Green Roofs: Benefits and Design Considerations

Green roofs, also known as vegetated roofs, are an effective stormwater management practice for improving urban sustainability. The Pennsylvania Stormwater Manual recommends vegetated roofs (green roofs) as an effective stormwater best management practice (BMP), highlighting their role in reducing runoff volume and peak discharge rates. Research at Penn State has demonstrated that green roofs can significantly decrease the amount of stormwater leaving a developed site by capturing and retaining precipitation, which is then evaporated and transpired back into the atmosphere through vegetation. Green roofs are typically characterized as extensive (having 2 to 6 inches of media and smaller vegetation) and intensive (having 6 inches or more of media and larger vegetation). This article will focus on extensive green roofs.

Extensive green roofs are a surface treatment for rooftops, typically less than 6 inches in depth, involving the addition of growth media and plants to create a sustainable green space over a flat or nearly flat roof deck (ideally between 1 and 5% slope). Extensive green roof advocates have claimed numerous benefits, including improved air quality, stormwater attenuation, water quality improvement, reduction of the "heat island effect," extended roof life, and aesthetic value.

The general design of a green roof consists of four distinct layers: a waterproof membrane and root barrier, a drainage layer, lightweight engineered growth media, and adapted vegetation (see Figure 1).

Stormwater Benefits

With the right selection of materials and vegetation, green roofs can effectively retain rain and snow while reducing the runoff peak and delaying its release from the building. In Pennsylvania, 50 to 60% of the rain falling on a green roof can be captured and cycled back into the atmosphere, thus never entering the stormwater system. These benefits, in combination with the limited open space in cities, make green roofs a practical method for easing the pressure on storm drainage systems. They intercept stormwater before it runs off a roof, which addresses the stormwater issue at the source rather than after the runoff has been collected. Penn State research suggests that a typical extensive green roof of 3.5 inches in depth will reduce the volume of runoff by about 55% in central Pennsylvania and attenuate the peak runoff rates to the level of the pre-development land use.

Green Roof Design Considerations

Components

1. Waterproof Membrane
Before a green roof can be installed, the roof must be covered with an impermeable membrane that will not let roots penetrate.

2. Drainage Layer
The drainage layer is an open, highly drainable material that quickly channels gravitational water to the downslope edge of the roof. The drainage layer is usually a thin layer (about 0.5 inches thick) of plastic netting or small chambers that receive the water flowing downward through the growth media and then channel this excess water to the edge or outflow location of the roof, where a downspout takes the water to ground level. There are a number of products available, including simply placing a 0.5-inch layer of pea gravel between the roof membrane and green roof media to serve as the drainage layer. If the roof slope is less than 2%, it is recommended to use filter fabric (lightweight nonwoven geotextile) in addition to the drainage layer beneath the growth media. If such fabric is used, it should allow easy penetration by roots but provide a durable separation between the layers.

3. Growth Media
Unlike garden soil, which is typically rich in organic material, green roof media consist of an engineered mix (80-95% lightweight aggregate and 5-10% organic matter) that performs several functions. It should be able to retain and drain the excess water and simultaneously provide nutrients and a suitable rooting zone for the selected vegetation. The medium should have low density, high water-holding capability, and be low in salts and heavy metals such as zinc, copper, and iron. Compost is typically not recommended to avoid nutrient runoff. The lighter weight of the media allows for retrofit installation on existing buildings and also reduces the need for extra structural support in new buildings. This media layer may also provide some insulation, depending on the thickness of the medium, its physical properties, and water content. The thickness of this layer also plays an important role in stormwater retention (see section below on Design Volume). A green roof with 3-4 inches of growth media can retain approximately one inch of water, which is equivalent to 0.6 gallons of water per square foot of green roof area. The minimum depth of the media should be 4 inches if the roof is not irrigated and 3 inches if it is irrigated. Most commercially available media use a lightweight base made from heating clay, shale, or slate, which is then mixed with another lightweight organic material. Ideally, the growth media should have a porosity of 40-65% and a field capacity (moisture remaining after drainage) of 40-60%.

4. Plants
The long-term success of green roofs depends heavily on the right selection of plants. The plants provide shade to the surface below the foliage, intercept rainfall, and slow the movement of runoff from sloped roofs. The environment on a green roof is harsh. Green roof plants are exposed to the hottest and driest conditions as well as the wettest and coldest conditions. Perennial plants that are drought-tolerant, low-growing, and rapidly spreading with fibrous roots are typically preferred for green roofs. Some of the popular plant options are succulents, especially the many varieties of sedums, common houseleek, creeping phlox, coral bells, etc. It is not desirable to irrigate (except during plant establishment period) or apply fertilizers on green roofs to prevent nutrient runoff. We recommend that you consult a Horticulturist for more information about appropriate plant materials in your area.

