One third of Harris County is now impervious cover. According to the Harris County Infrastructure Resilience Team, green roofs are one of several tools that can help mitigate flooding by converting impervious cover back into green space.
Also known as ‘vegetated roofs’ or ‘living roofs,’ they can reduce runoff by soaking up part of the rainfall during a storm and holding it back.
Green roofs consist of a waterproofing membrane, growing medium (soil) and vegetation (plants) overlying a traditional roof.
Well-designed, engineered and maintained green roofs provide multiple environmental, social, economic, and aesthetic benefits. They help:
- Manage stormwater
- Reduce energy use
- Improve biodiversity and provide habitat
- Address urban heat island effects
- Extend the life of a roof
- Improve aesthetics
Can Reduce Runoff by up to 65%
The U.S. General Services Administration (GSA) manages more than 2 million square feet of green roofs. Some date back to the 1930s. So, they have extensive experience.
GSA points out that “Most urban and suburban areas contain large amounts of paved or constructed surfaces which prevent stormwater from being absorbed into the ground. The resulting excess runoff damages water quality by sweeping pollutants into water bodies. Green roofs can reduce the flow of stormwater from a roof by up to 65% and delay the flow rate by up to three hours.
Extensive green roofs intercept and retain the first 1⁄2 to 3⁄4 inch of rainfall, preventing it from ever becoming runoff.
Green roofs mimic natural hydrological processes as part of a watershed management approach to drainage. They are just one of many tools to reduce impervious cover.
Cost/Benefit Analysis Shows Payback
Cost/benefit analysis shows that green roofs can provide payback in 6.2 years. Their longevity has the greatest effect on savings.
Of course, they cost more to install and maintain. But GSA says that the other benefits they provide (stormwater and energy reduction) more than compensate for the premium owners pay.
The City of Houston provides tax abatements and discounts for green stormwater infrastructure, such as green roofs, in recognition of the stormwater management benefits. Such savings can also provide significant value for owners.
But any cost/benefit analysis for owners/investors depends on many factors, such as the square footage and height of buildings. Energy and stormwater management benefits increase with size of a roof. Energy savings primarily benefit the higher floors in a multi-story building, i.e., those closest to the roof.
GSA notes that any increased market value of buildings with green roofs was not included in its cost/benefit analysis. GSA believes that if it were, it would show even greater benefit.
Benefits to Owners AND Community
Regardless, even though they provide a positive payback to owners, benefits to the community have the greatest positive impact. They provide net present value savings of almost $38 per square foot of roof, according to the GSA.
Green roofs are being increasingly used in urban areas where space constraints limit the use of other stormwater management practices. Such roofs can protect the roofing materials below by shielding harmful UV rays and protecting them from adverse weather events.
Importance of Drainage Paths, Right Plants
It is important that green roofs have a suitable drainage path for excess water that is not absorbed during larger storms. All green roofs should be designed to move excess water away from the building. This water can then be directed to rain gardens or cisterns.
In Texas, such roofs require special plant selection that can hold up to the region’s extremes in heat and humidity.
Not for Everyone
However, green roofs won’t work for everyone. The increased weight requires additional structural support. And the slope must be gentle. They’re primarily for flat or slightly sloping roofs. All those factors make retrofitting them to pre-existing buildings difficult. Generally, engineers must plan for them before construction of a building.
Posted by Bob Rehak on 1/14/23, first in a series on green infrastructure
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