putting a green roof system over his garage roof that was almost at street level (see below). The system, designed for flat or low-slope roofs, is installed by Henry-trained roofing or waterproofing contractors. “I think that as more and more of our waterproofing contractors gain experience with these systems, the price will come down and become more affordable,” Robertson says.