Green Roofs

Green roofs, rarely called vegetated or eco-roofs, are rooftops that have been at least partially covered with vegetation and a growing medium. Sometimes this technology can be used solely for aesthetic purposes, but often (and for the purpose of this Case Study) green roofs are used at least partially for stormwater management benefits. Well designed green roofs can act as a sponge, attenuating or retaining stormwater that would normally fall upon a completely impervious traditional roof. Where this technology really shines, however, is in the host of secondary benefits.

Types of Green Roofs:

Green roofs typically fall into two categories: extensive or intensive.

Extensive Green Roofs

• Relatively shallow (<6 inches of growing media)
• Lightweight (10-35 lb/ft² when wet)
• Typically planted with low maintenance vegetation, such as sedums or grasses

Intensive Green Roofs

• Deeper (>6 inches, sometimes several feet)
• Heavier (50-300 lb/ft² when wet)
• Can support shrubs or small trees
• May support foot traffic or serve as an amenity

You may not notice a hard divide between what qualifies as an extensive or an intensive green roof. A third classification, semi-intensive, is also used to describe the in-between area. Poetically, extensive green roofs are small, like lawns, while intensive green roofs are big, like gardens.

Another category of green roofs can be applied based on how they are built: modular or integral. Modular green roofs are typically tray systems with pre-planted units, which can be easier to construct and modify. Integral systems are built in place by layering materials, geotextiles, and membranes. Some research at the University of Florida has noted integral systems may withstand higher wind loads than modular systems.

Design Considerations:

Green roofs have many design considerations, moreso than other stormwater management technologies. We will start with the most important element and work our way through typical considerations.

Structural Design

The building or structure beneath the green roof must be able to support the additional load (obvious, I know). Green roofs can add a few hundred pounds of dead load per square foot. If the green roof is a retrofit to an existing structure, you may be limited to the kind of green roof you can use. A rule of thumb is that if a building can support an additional floor, it likely can support a green roof. Assure that your load calculation assumes the green roof is completely saturated, even if the drainage features make that situation unlikely. Florida Building Code Chapter 15 includes information on vegetated roofs.

Wind Resistance

The Florida Building Code includes definitions for “vegetative roof” systems and require them to meet the same wind uplift and fire resistance standards as other roofs. Hurricane safety concerns have been a main reason why Florida has been reluctant to adopt this technology more heavily.

Waterproofing and Layers

A green roof is a multi-layered assembly (think: roof sandwich) and one of the most important is the bottom waterproofing layer, which protects the building interior from moisture. Above that typically is a root barrier, to prevent vegetation from puncturing the waterproofing layer. Below are common layers considered in green roof designs from top to bottom:

• Vegetation
• Soil or engineered growth media
• Filter fabric, to keep soil from bottom layers
• Drainage media, to transfer excess water away
• Root barrier
• Waterproof membrane

Adequate drainage is critical for a successful green roof design, even though the purpose of them is to retain stormwater. It is best practice to include details for waterproofing around inspection chambers or drains as these can be common points of failure.

Stormwater Management

We finally made it. A key reason to design a green roof, at least in the context of this website.

Green roofs reduce the volume of runoff and the annual pollutant loading in a catchment. Both are centered around how much retention the green roof can offer.

To quantify your green roof’s benefit on water quantity, your approach depends on your hydrologic method. Most simply, a green roof reduces the curve number or rational coefficient for the area, which directly reduces peak runoff. In more advanced hydrologic methods, a green roof increases evapotranspiration and can reduce the total volume of runoff reaching downstream. Another method for quantifying green roof retention benefits is to reduce rainfall depths in an H&H model directly over the green roof area by an amount equal to the green roof’s retention depth.

Water quality is the other half of the stormwater management problem, and this approach varies depending on whether you are using presumptive criteria or annual nutrient loading criteria. BMPTRAINS (now BMPFAST), from UCF, is a commonly used annual nutrient loading calculator and this tool includes a treatment efficiency curve for green roof systems, which is a function of retention depth and area. For example, a green roof with 1-inch of retention depth could reduce annual nutrient loads by 60%, while a system with 6-inches of retention depth could reduce loads by 80%.

Retention depths are the name of the game with green roofs, and these can be increased by:

• Adding a reservoir layer
• Including a cistern
• Increasing the depth of your soil layer
• Utilizing a soil with a higher porosity

Planting

In other states, succulents (sedums) are popular for green roof applicants due to their hardiness and drought-tolerance; however, in Florida with our hot, humid, and rainy environment, they can be poor performers.

Your plant palette should be able to handle high sun exposure and high winds, while surviving in times of low soil moisture or intense rains. A green roof in Escambia County has found coastal dune grass species and wildflowers to perform well, while also offering aesthetics.

Overall, successful planting comes down to using hardy, adaptive plants that thrive in thin soil. Irrigation may be necessary for the first year, but your goal is to have a system that thrives on rainfall alone after that.

There are resources that can help, such as the Green Stormwater Infrastructure Maintenance and Planting Manual:

https://ffl.ifas.ufl.edu/media/fflifasufledu/docs/gsi-documents/GSI-Maintenance-Manual.pdf

Costs

A green roof can cost 30-60% more than a bare roof of the same size, and be much harder to maintain. Cost estimates may vary wildly as this is a relatively new technology and there is always risk involved in that. Approximations include $10-25 per square foot for extensive green roofs, and $25-50 per square foot for larger intensive options.

Benefits

Understanding the additional benefits of green roofs is important when talking about viability. The list of secondary benefits includes:

• Energy efficiency
• Urban heat reductions
• Habitat and biodiversity value
• Aesthetics and amenities
• Property value and community redevelopment
• Water reuse
• Air quality
• Noise reduction
• Scoring in sustainability frameworks (LEED)

When viewed solely in the lens of stormwater management, green roofs are consistently undervalued. Recently, many tools for quantifying the monetary benefit of these green infrastructure technologies are being used to frame the conversation in a more holistic way.

“In North America, the benefits of green roof technologies are poorly understood and the market continues to mature, despite the efforts of industry leaders. In Europe however, these technologies have become very well established. This has been the direct result of government legislation and financial support.” – greenroofs.org