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Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff.
Green (Vegetated) Roofs in the Chesapeake Bay Watershed
Chesapeake Bay Stormwater Training Partnership 2
Green (Vegetated) Roofs in the Chesapeake Bay Watershed
Outline:
Chesapeake Bay Stormwater Training Partnership 3
Green (Vegetated) Roofs in the Chesapeake Bay WatershedDefinition: Green Roofs (also known as Vegetated roofs, living
roofs or ecoroofs) are alternative roof surfaces that typically consist of waterproofing and drainage materials and an engineered growing media that is designed to support plant growth.
Function: Green roofs capture and temporarily store stormwater
runoff in the growing media before it is conveyed into the storm drain system.
A portion of the captured stormwater evaporates or is taken up by plants, which helps reduce the runoff volume and the peak rate of discharge from the roof.
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Green (Vegetated) Roofs in the Chesapeake Bay WatershedTypes of Green Roof:
There are two different types of green roof systems: • Intensive Green Roof Systems have a deeper growing
media layer that ranges from 6 inches to 4 feet thick, which is planted with a wider variety of plants, including trees.
• Extensive Green Roof Systems have much shallower growing media (2 to 6 inches), which is planted with carefully selected drought tolerant vegetation. Extensive vegetated roofs are much lighter and less expensive than intensive vegetated roofs and are recommended for use on most development and redevelopment sites.
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Green Roof: Basics
Intensive•Media depth greater than 8”•Weight varies•$$$$$• Level 2
Extensive•Media depth 2” – 8”•Weight 17 – 50 lbs/sf• $$• Level 1 or Level 2
1050K Street, Washington, DC Image courtesy of Timmons Group
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Regulatory Paradigm Shift
In addition to new stormwater requirements in the Bay Watershed that encourage green infrastructure practices like green roofs, there are also other regulatory factors:
EPA MS4 Stormwater Guidance: no net discharge;
DOD LEED Silver compliance mandate
UFC 3-210-10 LID manual (draft updates)
Energy Independence and Security Act Section 438 guidance document
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Rooftop Practices
What me worry? It’s just rainwater!
1/3 of total N load to the Bay is wet and dry atmospheric deposition (vehicle emissions, electric utility)
Roofing materials can leach (pH, Nitrate, Phosphorus, heavy metals) Clark et al. 2004
Rainwater is acidic and low in dissolved minerals - thus aggressive on roof materials (zinc, lead)
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Green Roof: Benefits Economic
• Heating and cooling energy use
• Extended roof life – anecdotal evidence from Germany shows 2-3 X roof life, from 20 to 40-60 years.
Social• Habitat – Living Space
• Aesthetics –view-scape
Environmental• Urban heat island reduction
• Temperature
• Stormwater
Image courtesy of Timmons Group
1050K Street, Washington, DC
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Vegetated (Green) Roof Performance in Virginia
Summary of Stormwater Functions Provided by Vegetated Roofs 1
Stormwater Function Level 1 Design Level 2 DesignAnnual Runoff Reduction (RR) 45% 60%
Total Phosphorus (TP) Removal 2 0 0
Total Nitrogen (TN) Removal 2 0 0
Channel Protection &Flood Mitigation 3
Use the following Curve Numbers (CN) for DesignStorm events:1-year storm = 64; 2-year storm = 66; 10-yearstorm = 72; and the 100 year storm = 75
1 Sources: CWP and CSN (2008) and CWP (2007).2 Moran et al (2004) and Clark et al (2008) indicate no nutrient reduction or even negativenutrient reduction (due to leaching from the media) in early stages of vegetated roofdevelopment.3 See Miller (2008), NVRC (2007) and MDE (2008)
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Vegetated (Green) Roof Performance in Maryland Green roofs are considered an alternative surface: A-
1 (MD SWM Manual Supp.1, Ch 5)
The effective runoff curve number for the rooftop area that is covered by a green roof is adjusted to reflect that of grassed or open space.
The capacity of the green roof to detain runoff is governed by the planting media thickness:
Effective RCNs for Extensive Green Roofs
Roof Thickness (in.) 2 3 4 6 8
Effective RCN 94 92 88 85 77
Additional treatment is needed to compensate for the loss of recharge volume (Rev) from rooftop areas.
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Pollutant ConcentrationsIn Green RooftopsDon’t Always Decline
The media can add nutrients and turbidity
Green roof, increasing media depth
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Green Roofs in the Chesapeake Bay Watershed:Green roofs are well suited for:
Commercial, institutional, municipal, and multi-family residential buildings;
Ultra-urban development (new and re-development)
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Green Roofs in the Chesapeake Bay Watershed: MarylandDesign Specifications: Conveyance:
• Runoff shall flow through and exit green roof systems in a safe and non-erosive manner;
• Convey the 2-year 24-hr storm without inundating the soil media;
• Flashing should extend 6 inches above the media and be protected by counter-flashing.
