Why And How To Install A Green Roof

Categories: Green

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

  1. house with green roofStormwater management:reducesstormwater 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 Impactssection 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



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
    • greenroof2a.jpg

  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:
  4. roof layers

    Cross section of typical green roof system

    image fromhttp://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
    roof diagram

    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.
  5. images.jpg
  6. 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.
    • green-roof-in-bloom.jpg

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