Green Roofs

Green Roof Layers

Green roof designs and components may vary depending on the type of system and product manufacturer. The list below includes the main components of a green roof. Extensive green roof systems may combine (or exclude) some of the optional layers.

• Structured roof deck: Whether retrofitting an existing building or planning for a new construction, the roof deck provides the structural support for the added weight of the green roof, as well as any additional loads. Structured roof decks most often consist of concrete (cast in place or precast), metal or timber.
• Waterproof membrane: Installed directly on the roof deck, the waterproof membrane is often applied in either a liquid form or in single-ply sheets.
• Root barrier: The root barrier prevents roots from puncturing or damaging the waterproof membrane. Depending on the green roof design and product type, the root barrier can vary in thickness and installation. Coatings or chemical additives like copper can be included as part of the waterproof membrane, in which case these two layers are combined into one. Some designs include this layer above the drainage layer to prevent roots from damaging or clogging the drainage layer.
• Insulation (optional): This optional layer can be installed either above or below the roof deck, and above the waterproof membrane, or may be combined as part of the drainage layer depending on the design and manufacturer. The added insulation helps to regulate the building temperature and if installed above the waterproof membrane, protects it from temperature swings.
• Drainage layer: The drainage layer is critical for green roofs, especially flat green roofs. This layer, usually made up of plastic sheets, fabrics, or synthetic materials, consists of channels that direct water through the green roof system to appropriate downspouts and gutters. For green roofs that require a lot of water, the drainage layer may also include a retention mat that consists of holding cups to provide excess water to the root systems and reduce stormwater runoff.
• Filter Fabric: A filter fabric prevents soil particles from clogging or blocking the drainage layer. This thin layer is made of geotextiles and holds the growing medium in place while allowing water to flow through to the drainage layer. Some designs, particularly extensive green roof systems, combine the drainage layer and filter fabric layer in one.
• Lightweight soil (engineered growing medium): A green roof’s soil must be lightweight, rich in nutrients, and be able to both absorb and drain stormwater. Engineered mixtures are customized for green roof conditions and environments. A suitable mixture may include 1/3 clean topsoil, 1/3 compost, 1/3 inorganic material.
• Vegetation: Common green roof plants include sedums and succulents that are hardy and do not require much irrigation. Plant selection is often determined by green roof design, climate conditions, maintenance requirements, and irrigation demands. Sedums and succulents with shallow roots are common extensive green roof plants, while grasses, shrubs and perennials generally may only be incorporated into intensive green roofs.

Schematic Design

• Consider the purpose and goals of the green roof for your project, and determine the type of green roof system that is most appropriate. Evaluate feasibility, goals, cost, use, and accessibility.  
• Evaluate the load requirements of the proposed green roof, and check the current load capacity on an existing building. This should be done by a structural engineer. 
• Research the cost implications for a variety of green roof types, including possible incentives. Grants or tax credits may be available to offset the added cost of green roofs. If project budget is a barrier, a shallow, simple, extensive green roof can provide many of the benefits of a green roof while keeping costs for installation and maintenance requirements low.
• Determine any local codes or regulations pertaining to green roof installation or design. 
• Consult with a green roof designer on type of roof, plant selection, irrigation requirements and installation method.

Design Development

• Perform an energy modeling analysis of the project incorporating the added insulation of the green roof. Added energy performance can contribute to EAc1 Optimize Energy Performance.
• Determine an appropriate green roof design or installation technology for your project. When selecting plantings, consider soil depth, climate, maintenance, and irrigation demands (both immediate and ongoing). Drip irrigationDrip irrigation delivers water at low pressure through buried mains and submains. From the submains, water is distributed to the soil through a network of perforated tubes or emitters. Drip irrigation is a high-efficiency type of microirrigation. is often used on intensive green roofs, or for vegetation requiring ongoing irrigation. Extensive green roofs can be very low-maintenance, especially if they have incorporated drought-tolerant plants such as sedums that require little to no irrigation.
• By using efficient or no irrigation after establishment, you can help achieve WEc1: Water Efficient Landscaping. In addition, green roof designs that are greater than 50% of the roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1. allow a project to achieve SSc7.1: Heat Island Effect—Roof.
• Incorporate native or adaptive plants in the design to promote biodiversity and maintain native habitats. If a project is pursuing LEED certification and achieves SSc2: Development Density and Community Connectivity, the green roof area may also contribute to other LEED credits such as SSc5.1: Site Development—Protect or Restore Habitat and SSc5.2 Site Development—Maximize Open Space. 
• If the design of the roof changes, double-check load capacity with your structural engineer.                                                                             The city of Portland, Oregon, has done extensive testing of the quantity and quality of stormwater runoff from this green roof on the Hamilton Terrace Apartment complex. City of Portland, Office of Sustainable Development
• Incorporate the green roof into the stormwater management plan, and consider it when making calculations for reduced stormwater rate and quantity, and improved quality. Reduction of stormwater runoff achieved by a green roof is dependent upon many factors including soil depth, soil type and retention capacity, as well as roof slope and level of saturation of the soil before a storm. Link the green roof design and benefits to other stormwater management strategies such as cisterns, rain barrels, rain gardens, etc as applicable.
• A green roof may contribute to a project achieving SSc6.1: Stormwater Management—Quantity Control and SSc6.2: Stormwater Management—Quality Control.
• Protect the roof from fire by including a 1–2 foot barrier of pavers or crushed gravel around the perimeter of the roof, and select plants that can resist drying out and don’t produce flammable oils or resins. (Flammable plants typically have gummy sap, high resin content, papery bark, aromatic needle-like leaves and stems containing oils and waxy surfaces.)
• Keep costs down by maintaining a simple design, shallower soil depth and less plant variety. Using local materials may also help lower cost.

Construction Documents

• Include a maintenance contract in the green roof specifications to ensure that the plants are properly established and cared for, and to verify that all systems are running as designed.
• Contract with a single-source supplier as mixing and matching systems may violate warranties.

Construction

• Perform a water impermeability test after the waterproof membrane is installed. This will take 24–48 hours to ensure that the membrane is secure and no leaks occur.
• Perform a leak detection test to determine roof areas that may not be sealed properly. A green roof can make it more difficult and costly to locate roof leaks, so it is best to do so before the green roof is installed. Many companies now use Electric Field Vector Mapping (EFVM) to determine areas that are not sealed properly or to locate a leak after the green roof has been installed. 
• When planting, use an erosion-prevention mat of jute or straw above the growing media, and create openings for vegetation. This will help the plants become established and have a better chance of surviving the harsher microclimate of the roof. The erosion mat will decay over time once the plants are established.

Operations & Maintenance

• Maintenance requirements depend on the size of the green roof, plant selection and design. For more involved green roof designs, the green roof contractor or installation company often provides a 1–2 year maintenance agreement.
• Ensure regular maintenance checks at least twice a year, and more often for intensive green roofs. Examine plant survivability, weeds, potential standing water or leaks, and proper functioning of drainage pipes and irrigation systems. Most extensive green roofs require the same maintenance attention of typical landscaping.