A blue roof is a system designed to store and to slowly release stormwater. .nyc/html/dep/pdf/green_infrastructure/stormwater_guidelines_2012_final.pdf.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 1 9.8 BLUE ROOFS Blue roofs are systems that are designed to provide stormwater detention. Rainfall onto the roof is managed using orifices, weirs, or other outlet devices that control the discharge rate of rooftop runoff . By reducing flow rates from rooftops, blue roofs are effect ive in reducing the size of downstream detention basins. N.J.A.C. 7:8 Stormwater Management Rules – Design and Performance Standards Nonstructural Strategies Not Allowed Water Quantity Yes Groundwater Recharge Not Allowed Water Quality Not Allowed Water Quantity Mechanism and Corresponding Criteria Peak Flow Reduction Maximum ponding depth Based on re quired structural calculations Partition requirement Roof slope > 2%
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 2 Introduction A blue roof is a system designed to store and to sl owly release stormwater. Flow rates are controlled by orifices, weirs, or other outlets. Because the flow rate leaving a blue roof is less than that of a conventional roof, they reduce the need for peak flow reduction of roof runoff at other at-grade facilities. Blue roofs are most effective on sites where roofs make up the majority of the impervious surface on the site or for sites that have little space available at-grade to build other BMPs. Blue roofs may be used on newly co nstructed buildings or can be cons tructed as retrofits on existing structures. Blue roofs are recommen ded for relatively flat roofs, as steeper roofs result in greater ponding depths. Their use may be limited by the st rength of the roof and bearing capacity of the foundation; therefore, it is important to consider st ructural integrity before deciding whether to use a blue roof. Blue roofs should also be located away from trees and ot her sources of debris to prevent excessive ponding due to clogging of outlets. A blue roof used to meet any State requirement must have a maintenance plan and, if privately owned, must be protected by easement, deed restriction, or dinance, or other legal measures that prevent its neglect, adverse alteration, or removal. Applications Blue roofs may be designed to convey st orm events larger than the Water Quality Design Storm; however, regardless of the de sign storm chosen, all blue roofs must be designed for stability in accordance with the New Jersey Department of Agriculture publication, Standards for Soil Erosion and Sediment Control in New Jersey, adopted December 2013 pursuant to the Soil Erosion and Sediment Control Act of 1975 as amended (N.J.S.A. 4:24-29 et seq.) and N.J.A.C. 2-90-1.1 et seq., as required by N.J.A.C. 7:8. Design Criteria Basic Requirements Roof Slope For roofs with slopes greater than 2%, partit ions are required to avoid excessive ponding depths. An illustration detailing one possible config uration is found on the following page. It is not intended to limit the design. Partitions must be designed with orifices or ot her perforations to allow water to drain from the system. If no orifice is proposed in a partit ioned area, weep holes must be located at the bottom of any partition to prevent permanent ponding of stormwater on the rooftop. The weep holes must be included in all routing calculations.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 3 Watertight seals are required between different sections of partitioned areas, as well as between the partitions and the roof membrane to avoid seepage of water. Take note that it may be necessary to account for the volume occupied by partitions in the event that a large quantity or a thicker partition is proposed. Blue Roof with Partitions Storage Volume No standing water may remain on a blue roof 72 hours after a ra in event in order to allow for sufficient storage for the next rain event. Addi tionally, storage in excess of 72 hours may render the blue roof ineffective and may result in anaer obic conditions, odor, and both water quality and mosquito breeding issues. Waterproofing System A waterproofing system must be included in the design to ensure that water does not damage the roof or leak into the building. If the waterproofing method selected requires weight to hold it in place, the void space of the material must be used when calculating the stor age volume. The weight of this material must also be considered when determining structural requirements.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 4 Outlet Structure The minimum diameter of any outlet orifice in a bl ue roof is 2.5 inches, as required by N.J.A.C. 7:8-5.7(a)4; additional information regarding outl et structures can be found in the Residential Site Improvement Standards at N.J.A.C. 5:21-7. The illustrations below represent, respectively, a profile section through a blue roof with a sl ope greater than two percent and one with a slope less than or equal to two percent. The illustrati ons show possible configurations and flow paths and are not intended to limit the design. Blue Roof with Partitions – Profile Section Blue Roof without Partitions – Profile Section All blue roofs must be designed with unrestricte d overflow outlets to ensure that there will be no standing water in excess of the maximum design ponding depths. At least two drains are required for roofs up to 10,000 sf, and at least four are required for all roofs over 10,000 sf. A debris cover is required around all drains on a blue roof, as shown in the detail on the following page.