image source: Penn State Green Roof Research Center

Green roofs or vegetated roof covers (also referred to as living roofs, nature roofs and eco-roofs) are a thin layer of living plants growing on top of a roof. A green roof is not a collection of potted plants to decorate a roof space but rather an extension of a conventional roof which involves instillation of a layered system of membranes, substrate and plants.

Green roofs are not a new technology. Historically green roofs have been used for hundreds of years if not thousands to help keep houses in colder climes warm and houses in warmer climes cooler. In the 1960s northern Europeans began to popularize green roofs as a means of improving urban environmental quality through reduction of stormwater runoff and the addition of green space.

Local Example

In this video, David Yount, Green Sanctuary Committee Member at the Unitarian Universalist Congregation of Duluth, comments on the installation and use of a green roof.

Applications

Installation of a green roof provides a suite of advantages, to include:

1. Stormwater management: reduces stormwater runoff so there is less water directed into storm drains
   • Green roofs will intercept between 15 and 90% of rooftop runoff. Absorption of runoff into a green roof system will vary between 50-60% and is related to the type of growing medium and plant cover variability. \ variation in absorption rates can be as great as 50% based on differences in temperature, wind, evapotranspiration rates and plant uptake 
   • Visit our Water Quality Impacts section to find out how how land disturbance that creates impervious surface and other human activities can cause increased runoff and pollutant loads to streams and lakes.
2. Energy conservation – reduces the need for energy to heat house in winter and cool in summer. The vegetation layer and trapped air help insulate a buildings interior, providing up to 25% additional insulation when the soils are dry to slightly most. Additionally, the plants natural evapotranspiration process results in cooling of roof surfaces in summer. Heat loss due to the slowing of winter winds by friction created by the plant surface can reduce heat loss by 50%
3. Urban heat islands – Cities have microclimates characterized by the city area being warmer than surrounding countryside. Green roofs absorb heat and lower ambient temperature thus aiding in the reduction of urban heat island impact
4. Economics- green roofs installed correctly can extend the life of a conventional roof by a factor of three times. The green roof protects the roof surface from UV light, large temperature fluctuations and normal wear and tear associated with exposed surface roofs. As noted earlier green roofs will provide energy savings.
5. Natural Benefits – Green roofs can incorporate food gardens that can additionally stress organic methods. Careful plant selections can favor habitat creation utilizing appropriate threatened native species  or as a food supply for birds and favorable insects
6. Aesthetic and social benefits – Green roofs provide pleasant sights, smells and sounds that promote favorable social interaction and are beneficial to quality of life in urban areas 
7. Air Quality improvements – Plant photosynthesis reduces the negative impact of carbon dioxide and produces oxygen. Plants also help filter out airborne pollutants
8. Sound adsorption -  green roofs are good sound insulators reducing the impact of noise to areas below the roof

Examples

Materials and Installation

1. Analyze the structure
   The first, very important step to take when contemplating the installation of a green roof is to involve licensed architects or engineering service (structural, mechanical and electrical). Individuals assigned to the project must have ample familiarity with the requirements of green roof technology. Their initial task will be to determine the structural soundness of the building and its roof (be it from new building plans or a proposed retrofitted roof on an existing building) to support a green roof. Their analysis should take into consideration:
   
