Guide to Green Roofs – Rooftop Gardens

Green roofs, also known as rooftop gardens, are an extension of the existing roof for both residential and commercial buildings that is partially or completely covered with vegetation. By turning your roof into a beautiful, functional garden you can gain a long list of benefits like long-term cost savings and positive return on building investment.

The motivation for installing a green roof systems is varied, as they offer plenty of benefits over traditional roofing. For some, green roofing is a necessary step on the path to LEED certification. Some cultivate green roof systems simply out of a desire to be environmentally responsible. Plenty of others are looking for ways to conserve on energy as a means of reducing costs.

Every building’s roof is unique, and when planning on constructing a rooftop garden it is necessary to consult with a seasoned green roof construction company so a customized design can be created to fit the unique demands of the roof. Green roof systems will vary according to region, climate, building and green roof type and design. Some contractors offer modular green roof systems where layers, including the plants, are prepared in movable interlocking grids, or each green roof component/layer may be installed separately.

However the green roof is installed there are some important aspects to consider before beginning the process, including the roof’s slope, structural load capacity, and existing roof materials. Additionally, it is necessary to review the drainage system, waterproofing, and electrical and water supply currently in place.

Green Roof Components

Green roof systems involve layering different materials in order to provide complete water proofing. Rooftop gardens typically have 6 to 7 different layers. Starting from the bottom up, a green roof consists of a high quality waterproof membrane, a root repellant system, an optional layer of insulation, a drainage layer, a filter cloth for fine soils, a lightweight growing medium or soil substrate, and plants. For a detailed diagram of the layers, see intensive and extensive green roofs below.

Furthermore, in order to provide additional drainage, fire control, and access to the roof for maintenance, a shallow layer of gravel or pebbles is usually used around the outside perimeter of the rooftop garden.

Green Roof Benefits

Historically, green roofs have been used to insulate buildings by preventing heat from moving through the roof. Technological advancements have allowed rooftop gardens to do much more, including absorb rainwater, lower urban air temperatures and combat heat island effects.

Green Roofs provide the following benefits:

  • Reduce heat loss and energy consumption in winter conditions
  • Reduce cooling loads on a building
  • Lower urban air temperatures (requires concentration of green roofs in metropolitan area)
  • Reduce stormwater run off
  • Provide increased sound insulation
  • Improve building’s LEED rating
  • Increase roof lifespan

Types of Green Roofs

There are two types of rooftop gardens: intensive and extensive. Primarily, the differences lie in the accessibility and amount of maintenance required. Plant selection also differs for both types of rooftop gardens and depends on a variety of factors, including climate, depth of planting medium, load capacity, height and slope of the roof, and type of irrigation system.

Intensive Green Roofs

Intensive green roofs typically grow large plants or maintain conventional lawns. In order to create an effective environment for sustaining large plant life a reasonable depth of soil is required, as well as robust irrigation and regular maintenance. Intensive rooftop gardens are more like a park intended for human interaction with easy access and elaborate landscapes, such as small trees, shrubs, fountains, and walking paths. As a result, intensive roof gardens are more labor-intensive, hence the development of the name, and require additional engineering to conform the roof to load requirements. Ideally, intensive green roofs should be constructed on flat roofs or on a mild roof slope of up to 3%.
 

Extensive Green Roofs

Extensive green roofs differ from intensive rooftop gardens in that they are designed to be more self-sustaining, require minimal maintenance, and are not intended for recreation or regular foot traffic. The plant life also differs. Extensive roof gardens are generally landscaped with drought tolerant, self-seeding sedums, grasses, mosses and meadow flowers which require very little irrigation, fertilization or maintenance. Usually extensive green roofs do not require reengineering of the existing roof structure and can be constructed on roofs with slopes up to 33%. Furthermore, extensive green roofs are less costly than intensive ones due to labor and materials costs, as well as maintenance requirements.
 

Ideal Plants for a Green Roof System

Generally speaking, the plants you select for green roofing should be ecologically compatible with your construction location and resistant to weather, insects and disease. Ideal plants for a green roof system are very hardy, fast growing, non-invasive, and fire resistant. For minimal maintenance, they should have a shallow, fibrous root system and be resistant to damage when walked on. Succulent groundcovers like Delosperma, Jovibarba and Sedum are top choices for green roofing.

Sedum is a low-growing, shallow-rooted, perennial plant that is very tolerant of heat, sun, wind, drought, salt, disease and insects. The hardiness of this plant makes it a great option for green roof systems. A sedum green roof is generally comprised of a waterproof membrane, a drainage layer, and a blanket of planted sedum. Don’t let the “green” roof name fool you – sedum is available in a range of colors. Relative to other green roof systems, a sedum roof requires minimal maintenance and is durable enough to be walked on without causing damage to the foliage

Green Roof Cost

The cost of a green roof – rooftop garden – requires an initial investment that should not overshadow its long term benefits. A properly designed green roof can greatly reduce heating and cooling, as well as extend the life of the roof. Typically, an extensive rooftop garden with all the necessary layers/components can be installed for $15-25 per square foot. Intensive green roofs require a greater investment than extensive, but provide the additional benefits of accessibility and larger plant life. Costs can vary according to the roof’s specific requirements. It is important to speak with a qualified green roofing contractor about the range of costs, benefits and roofing requirements for different green roof systems and designs.

Kirberg is a full-service commercial roofing company with over 90 years of experiencing operating in the Midwest. As an innovator, Kirberg provides green roofing to buildings of all sizes. The Kirberg team of seasoned roofing experts is standing by to help answer any of your questions and would be happy to provide a free consultation about green roofs, LEED certification or any other facet of green roof construction that you may be interested in. Contact us today at 866.547.2374 to get started!

Interested in learning more?
Check out Kirberg’s published green roofing article in the National Roofing Contractor’s Association magazine.