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Residential Green Roof Systems

Roof-top plants provide a pleasing aesthetic and environmental benefits

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Green roofs, also called living or planted roofs, are systems of living plants and vegetation installed on the roof of an existing or new structure. The green roof concept is not new. The Hanging Gardens of Babylon constructed around 500 B.C. were perhaps one of the first green roof systems. Planted components of a prevegetated modular green roof system. Photo courtesy of LiveRoof, LLC Terrace structures were built over arched stone beams and waterproofed with layers of reeds and thick tar on which plants and trees were placed in soil.

Popular in Europe for decades, technology has improved upon the ancient systems, making green roofs available in and appropriate for nearly all climates and areas of the United States. All green roof systems consist of four basic components: a waterproofing layer, a drainage layer, a growing medium, and vegetation. Some green roofs also include root retention and irrigation systems, but these are not essential.

Green roof systems are often broken down into two types—extensive and intensive systems. An extensive system features low-lying plants such as succulents, mosses, and grasses. They require relatively thin layers of soil (1-6 inches), and plants usually produce a few inches of foliage. Extensive systems have less of an impact on the roof structure, weighing 10-50 pounds per square foot on average, and are generally accessible only for routine maintenance. Most residential applications are composed of extensive green roof systems. Intensive systems feature deeper soil and can support larger plants including crops, shrubs, and trees. Intensive systems can be harder to maintain, depending on the plants used, and are much heavier than extensive systems—they range from 80 to more than 120 pounds per square foot. Intensive systems are typically designed to be accessible to building inhabitants for relaxation and/or harvesting.

There is a wide variety of materials used for each component of the green roof system, depending on the chosen plants, type of system employed, climate, and underlying structure. Growing mediums include soils, peat and other organic materials, gravel, and other aggregates. A drainage layer is required to adequately distribute water and prevent pooling. To minimize the weight of the system, drainage layers are often made from plastic or rubber, but may also be made of gravel or clay. The drainage layer may or may not include filter media to ensure aeration. The waterproofing membrane is a critical component of the system and should include a root barrier to ensure the underlying roof surface is not compromised. If the weatherproofing material is not root-resistant, an additional layer must be applied to serve this purpose.

Plants used in green roof applications must be easy to maintain and tolerant of extreme weather conditions including heat, freezing, and drought, and must have relatively shallow, fibrous root systems. The plants should also be resistant to diseases and insects, and not generate airborne seeds in order to protect surrounding plantings. Climate-appropriate succulents, mosses, and grasses are often best suited for extensive green roof systems. These types of plants are available in a variety of colors, in both deciduous and evergreen options. Many nurseries throughout the country specialize in vegetation for green roofs.


Energy Efficiency

The added mass and thermal resistance of green roofs reduces the heating and cooling loads of the building. These systems reduce the ambient temperature around the roof, decreasing the building’s urban heat island effect; reduce the ambient temperature of the roof’s surface; and slow the transfer of heat into the building, reducing cooling costs. They also provide added insulation to the roof structure, reducing heating requirements in the winter.

Environmental Performance

Green roofs reduce stormwater runoff by absorbing and retaining the water in the soil medium for plant growth. The plants can filter pollutants and carbon dioxide from the air and rain water. These systems reduce rooftop temperatures and can reduce air and noise pollution. They also serve as living habitats for birds and other wildlife.

Quality and Durability

Vegetation protects the roof from extreme temperatures, ultraviolet radiation, and harsh weather conditions, resulting in a longer lasting roof system.


Medium

Green roof systems can be implemented in new and existing construction. The structural design of the home must be carefully considered to accommodate the additional loads. Roofs do not need to be flat to support green roof systems, but different systems have varying pitch recommendations and limitations, which should be considered during the design phase. The systems also require selection of appropriate plantings for the climatic region. Flood testing of the roof membrane should also be conducted prior to placement of the green roof system.


Costs for research, design, and materials of the green roof system and structural support are higher than a conventional roofing system. Extensive systems can cost as little as $7 a square foot, though ranges tend to be $10-15 for extensive, and $15-25 per square foot for intensive systems.


There will be some additional costs involved with maintaining the roof top plantings, but overall maintenance of the roofing membrane will be reduced. Since planted roof systems increase the life-span of the roof, repairs and replacement should be minimized.


The IRC does not cite green roofs. They are accepted under the IRC Section R104.11 for alternative materials, design, and methods of construction and equipment. Local plan reviewers and inspectors might not be familiar with green roofs and may require additional inspections and testing of the waterproofing membrane.


None available


In built-in-place systems, the roof structure must be covered with a waterproof roofing membrane system and tested for waterproofing. A drainage layer system and growing soil medium are then placed, and appropriate plantings for the climatic region are installed. In modular systems, the required roofing systems are contained in pre-set grids which are installed on top of prepared roof surfaces.


Manufacturer warranties vary and may depend on maintenance agreements. Some are all-inclusive and cover the entire system; others cover components such as waterproofing and plantings separately.


Green roofs reduce stormwater runoff by absorbing and retaining the water in the soil medium for plant growth, rather than piping all water directly into a storm sewer system. Results of an evaluation of green roofs in the Seattle area found that green roofs reduced rainwater runoff from 65 to 94 percent (Fall 2007, The Green Roof Infrastructure Monitor, www.greenroofs.org). A green roof might reduce overall development costs for large multifamily buildings; although the green roof would cost more than a conventional roof, stormwater connection fees and related infrastructure costs might be greatly reduced or eliminated. Green roofs reduce the urban heat island effect by reducing roof top temperatures. Green roofs can also reduce air and noise pollution.

Vegetation protects the roof from extreme temperatures, ultraviolet radiation, and harsh weather conditions, resulting in a longer lasting roof system. Reducing daily temperature swings also decreases the amount of expansion and contraction of the roof waterproofing membrane, prolonging its life. Green roofs can lower the energy requirements of buildings by reducing the temperature of the roof’s surface and slowing the transfer of heat into the building. They also provide added insulation to the roof structure, mainly by trapping a layer of still air against the roof surface, thus reducing heating requirements in the winter. Appropriate plant species selection provides needed habitat for local and migratory butterflies and other wildlife populations. Green roofs in California’s Bay Area have been coordinated to be part of a larger wildlife corridor stretching from the Pacific Ocean to the Sierra Nevada Mountains. Green roofs are also aesthetically pleasing, especially in urban environments.

While extensive green roof systems weigh no more than slate roofs, intensive systems are much heavier than traditional roofs, so structural design is an important consideration. Like standard gardens, the plantings also require maintenance, the level of which depends on the species chosen and their hardiness in the climate.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.