   • The buildings position and structural weight loading capacity. In northern environments snow loading and roof design features to accommodate snow must be considered in the analysis. The roof loading factor above snow loading should exceed the saturated per square foot weight of the system being installed plus a safety factor suggested by consultant engineers
   • The condition of roof structure:
     • When was new roof last installed, its condition and are repairs needed ( these concerns apply if a green roof structure is being proposed as a retrofit to an existing building)
     • Height above ground
     • Roofs orientation and slope or pitch
   • Availability of electrical and water supply
   • Accessibility of roof for instillation and required maintenance of green roof
2. Design plan development
   Once the decision is made to proceed, a design plan needs to be developed that will include additional input from professionals such as landscape architects, horticulturists, green roof system suppliers, and construction companies to be involved. The amount of involvement required will depend on the complexity of the project. Design should:
   • Set specific goals for the green roof to achieve e.g. provide a surface that is 75% permeable
   • Reduce weight when possible
   • Provide a planting plan and plant list that includes hardy native shallow rooted plants. The plan should take into account building orientation and local climate conditions.
   • Be a simple design that utilizes durable standard sized materials and fittings to reduce waste and facilitate maintenance needs.
   • Identify and place any irrigation system (preferably drip-irrigation) required beyond normal rainfall to support  roof vegetation
   With extensive green roof structures characteristics of plants to be selected is a most important consideration:
   • Use a variety of plants; a mix of suitable plants increases the success rat of establishing an self-maintaining green roof.
   • Spreading, low growth habit with fibrous root system
   • Hardy  in local area with high tolerance for drought
   • Require no special nutrient or watering conditions. In areas prone to acid rain, buffering of thin soils might be required
   • Require a low maintenance effort-trimming and weeding
   • Do not produce wind blown seeds that can invade other gardens
   • Provide the desired aesthetic impact  
   With intensive green roof structures:
   • Provide for a work area for production of mulch and for composting. A small greenhouse is also a useful addition
   • Provide public space to encourage accessibility and social activity. Do not ignore need for garbage disposal and recycling
   • Provide habitat for desirable urban birds and insects
   • Pave as little area as possible and explore the use of pervious pavement when pavement is needed
3. Construction Materials
   Modular systems – there are a number of  modular systems available, these systems:
   • Are made up of individual plastic modular, interlocking containers about two feet squared and loaded with a drainage, system, growing medium and vegetation prior to placement
   • are quick to install
   • Are ideal for extensive green roofs, however, deeper modules make the system adaptable to intensive green roofs The growing medium is engineered product containing some natural soil, modular growing medium depths can vary between 2-18 inches deep
   • Are lightweight , weighing in at between 15-50 pounds per square foot when fully saturated
   • The modules can normally be placed on existing roof surfaces.  Individual modules can be removed and replaced allowing quick repair or rotated  for seasonal effect
   • Can be installed at any time of the year because the modules are pre-planted. Since the plantings are established prior to instillation there is a high level of success
   • Must be watered and weeded for the first year. In areas prone to acid rain, the application of buffering compounds can be required to prevent acidification of thin soils
   Built-In-Place systems  -  are the classical and probably commonest green roof systems found, these systems:
   • Are best used for  intensive green roofs
   • Require detail design plans and a long installation time due to accommodate the sequenced incorporation of garden materials and landscapes
   • Require plants  to be pre-established in pots for one to two years prior instillation on the roof
   • Have a growing medium that is soil based with a depth of at least four to eight inches (normal range 8-24 inches deep)
   • Have a weight loading on the roof structure of 40 – 200 pounds per square foot when fully saturated and thus may require additional structural support being added to the building
   • Provide for display of a diverse plant selection and the incorporation of public spaces. In areas prone to acid rain, steps may need to be taken to prevent acidification of soils.
   Materials  are deployed on the roof in a series of layers in the following sequence:

   Cross section of typical green roof system image from: http://www.greenroofs.org

   
   
   • Supporting roof structure
   • Waterproof and root repellent membrane(s)
   • Insulation
   • Drainage, aeration, water storage  and a root barrier system
   • A lightweight growing medium that is not necessarily soil based
   • Collection of specialized mix of plants with an ability to survive harsh environmental conditions at the roof top
   • The diagram below provides suggested methods and dimenstions dealing with roof penetrations, buffer strips and roof edges
   

   1. Wall cap flashing	9. Waterproof membrane
   2. Drain rock, paving slab or other buffer equivalent	10. Thermal insulation
   3. Wood, steel or concrete curb/edging (optional)	11. Vapor barrier
   4. Planting	12. Area drain
   5. Growing medium	13. Structural slab
   6. Filter layer	14. Building Interior
   7. Drainage Layer	15. Wall flashing
   8. Protection layer and root barrier
   Source: Stormwater Source Control Design Guidelines 2005; Greater Vancouver Regional District, BC, CA.

