Question: I am building a new home and am interested in having a “green roof.” What are the main benefits of green roofs? Can you recommend some good resources to learn more?
First let’s look at some basic information on green roofs and how to decide if a green roof is a good option for your new house.
What is a Green Roof?
A vegetated roof (commonly known as a green roof, living roof or eco-roof) is an alternative to conventional roof surfaces that is designed to support plant growth. It is a best management practice (BMP) used to reduce stormwater runoff and pollution. Vegetated roofs capture, filter, and store stormwater briefly before it flows into storm drains. Some of the captured stormwater evaporates or is used by the plants, which reduces the volume, velocity and runoff of water and pollutants reaching sewers and waterways.
Vegetated roofs consist of a layered system of roofing that includes vegetation, a lightweight growing medium, filter membrane, drainage layer, waterproof and root barrier, thermal insulation, and structural roof support. There are three categories of green roofs:
- Extensive vegetated roofs: the most common type, have a shallow (4-6“ deep) growing medium, limited plant diversity, often no need for watering after plants are established, and require less structural support. They are low maintenance and are well suited for large areas.
- Intensive vegetated roofs: have a deeper (10” to 4’) growing medium and potential for a greater diversity of plants. They require watering or an irrigation system, more maintenance, more structural support, and a higher initial investment.
- Semi-intensive vegetated roofs combine features of both the extensive and intensive types.
Benefits of a Green Roof
Green roofs have gained popularity recently, because they can conserve energy, save money, manage stormwater, improve air, and water quality and add plant diversity and habitat to local ecosystems. Vegetated roofs can:
- Reduce temperatures on structures up to 20°F in summer and provide more insulation to the house interior in winter, resulting in reduced energy costs.
- Extend the life of your roof by reducing temperature fluctuations that damage roof materials. One German study shows that green roofs last two times longer than a traditional roof.
- Reduce noise from traffic and other urban sounds.
- Mitigate stormwater runoff, which can prevent drainage and runoff problems in your yard as well as limit the amount of runoff and pollutants flowing into local waterways.
- Moderate the heat island effect by shielding buildings from high heat levels produced by the sun. According to the EPA, a green roof surface can be as much as 90°F cooler than a traditional dark rooftop.
- Improve air quality by reducing smog and ozone produced at high temperatures. Green roof plants also sequester carbon dioxide and release oxygen into the air.
- Act as a temporary habitat for birds, bats, insects, and some other animals as they move from one natural area to another.
- Provide attractive additional space for gardening and other recreational activities.
A University of Michigan study compared a 2,000 square meter conventional roof with a green roof and found that over an estimated lifespan of 40 years, a green roof would save about $200,000, of which nearly two-thirds would be due to energy costs.
For more information, see Green Roofs for Healthy Cities for comprehensive list of benefits and Frequently Asked Questions.
The Right Plants for Green Roofs
For plants to survive on a green roof, they must be perennial or self-seeding, low growing, drought tolerant, densely planted, climate resilient and appropriate for the plant hardiness zone in your area. Green roofs require a specially made lightweight mixture of inorganic and organic growing medium due weight restrictions. Extensive vegetated roof plants need to be able to thrive in a thin layer of low nutrient growing medium. With the more soil depth, intensive vegetated roofs can accommodate a more diverse variety of plants, including small trees and shrubs.
Is a Green Roof Right for You?
Before deciding to incorporate a green roof into your new house design, there are several considerations to take into count. Be sure to consult an expert to help you make the right decisions for your house. Start by thinking through the topics below:
What is the primary purpose of installing a green roof on your house?
Reduced heating and cooling costs, better stormwater management, providing a garden habitat for birds and other wildlife or other benefits? Your objectives will help determine whether to proceed and the type of roof to install.
Can the house design accommodate a vegetated roof, or can it be adapted for this purpose?
Vegetated roofs can be installed on flat or slightly sloped roofs and must be built to certain engineering specifications depending on the type of roof and the added weight of this roofing system.
Are you willing to pay the cost of installing the roof, the plantings and continued maintenance?
These costs will depend on the site conditions, type, dimensions and design of roof, roof materials, and plantings.
How will you provide initial and continued maintenance for the roof?
Maintenance can range from minimal weeding and watering for extensive vegetated roofs to the need for an irrigation system and more frequent and time-consuming upkeep for intensive vegetated roofs.
Be sure to check out the Charlottesville City Hall and Downtown Police Station green roofs and other examples in the Mid-Atlantic. To promote the use of green roofs, the City of Charlottesville has reduced green roof building permit fees for commercial/non-residential and residential properties in the city by 50%.
If you find out that a green roof is not right for your new or existing home, there are other options you can consider. Cool Roofs and other green roof alternatives can achieve some of the same benefits as green roofs but require less initial costs. If you want to learn more, read through the references listed below.
References
“Basic Principles of Watershed Restoration and Stormwater Management in the Chesapeake Bay Region,” Dindinger, J. et al., Virginia Tech, Virginia Cooperative Extension, Publication No. SPES-195, 8 Dec. 2020.
“Best Management Practice Fact Sheet 5: Vegetated Roofs,” Sample, D. et al., Virginia Tech, Virginia Cooperative Extension, Publication No. 426-124, 11 Dec. 2019.
“Best Management Practices for Stormwater Runoff,” Piedmont Master Gardeners, Ask a Master Gardener Blog, 18 May 2021.
“Green Roofs,” Susan Barton, Extension Specialist, University of Delaware, Agriculture & Natural Resources, Fact Sheet & Publications, Bulletin #128, UD Cooperative Extension, Revised 31 Jan. 2009.
“Green Roofs for Stormwater,” Jarrett, A., Professor Emeritus, Pennsylvania State University, PennState Extension, Updated 24 May 2016.
“Pioneering Higher Ground: Green Roof Lessons for Planting Design”. Hansplant, L., Landscape Architect, Roofmeadow, Ecological Landscape Alliance (ELA), 15 May 2019.
“Urban Stormwater: Terms and Definitions,” Sample, D., et al., Virginia Tech, Virginia Cooperative Extension, Publication No. 426-119, 14 Jul. 2020.
“Vegetated Roof,” Virginia Department of Environmental Quality (DEQ), Stormwater Design Specification No. 5, Version 2.3, 1 Mar. 2011.
“What is A Green Roof?” National Parks Service, U.S. Department of the Interior, Technical Preservation Services, Accessed 2021.
