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Minnesota Stormwater Manual

Green roofs terminology and glossary
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Glossary

  • Ballast: A material used to hold loose laid roofing materials in place [1]
  • Built in place green roof: in a built-in-place green roof, the layers of the green roof are installed on-site, as opposed to green roof trays or mats, which are delivered pre-assembled
  • Cation exchange capacity: The quantity of positive ions (cation) that can be absorbed by growing medium. [2]
  • Compaction: increase in growing medium density
  • Curtain wall: A non-structural outer wall, often of glass or steel
  • Dead load: All permanently placed materials on and below the roof (e ,g. green roof materials, green roof plants, ceiling fans) [3]
  • Descriptive Specifications: Require specific products and methods of installation. Compared to performance specifications, these give the designer the most control, but since they allow the contractor less flexibility and innovation, they can also be less cost effective.
  • Drain access chamber: removable cover installed around roof drains to protect the drains from clogging while still allowing for easy access to the drains
  • Drainage layer: Drainage layers, such as, for example, drainage aggregate, drainage sheets, and drainage mats, convey water across the roof surface under the growing medium and filter fabric, and are available in a range of different materials and drainage capacities The drainage layer must be selected to drain well enough so as to not compromise underlying waterproofing or building, but also retain water in a manner that meets stormwater management and vegetation goals. The choice of the drainage layer will have a significant impact on the peak rate and time delay of discharges from the green roof (Taylor and Gangnes, 2007)
  • Erosion protection layer: erosion control blanket, netting, or tackifier adequate to protect green roof from wind and water erosion.
  • Evapotranspiration: water conveyed to the atmosphere through evaporation and plant transpiration.
  • Extensive green roof: an extensive green roof has growing medium that is 6 inches or less deep
  • Filter fabric: a lightweight, rot-proof material laid over or included as part of a drainage layer to keep the growing medium in place and thereby prevent fine particles from blocking the drainage system. [4]
  • Flashing: the weatherproof material installed between roof sheathing (or wall sheathing) and the finish materials to help keep moisture away from the sheathing [5]
  • Growing medium: the particulate matter or substrate that anchors the plant roots to sustain the plant growth [6]
  • Intensive green roof: an intensive green roof has growing medium that is more than 6 inches deep
  • Irrigation system: Systems which deliver moisture to the growing medium making it available for plant use [7]
  • Leak detection system: Leak detection systems allow for detecting and pinpointing the exact location of leaks in the waterproof membrane
  • Lifecycle cost: The capital and operational cost of a construction item or system during the estimated useful life of the building [8]
  • Live load: All equipment and people on the roof. They are not permanent elements [9]
  • Maximum media density: the density of a mixed media material determined after it has been subjected to a specific amount of compaction and hydrated by immersion to simulate prolonged exposure to both foot traffic and rainfall (ASTM E 2399-05)
  • Maximum Media Water Retention (MMWR): terminology used in ASTM E 2399-05 for FLL Maximum Water Capacity (MWC); the quantity of water held in a media at the maximum media density. This is a useful measure of the capacity of a media to hold water under drained conditions.
  • Maximum Water Capacity (MWC): terminology used in FLL guidelines for Maximum Media Water Retention (MMWR); the quantity of water held in a media at the maximum media density. This is a useful measure of the capacity of a media to hold water under drained conditions.
  • Membrane protection layer: A material used to protect the waterproofing membrane and/or insulation layer during the installation of a green roof [10]
  • Modular green roof: A green roof system which combines one or more layers in a single pre-manufactured product (e.g. drainage, growing medium, and plants) [11]
  • Performance Specifications: allow the contractor to choose what products to install and how to install them as long as specified performance goals are met. Performance specifications allow the contractor more flexibility and innovation compared to descriptive specifications, and therefore often result in more economical bids.
  • Pre-grown mats: Mats filled with growing medium and delivered to the site pre-grown with green roof vegetation, typically Sedums. Pre-grown mats are delivered in rolls like sod. The mats can be biodegradable or permanent
  • R factor: The measure of thermal resistance. R = K·m²/ [12]
  • Root barrier: green roof component that prevents plant roots from damaging the waterproofing membrane. (link to description in 5E)
  • Water Permeability: the coefficient, which when multiplied by the hydraulic gradient will yield the apparent velocity with which water, at 68°F (20°C) will move through a cross-section of media. The conditions created in this method apply to freely-drained media where the free water surface is level with the upper surface of the media layer (such as, impending accumulation of water above the surface of the media).
  • Water retention layer: Typically a water holding fabric or a plastic sheet with cup-like depressions, the water retention layer holds water for later use by plants.
  • Wind uplift: a net upward force that occurs when the pressure below a roof is greater than above it.

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