   The variety of available materials to accomplish each of these functions is numerous and there are new entries into field on a regular basis. Employment of professional consultants (Architects and Horticulturists) and reputable suppliers, while adding expense to the project is highly recommended and critical to success.

   Past experience and technical expertise in the planning, engineering and instillation of green roofs should be primary criteria for selection of these professionals. With such help, the end product achieved should be cost effective and provide optimal functionality.
4. Cost
   • Costs per square foot (2005 dollars)
     • Intensive green roofs should cost between $16 and $35 per square foot
     • Extensive green roofs should cost between $7 and $35 per square foot
   • Installations of green roofs are sometimes eligible for tax relief, energy conservation credits or lower storm water charges from municipalities and should be investigated.

Suggested References: Guidebooks, websites and pamphlets

1. RoofBloom
   A Minnesota resource for homeowners interested in having a green roof on their garage, porch, shed, or other small outbuilding. Site includes "Green your Garage" guide.
2. Design Guidelines For Green Roofs
   by Steven Peck and Monica Kuhn
   A Canadian comprehensive guide from the architects point of view to the development of green roofs. The publication contains a strong section on relative costs and their variables and some Canadian case studies.
3. Introductory Manual for Greening Roofs
   by Cornelia Oberlander, Elisabeth Whitelaw and Eva Matsuzaki for the Canadian Department of Public Works and Government Services.
   Another outstanding comprehensive guide to help develop a thorough understanding of designing and constructing a green roof. Strong sections on suppliers of systems and related products, resources and history.
4. Green Roofs: Stormwater Management From the Top Down
   by Katrin Scholz-Barth in Environmental Design+Construction –The Premier Source for Integrated High-Performance Building. A good overview of Green Roof development from a stormwater management perspective.
5. Great Lakes WATER Institute (University of Wisconsin) Green Roof Project
   One step toward a green facility. An extensive web site with buttons that allows investigation from the development of the institute’s green roof project to  a wealth of information about the siting, design, instillation of green roofs and applicable plants lists along with suggested  planting plans.
6. The Greenroof Projects Database
   This is a database of green roof projects from around the world.
7. Stormwater Source Control Design Guidelines 2005-Green Roofs
   A thorough discussion of the use and instillation of extensive green roofs
8. Toronto, Ontario, Canada
   has a new website dedicated to promoting the use of green roof technology to mitigate a variety of urban environmental impacts
9. Greenroofs.com
   bills itself as the international greenroof industry’s resource and online information portal with a goal of promoting organic greenroof architecture and technologies. The website includes a Directory of Manufacturers, Suppliers, Professional Services, Organizations, Students & Green Resources
10. The Scandinavian Green Roof Institute
    in Sweden includes many resources and publishes a journal.

Tips and Wisdom

The greatest wisdom when contemplating the instillation of a green roof is to realize the project is not a DIY project and the owners vision is best translated into reality with the help of an experienced consultant-most likely a group of consultants with different expertise.

The owners familiarity with green roof construction and instillation will aid communication of their vision to chosen consultants. Being conversant with green roof technology will also encourage useful participation of the owner in planning and problem solving as project proceeds.  

