Green roofs
Walking around American hometowns we can see many new protections against increased flooding, hotter and drier summers, and other deteriorating weather conditions. One of the most effective but least obvious is a grass roof.
Planting vegetation on a roof isn’t new. But covering a home or office building with soil and vegetation - on top of layers that drain and filter the runoff and protect the structural roof - is more complicated than the Hobbits make it look.
Why a green roof?
Cools the area A green roof absorbs heat, reducing the air temperature nearby and moderating the city’s heat island effect. In Chicago the average temperature of green roof areas in the heat of the day was up to 31% cooler than other roof types.
Saves some money by acting as extra insulation. Green roofs reduce the cost of heating or air conditioning the building.
Reduces local flooding by holding rainwater and releasing it slowly. In Fort Wright, Kentucky, a big green roof releases rainwater into a nearby wetland, then a retention basin, a detention basin, step pools, and finally into the local Creek.
Purifies the air. Yes, there’s evidence that vegetation on a roof cleans some contaminants from the air.
Protects roofing. A green roof, properly installed can extend the life of underlying roofing by shielding it from slow damage from sun, wind and rain.
Provides usable space. Look at an aerial photo of any urban area, and we’ll see a vast area of unused roof space. Where people can conveniently reach a flat vegetated roof, we often see a small patio, meeting area, even a mini golf course among the greenery. For some it may be a getaway space: “Honey, I’m going up to weed the roof.”
Can look interesting. Of course most flat green roofs can’t be seen from below, but there are ways! As you enter the public library in Ballard, Washington, you’re confronted with a periscope, like on a submarine. Turn the scope and you’re looking around the library’s roof, covered in vegetation.
There are some disadvantages of course. Green roofs are costly, and they weigh a lot, putting stress on the structure. The upfront cost of installation can be high, although a variety of grants, rebates, and subsidies are available from local, state and federal authorities. And there’s maintenance. Growing conditions need to be checked and pernicious weeds removed from time to time.
Choices
It’s important to opt for the right greenery and design. Without any shade, a thirsty plant may not be suitable in the conditions. The wrong type or depth of soil and drainage can quickly cause plants to dry and shrivel. That’s why there are a fast-growing cadre of experts, many of them listed at greenroofs.org.
Drought-tolerant species are a must. One green roofer, Roof Top Sedums, has custom grown more than 60 green roof projects in Iowa, Kansas, Nebraska and parts of Illinois and Missouri. They’re expanding to all of Missouri, including St. Louis, Jefferson City, and Columbia.
To see how a green roof is installed check the photo series at greenroofblocks.com.