Volume 2-05, Chapter 3 - City of Wichita

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Section 3.4.5 - Green Roof 3.4.5.1 General Description Green roofs (also referred to as vegetated roofs, ecoroofs, roof gardens, or roof meadows) are vegetated roofs used in place of conventional roofing, such as gravel-ballasted roofs. They are used as part of sustainable development initiatives, along with narrow streets, permeable pavement, and various infiltration devices. There are two main types of green roofs. The first is what is called roof gardens or intensive green roofs. They may be thought of as a garden on the roof. They have a greater diversity of plants, including trees and shrubs, but require deeper soil, increased load bearing capacity, and require more maintenance. The second has been referred to as roof meadows or extensive green roofs. The vegetation is limited and similar to an alpine meadow, requiring less soil depth and minimal maintenance. Due to the considerably greater costs and structural design requirements of intensive green roofs, only the second type of green roof, the roof meadow or extensive type is discussed in this manual. The green roof is designed to control smaller storms by intercepting and retaining or storing water until the peak storm event has passed. The plants intercept and delay runoff by capturing and holding precipitation in the foliage, absorbing water in the root zone, and slowing the velocity of direct runoff by increasing retardance to flow and extending the flowpath through the vegetation. Water is also stored and evaporated from the growing media. Green roofs can capture and evaporate up to 100 percent of the incident precipitation, depending on the roof design and the storm characteristics. Monitoring in Pennsylvania, for instance, showed reductions of approximately 2/3 in runoff from a green roof (15.5 inches runoff from 44 inches of rainfall). Furthermore, runoff was negligible for storm events of less than 0.6 inches. A study done for Portland, Oregon, indicated a reduction in stormwater discharges from the downtown area of between 11 and 15% annually if half of the roofs in the downtown area were retrofitted as green roofs. Green roofs also: • reduce the temperature of runoff; • reduce the “heat island” effect of urban buildings; • help insulate the building; • improve visual aesthetics; • protect roofs from weather; • improve building insulation; • reduce noise; and, • provide habitat for wildlife. As with a conventional roof, a green roof must safely drain runoff from the roof. It may be desirable to drain the runoff to a rainwater harvesting system such as rain barrels or other stormwater facilities such as rain gardens and swales. Page 3 - 208 Volume 2, Technical Guidance
Section 3.4.5 - Green Roof Significant removals of heavy metals by green roofs have been reported, but there is not enough evidence to include removal rates at this time. 3.4.5.2 Design Criteria and Specifications • An architect or structural engineer must be involved to ensure that the building will provide the structural support needed for a green roof. • Generally, the building structure must be adequate to hold an additional 10 to 25 pounds per square foot (psf) saturated weight, depending on the vegetation and growth medium that will be used. (This is in addition to snow load requirements.) These loads are for preliminary planning only and may vary significantly from design loads. • Green roofs can be used on flat or pitched roofs up to 40 percent. Although, on a roof slope greater than 10 degrees, the green roof installer needs to ensure that the plant layer does not slip or slump through its own weight, especially when it becomes wet. Horizontal strapping, wood, plastic, or metal, may be necessary. Some commercial support grid systems are also available for this purpose. • A green roof typically consists of several layers, as shown in Figure 3-62. A waterproof membrane is placed over the roof’s structure. A root barrier is placed on top of the membrane to prevent roots from penetrating the membrane and causing leaks. A layer for drainage is installed above this, followed by the growth media. The vegetation is then planted to form the top layer. Details of the various layers are given below. • Waterproof membranes are made of various materials, such as synthetic rubber (EPDM), hypolan (CPSE), reinforced PVC, or modified asphalts (bitumens). The membranes are available in various forms, liquid, sheets, or rolls. Check with the manufacturer to determine their strength and functional characteristics of the membrane under consideration. • Root barriers are made of dense materials or are treated with copper or other materials that inhibit root penetration, protecting the waterproof membrane from being breached. A root barrier may not be necessary for synthetic rubber or reinforced PVC membranes, but will likely be needed for asphalt mixtures. Check with the manufacturer to determine if a root barrier is required for a particular product. • The drainage layer of a green roof is usually constructed of various forms of plastic sheeting, a layer of gravel, or in some cases, the growth medium. • The growth medium is generally 2 to 6 inches thick and made of a material that drains relatively quickly. Commercial mixtures containing coir (coconut fiber), pumice, or expanded clay are available. Sand, gravel, crushed brick, and peat are also commonly used. Suppliers recommend limiting organic material to less than 33% to reduce fire hazards and prevent long-term degradation. The City of Portland, Oregon has found a mix of 1/3 topsoil, 1/3 compost, and 1/3 perlite may be sufficient for many applications. Growth media can weigh from 16 to 35 psf when saturated depending on the type (intensive/extensive), with the most typical range being from 10-25 psf. Volume 2, Technical Guidance Page 3 - 209
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CHAPTER 3 STORMWATER CONTROLS TABLE
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Chapter 3 - Table of Contents 3.2.8
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Chapter 3 - Table of Contents LIST
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Section 3.2.5 - Enhanced Swale 3.2.
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Section 3.2.5 - Enhanced Swale 3.2.
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Section 3.2.7 - Soakage Trench 3.2.
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Section 3.2.10 - Stormwater Wetland
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Section 3.3 - Secondary TSS Treatme
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Section 3.3.2 - Gravity (Oil-Water)
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Volume 2-05, Chapter 3 - City of Wichita

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