Some of the types of knowledge that will need to be gathered, decisions that need to be made and questions that will need to be asked if a project is to proceed successfully are summarized below: 

1. Supporting Roof Structure
   • Is the existing roof structurally capable of accommodating proposed landscape and people?
   • If required, can an existing roof be simply and cost effectively be strengthened or will costly changes to a buildings foundations be required?
   • A variety of calculations, decisions and coordination efforts that relate to items like weight loading of roof based on saturated soil not dry soil, placement of heavy components to take advantage of existing structural features and coordinating landscape and structural  requirements for new roofs
   • What is the slope or pitch of the roof? If the slope is greater than 20° slippage and slumping of soil and plant materials may occur. Some type of stabilization system will need to be engineered to prevent gravity creep of heavier materials. A slightly sloping roof (5° {1:12} to 20° {4:12} ) is not a problem and has the advantage of aiding in drainage of excess water from plant root zones during storm events. Roofs with less than a 2° slope may require special drainage to avoid constantly waterlogged soil 
   • Placement locations for tree and safety harness anchors
   • Compliance with building codes  and obtaining any necessary permits or inspections
   • Study climatic conditions on the roof to understand hours of sunshine through varying seasons, range of wind force and direction and potential sources of glare. Such information relate to plant selection and creating a people friendly environment if the roof is to be accessible
   • Other garden elements, which will impact landscape and people, include such items as location of pavement, planter boxes, work spaces, research areas, electrical supply, roof-top fans /hoods and stormwater collection-storage facilities and pumps to move this water.
2. Membrane Layer
   • Choice will be dictated mostly by past experience with available materials and knowledge with condition of roof, available budget  and ease of repair that is required
   • Be certain that selected membranes are chemically compatible with each other             and other roofing materials
   • The membrane layer will be protected from damage occurring during instillation and from UV light by the addition of a protection board
   • Root barriers may or may not be recommended depending on plant selection
3. Growing Medium
   • Most suitable growing mediums are engineered and thus commercially provided and contain little natural soil. A typical soil is made from:
     • Gravel
     • Sand
     • Crushed brick
     • A little natural soil
     • Lightweight expanded clay aggregates (L.E.C.A.)
     • Peat
     • Organic matter
   • These soils are relatively lightweight, when wet they should weigh in the range of 55 pounds per cubic foot
4. Drainage System
   • Drainage systems are usually light weight prefabricated mats that will be placed to cover the entire roof. They often will incorporate a filter fabric to retain fine materials from clogging lower layer.
   • The drainage system should direct rainwater to planted areas where the roots can intercept water and slow runoff. Excess stormwater should be directed to a water collection- storage facility for later use in irrigation of green roof or other permissible uses
5. Plant Materials
   • Plant selection is a critical factor in success of a green roof. Shallow rooted, regenerative plants that are resistant to direct sunlight, drought, frost and snow cover are most desirable. Make certain that chosen plant species are sufficiently hardy for planting in local area. Decisions should be based on the USDA hardiness zone for the local area. Consultants should be familiar with  determining hardiness zone and selecting appropriate plants. Examine the selection of potential plant species tried at the Great Lakes WATER Institute (U. of Wisconsin-Milwaukee) Green Roof Project.
     • Extensive gardens will limit choice to succulents like sedum, some alpine plants and shallow rooted grasses.
     • With intensive green roofs the diversity increases primarily because of the deeper soils that can be used. Even trees can be planted provided root balls are anchored to provide stability.
   • Vegetable garden spaces can be incorporated into intensive green roof plans providing there is adequate water available
   • Propagation from seed, cuttings and plant plugs usually occurs off-site to encourage development of strong root systems
   • To establish plants, regular irrigation throughout the first couple of years must occur. A light weight drip irrigation system directed to the root zone is efficient. If a drip irrigation system is not provided then hose bibs must be available for manual watering.

Limitations

There are few limitations to adding a green roof to a building, however, serious constraints are:

• Soundness of chosen building, particularly the structural capacity of the roof
• Roof leaks due to incorrect installation practices
• Capital costs are high and a realistic budget within means must be developed. Often the budget may not support  the original vision of the owner and compromises must be made
• Consultants must be carefully chosen, with choice being based on their technical knowledge and experience with green roofs