Design Volume

The amount of water a green roof can retain depends on the water-holding capacity of its growth media and the depth of the media layer. This capacity, known as plant available water (PAW), is the difference between field capacity and the permanent wilting point (water remaining in the media that is unavailable for plants). Typically, PAW is assumed to be 0.25 in/in, unless supported by independent testing indicating a higher value. The design volume (DV) retention provided by a green roof is calculated using the formula:

DV= PAW* d_media*A_gr/12 

where DV = Design volume (cu ft)

PAW = Plant available water (in/in)

d_media = Depth of the media layer (in)

A_gr = Green roof area (sq ft)

Peak Flow Attenuation

Green roofs can attenuate peak discharge rates, with effectiveness varying by storm size and duration. Penn State Research has found that a green roof can reduce peak flows to pre-development levels for storms with return periods between 2 and 100 years. However, green roofs are more effective at mitigating peak flows from smaller storms than larger ones. Adding a storage reservoir is generally more efficient at peak flow reduction than increasing media depth. For design calculations, a green roof area is considered as a pervious open space. Predicting the runoff response from green roofs is often challenging due to variations in the media’s water-holding capacity and antecedent moisture conditions. If direct rainfall-runoff relationships cannot be determined, the NRCS Curve Number method (using CN = 86) may be used to estimate hydrologic response in green roofs.

Water Quality Benefits

Vegetated roofs help reduce nonpoint source pollution by minimizing direct runoff from rooftops, with their effectiveness depending on size, design, and substrate composition. For optimal water quality benefits, they should use engineered media with 100% mineral content and avoid ongoing fertilization after the initial 18-month plant establishment period.  Although there are some concerns about nutrient runoff from the substrate in the initial period, pollutant removal efficiency improves over time, reaching its peak performance around five years after installation (PA BMP Manual).

Green Roof Maintenance

Generally, green roofs do not need much maintenance. Most of the maintenance is required during plant establishment. During this time, frequent watering may be necessary, particularly in dry seasons, along with regular weeding until a dense plant cover (60–75%) is achieved to suppress weed growth. The roofs should be checked once or twice each year to make sure the plants are healthy and to pull out weeds that may have made their way onto the roof. Low-maintenance plants can help reduce ongoing care, particularly in terms of fertilization, irrigation, and weeding. The drainage features (pipes, gutters, inlets, etc.) should be checked regularly for blockage and to ensure they are free of vegetation and other debris.

Disadvantages

The two most frequent concerns about using green roofs are the cost and the possible need to provide additional reinforcement of the structure. Green roofs have higher initial installation and labor costs than conventional roofs, but the resulting reductions in heating and cooling expenses can help offset the upfront investment over time. Extensive vegetated roofs will typically cost $8 to $15/ sq ft, including design, installation, and warranty service. When designing a green roof, it is essential to account for the building's structural capacity. Extensive green roofs typically add 15 to 30 pounds per square foot to a rooftop (Department of Energy and Environment) and require less structural reinforcement compared to intensive systems. A structural engineer should always evaluate the building’s load-bearing capacity and assess additional reinforcement requirements before installing a green roof.

Summary

Green roofs provide a sustainable solution for urban stormwater management by reducing runoff, improving water quality, and attenuating peak flows. Their design integrates multiple components, including waterproof membranes, drainage layers, lightweight engineered media, and carefully selected vegetation to maximize performance and longevity. Despite higher upfront costs, green roofs offer energy savings, extend roof lifespan, and contribute to urban biodiversity. With proper planning and design, green roofs can serve as an effective best management practice in urban and suburban landscapes.

Sources and Additional Information

Penn State Center for Green Roof Research. Accessed March 4, 2025

Peter A. Johnson. Green Roof Performance Measures: A Review of Stormwater Management Data and Research (PDF). September 2008. Chesapeake Bay Foundation – Anacostia River Initiative

Pennsylvania Stormwater Best Management Practices Manual. Chapter 6 – Structural BMPs. Accessed March 4, 2025.

North Carolina Department of Environmental Quality Stormwater Design Manual. C-8: Green Roof (PDF). Accessed March 4, 2025.

Philadelphia Water Department Stormwater Management Practice Guidance: Green Roofs. Accessed March 7, 2025.

HEC-RAS Hydraulic Reference Manual - NRCS Curve Number Method. Accessed March 7, 2025.

Department of Energy and Environment - Green Roofs (PDF). Accessed March 12, 2025.

Author: Femeena Pandara Valappil, Assistant Professor and Extension Specialist, Department of Agricultural and Biological Engineering
Reviewer: Andy Yencha, Extension Educator, Renewable Natural Resources
This article was modified from an original version written by Dr. Albert R. Jarrett, Professor Emeritus of Biological Engineering.