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 5 All blue roof systems must drain within 72 hours. Safety All blue roofs must be designed to safely co nvey system overflows to downstream drainage systems. The design of the overflow struct ure must be sufficient to provide safe, stable discharge of stormwater in the event of an over flow. Safe and stable discharge minimizes the possibility of erosion and flooding in down-gradient areas. Therefore, discharge in the event of an overflow must be consistent with the current version of Standards for Off-Site Stability found in the Standards for Soil Erosion and Sediment Control in New Jersey , as required by N.J.A.C. 7:8. The following illustration depicts one configuratio n for an overflow but is not intended to limit the design. Blind connections to downstream facilities are prohibited. Any connection to downstream stormwater management facilities must include access points such as inspections ports and manholes, for visual inspection and maintenance, as appropriate, to prevent blockage of flow and ensure operation as intended. All entrance po ints must adhere to a ll Federal, State, County and municipal safety standards such as those for confined space entry.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 6 Roof Equipment Any equipment, mechanical systems, or roof furn iture located within the ponding zone must be waterproofed to prevent damage. The loss of storage volume due to any objects on the roof must be considered during design to ensure that the blue roof functions as designed. Designing a Blue Roof Blue roofs may be designed in many different ways. Below are two examples of how a blue roof can be designed using partitions or outlets. The examples below show possib le configurations and flow paths and are not intended to limit the design. Example 1: Blue Roof Designed with Partitions A 40,000 sf blue roof is being designed based on the following parameters: Roof area: 40,000 sf Roof width, W: 125 feet Roof length, L: 320 feet 2-year storm rainfall, P: 3.31 inches 10-year storm rainfall, P: 5.06 inches 100-year storm rainfall, P: 8.51 inches Maximum ponding depth: 6 inches Roof slope: 2% Existing land cover Lawn, Good Condition Hydrologic soil group C Existing curve number 74 Existing time of concentration 20 minutes Step #1: Calulating the Distance Between Partitions This roof is being designed with a series of parti tions; therefore, the ponding area will be split into sections, each with a different maximum el evation but with the same maximum depth. The distance between the partitions may be calculated as follows: 25 ft Because the roof is 125 feet wide, it can be divi ded into five sections, each separated by a partition.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 8 Example 2: Blue Roof Designed with Orifices A 20,000 sf traditional roof is being retrofitted as a bl ue roof to reduce peak flow rates leaving the site. The blue roof has a single drainage area based on the following parameters: Roof area: 20,000 sf Roof width, W: 100 feet Roof length, L: 200 feet 2-year storm rainfall, P: 3.31 inches 10-year storm rainfall, P: 5.06 inches 100-year storm rainfall, P: 8.51 inches Maximum ponding depth: 9 inches Roof slope: 1% Step #1: Calculating the Ponding Area Ponding will only occur on a section of the blue roof. The ponding area can be calculated based on the dimensions of the roof as follows: 15,000 sf Step #2: Designing Orifices and Overflows The peak flow rate reductions will be determin ed by the size and number of orifices. Many commercial drains are available which have pre- determined flow rates. For this example, we will assume that each drain is designed as a vert ical orifice which slowly releases water from the rooftop, and water depths in excess of the maximum ponding depth are freely discharged (modeled as a weir). It is also assumed that th e rooftop is designed with crushed stone with a porosity of 0.4 to hold the waterproofing membrane in place. Using 4 drains, two located at the bottom of the roof each with an orifice with a diameter of 5 inches and two elevated 6 inches above the bottom of the roof with an orifice diameter of 12 inches, the blue roof achieves the peak flow reductions provided in the table below: Storm Event Traditional Roof Peak Flow (cfs) Blue Roof Peak Flow (cfs) Peak Flow Reduction Rate 2-year 1.48 0.67 55% 10-year 2.27 0.95 58% 100-year 3.84 2.70 30% These flow rate reductions apply to the entire roof area, though only the 15,000 sf area calculated above will have ponding. Retrofitting an existing roof to provide peak flow reductions could be used as part of a mitigation plan in instances in which a variance or exemption for a