Treatment:• Planting media shall be non-soil engineered mixes (App.
B.4)
• Individual layers (root barriers, drainage mats, separation geotextiles) (App B.4)
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Green Roofs in the Chesapeake Bay Watershed: MarylandDesign Specifications:
Structures:• Roof structure shall be capable of bearing the maximum
predicted dead and live loads associated with green roof systems. Standardized media weights and procedures (e.g., ASTM E-2397-05, E-2399-05) shall be used to establish the dead load bearing capacity of the roof.
• Green roofs with pitches steeper than 2:12 shall include supplemental measures (e.g., slope bars, rigid stabilization panels, reinforcing mesh) to enhance stability and prevent media sliding.
Landscaping: Vigorous, drought tolerant vegetative cover.
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Green Roofs in the Chesapeake Bay Watershed: VirginiaDesign Specifications:Conveyance:
• The drainage layer below the growth media should be designed to convey the 10-year stormwithout backing water up to into the growing media.
• Flashing should extend 3 inches above the media and be protected by counter-flashing.
Treatment:• Planting media and roof elements covered by specifications (comparable to MD).
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Green Roofs in the Chesapeake Bay Watershed:Other applicability considerations:Pitch of roof:
• Tv is maximized on flat roof;• Effective Tv diminishes with increased pitch (VA DCR
Spec No 5)• Minimal pitch is needed to promote positive
drainage);
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Green Roofs in the Chesapeake Bay Watershed:Other applicability considerations:
Structural capacity of roof;
Roof Access: construct & maintain;
Setbacks from electrical and HVAC systems, and vegetation-free zones (firebreaks, perimeter, etc.)
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Vegetated (Green) Roof
Level 1 Design (RR:45; TP:0; TN:0) Level 2 Design (RR: 60; TP:0; TN:0)Tv = 1.0 (Rv)1 (A)/12 Tv = 1.1 (Rv) 1 (A)/12Depth of media up to 4 inches Media depth 4 to 8 inchesDrainage mats 2-inch stone drainage layer
No more than 20% organic matter in media
No more than 10% organic matter in media
All Designs: Must be in conformance to ASTM (2005) International Green (Vegetated) Roof Stds.
1Rv represents the runoff coefficient for a conventional roof, which will usually be 0.95. The runoff reduction rate applied to the vegetated roof is for “capturing” the Treatment Volume (Tv) compared to what a conventional roof would produce as runoff.
Overall Sizing: Tv = (RA*D*P)/12Tv=Treatment Volume(ft3); RA=green roof area(ft2); D=media depth(in.); P=media porosity (0.3 or as specified)
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Green Roof: Elements• Created;or• Manufactured System
Images courtesy of Timmons Group
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Green Roofs in the Chesapeake Bay Watershed:
Comparison of Green Roof in Different Water Resource Settings
PracticeSpecNo.
KarstTerrain 1
CoastalPlain 2
TroutWaters 3
Ultra-Urban 4 Hotspots 5
Vegetated Roofs 5 Preferred Accepted Accepted Preferred Accepted
KEY
Preferred Practice: widely feasible and recommended Accepted Practice: can work depending on site conditionsRestricted Practice: extremely limited feasibility Prohibited Practice: do not use due to environmental risk
NOTES: SS = small scale applications LS = large scale applications1 CSN Tech Bulletin No. 1 2 CSN Tech Bulletin No. 2 3 CSN Tech Bulletin No. 64 CSN Tech Bulletin No. 5 5 CWP (2004)
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Construction Costs on a Per Acre Treated Basis
$650K - $1.1M $800-$7K$14K -$44KNeed to consider the life cycle costs of the Building and roof to assess total cost/benefit
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Green Roofs in the Chesapeake Bay Watershed
Survey of stormwater practitioners indicates that Green Roofs are the least used practice of all structural and non-structural BMPs;
The same survey reveals that training on Green Roofs is in the bottom three of preferred training topics of 10 commonly used practices (with disconnection and sand filters);
Is this due to cost? Complexity? Design by others?
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Green Roof: key to successCollaboration!
• Architect• Landscape architect• Structural engineer• Civil engineer• Plumbing engineer• Mechanical engineer
Images courtesy of Timmons Group
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Green Roof: O & M
Extensive Roofs• Periodic weeding
•Especially woody vegetation
• Annual mowing
• Annual soil test for nutrient needs
• Water if greater than 3 -4 weeks between rain events
Images courtesy of Timmons Group
LiveRoof - Holgren Residence, MN
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Images courtesy of Timmons Group
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