Green roofs—or vegetated roofs—have grown in popularity in recent years. Installations have nearly doubled since 2008, and across the US, almost 17.5 million square feet of roofs are considered green. Most growth has occurred in urban areas, with many cities using green roofs to combat heat and reduce storm water runoff.
We have worked on many projects with green roofs, including office buildings like One Place and adaptive reuse projects like Market One. In our experience, green roofs offer many benefits to owners and communities but require careful planning and maintenance.
This article will help you decide if a green roof is right for your project by discussing different types of green roofs, their benefits, and a few considerations before installing.
Types of Green Roofs
Green roofs typically consist of a waterproof membrane, a growing medium, and vegetation overlying a traditional roof. They come in one of three types:
- Extensive
- Semi-Intensive
- Intensive
Extensive roofs are the shallowest option. Typically, they consist of trays containing three to four inches of growing medium that hold shallow-rooted vegetation. Due to their simplicity, they are one of the most popular options.
Semi-intensive refers to roofs with a slightly deeper growing medium, usually three to four inches. These roofs can contain grasses and some perennials.
Lastly, intensive roofs are anything beyond eight inches of growing medium. These roofs can support most vegetation, including trees, and act as occupiable gardens.
Benefits of Green Roofs
Green roofs offer numerous benefits and can be part of a holistic sustainability strategy. When well-designed and maintained, they help mitigate urban heat, reduce energy costs, manage storm water, and promote occupant well-being.
1. Heat Management
Cities tend to experience higher temperatures in the summer than rural or suburban areas. Building materials with high emissivity ratings—including concrete, brick, and asphalt—warm the surrounding air, a phenomenon known as the urban heat island effect. Combating this issue has become a strategic goal for many cities.
Vegetation is a city’s greatest guardrail against heat islands, with plants and trees releasing water through their leaves to cool surrounding areas. In high-density urban environments, increasing street-side plantings or ground-level green space can be difficult, making green roofs a logical solution.
In urban areas, rooftop vegetation can held modulate temperatures.
In addition to cooling the surrounding area, green roofs help modulate interior temperatures. Vegetation protects roof membranes from UV rays while providing additional insulation. It also helps reduce the energy needed to cool air by lowering the temperature surrounding rooftop air conditioning units.
Together, these effects result in lower-than-average energy costs. A study by the City of Chicago estimated that whole-scale greening of the city’s rooftops could save nearly $100 million a year.
2. Storm Water Runoff Management
Green roofs can also help with storm water management. Whether on roofs or at ground level, vegetation acts as a sponge, absorbing water and releasing it through evaporation.
In urban areas with high concentrations of hardscape, increased vegetation can reduce runoff and the stress on sewer systems during peak flow periods.
With climate change, heavy rain events and flash flooding are becoming more common. Green roofs are one way to combat these issues and make cities more resilient to extreme weather.
3. Occupant Well-Being
Green roofs also benefit occupants. In urban areas, they can provide a much-needed green space for recreation or socializing. Including one in a multifamily, mixed-use, or commercial development can help attract and retain tenants, especially in competitive markets.
Even when non-occupiable, they can provide a visual connection to nature. Cultivating our innate love of nature—a concept known as biophilia—is crucial to human well-being.
Green roofs can be an attractive amenity for tenants.
Studies show that views of nature help reduce stress and improve moods. They can help improve recovery times in hospitals and boost test scores in schools. No matter your project type, providing views of greenery has a tangible impact on occupants.
Considerations When Planning a Green Roof
Like all building systems, green roofs require careful planning during design. Structural loads are perhaps the most crucial consideration.
More intensive systems involve greater quantities of growing material, increasing the roof’s weight. Roofs should be robust enough to support the growing material and vegetation.
An architect, working with a landscaping professional, can help plan the position of the plants to minimize stress on the structural system. For example, it helps to plant trees over structural columns rather than bays.
Beyond structural concerns, consider how you will maintain the roof. You may need to assign staff to monitor the roof, especially in dry seasons. Dried vegetation can be a fire hazard, and you should be prepared to stay on top of watering.
North Ridge Pavilion, one of the first green roofs in Iowa, is still lasting today.
Irrigation systems can be one solution. Although they increase water consumption, they can make maintenance more manageable. Another solution is to install a rainwater collection system for watering the roof.
Lastly, it helps to consider your sustainability goals. Despite the benefits, green roofs may not fit every project. Those more focused on on-site energy generation may use their rooftop for a solar array. Others may focus more on rainwater harvesting. Your architect should help you weigh the pros and cons and determine the right path for your project.
Green Roofs and Green Building Certification Systems
Greens roofs offer multiple public and private benefits, especially in dense, urban areas lacking greenspace. In addition to their environmental benefits, they improve occupant well-being and provide recreational opportunities. They are also a way to demonstrate your organization’s values and your commitment to sustainability.
If you are considering a green roof, your architect can help you evaluate your options. While an extensive system is the simplest approach, intensive systems work well for those wanting a more robust garden space. Intensive systems should be discussed early in the design process to allow your architect to plan roofing and structural systems accordingly.
Green roofs are just one component of a sustainable design strategy. Their impact on urban heat islands and storm water runoff can help you achieve points with green building certification systems like Leadership in Energy and Environmental Design (LEED). To learn more, read about the most popular certification systems and their requirements.