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 9 separate project must be granted for the water quantity standard, or a way to reduce impacts due to flooding or combined sewer overflows. Considerations A number of factors should be considered when using a blue roof to manage stormwater runoff. Access Blue roofs, like all other stormw ater BMPs require maintenance; therefore, it is important all maintenance personnel are able to access the roof. Additionally, if the roof is open to the public, measures should be included to ensure safety. Many large buildings use the roof as the access point for window washing; therefore, the blue roof should be designed to provide access and safe working conditions for maintenance personnel. Structural Strength Blue roofs may add a significant amount of weight to a roof including the dead load of the blue roof components, as well as the added live loads as a result of rainfa ll, maintenance personnel, and equipment. Therefore, the structural load capacity of the roof is a crucial co nsideration when designing a blue roof. Leak Detection A method for detecting leaks should be included in the blue roof™s maintenance plan. Location of physical leak detection systems will depend on the ma nufacturer™s specifications and should be placed accordingly. Longevity When designing a blue roof, it is important to consider the longevity of all parts of the system, as well as the roof and all structural supports. The waterproofin g layer of a blue roof is especially important, as failures can create serious structural problems. Care must be taken during construction to ensure that the waterproofing membrane is not damaged. In some instances, a support layer may be required between the membrane and the ballast to prevent localized damage or distribute the weight from various live and static loading conditions. Additiona lly, the lifespan of a blue roof may be shortened by repeated clogging of the outlet stru ctures by debris. It may be nece ssary to increase the frequency of monitoring if clogging becomes chronic. Other Rooftop Uses Blue roof systems may be designed to allow other us es of the roof. Any equipment stored on a blue roof should be protected to ensure that it is not damaged as a result of pond ed water. A blue roof system may also be designed with solar panels, prov ided that the solar panels are designed to allow rainfall to reach the roof surface and will not be da maged by ponded water. All pedestrian walkways
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 10 should be designed to avoid the blue roof™s pond ing area to prevent damage to the waterproofing membrane. Maintenance Regular and effective maintenance is crucial to ensure effective blue roof performance. A blue roof used to meet any State requirement must have a maintenance plan. For projects that are major developments, there are a number of required elements in all mainte nance plans, pursuant to N.J.A.C. 7:8-5.8; these are discussed in more detail in Chapter 8: Maintenance of Stormwater Management Measures. Furthermore, maintenance activities are requi red through various regulations, including the New Jersey Pollutant Discharge Elimination System (NJP DES) rules, N.J.A.C. 7:14A. Specific maintenance activities for blue roofs are presented below; thes e activities should be included in the maintenance plan. General Maintenance Blue roofs should be inspected at least fo ur times annually and after every storm event exceeding 1 inch of rainfall. Blue roofs located in areas with significant tree cover or in areas with a high potential for airborne debris may require more frequent inspections due to the potential for clogging. All structural components should be inspected for cracking, spalling and deterioration at least once annually. Disposal of debris, trash, sediment and othe r waste material must be done at suitable disposal/recycling sites and in compliance with all applicable local, state and federal waste regulations. Access points for maintenance are required on a ll blue roofs; these access points should be clearly identified in the maintenance plan. In addition, any special training required for maintenance personnel to perform specific tasks should be included in the plan. Drain Time The design drain time for the maximum design st orm runoff volume must be indicated in the maintenance plan. If the actual drain time is longer than the design drain time, the blue roof must be evaluated and appropriate measures must be taken to return the blue roof to the as-built condition. If the blue roof fails to fully drain within 72 hours, corrective action must be taken and the maintenance manual revised accordingly to prevent similar failures in the future.
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New Jersey Stormwater Best Management Practices Manual September 2017 Chapter 9.8 Blue Roofs Page 11 References City of New York. July 2012. Guidelines for the Design and Construction of Stormwater Management Systems. New York City Department of Environmental Protection. New York, NY. http://www.nyc.gov/html/dep/pdf/green_infrastructure/stormwater_guidelines_2012_final.pdf City of New York. Rooftop Detention. New York City Department of Environmental Protection. New York, NY. http://www.nyc.gov/html/dep/pdf/rooftop_detention.pdf City of Philadelphia. February 2014. City of Philadelphia Stormwater Management Guidance Manual. Philadelphia Water Department. Philadelphia, PA. http://www.pwdplanreview.org/upload/pdf/Full%20Manual%20(Manual%20Version%202.1).pdf Katiyar, Nitin, et al. June 21, 2012. Performance Evaluation of Blue Roofs to Mitigate CSO Impacts (PowerPoint Slides). http://www.ohiowea.org/docs/Th1100Storm_Perf_Eval_Blue_Roofs.pdf
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