Green (Vegetated) Roofs in the Chesapeake Bay Watershed

Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff.

http://www.cbstp.org

Green (Vegetated) Roofs in the Chesapeake Bay Watershed  Outline:

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Green (Vegetated) Roofs in the Chesapeake Bay Watershed Definition:  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. Chesapeake Bay Stormwater Training Partnership

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Green (Vegetated) Roofs in the Chesapeake Bay Watershed Types 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 



1050K Street, Washington, DC Chesapeake Bay Stormwater Training Partnership

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

Image courtesy of Timmons Group 5

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  UFC

LEED Silver compliance mandate 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! 

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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.

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Social • Habitat – Living Space • Aesthetics –view-scape

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1050K Street, Washington, DC

Environmental •

Urban heat island reduction

•

Temperature

•

Stormwater

Image courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership

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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 Design Annual Runoff Reduction 45% 60% (RR) Total Phosphorus (TP) 0 0 Removal 2 Total Nitrogen (TN) 0 0 Removal 2 Use the following Curve Numbers (CN) for Design Storm events: Channel Protection & Flood Mitigation 3 1-year storm = 64; 2-year storm = 66; 10-year storm = 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 negative nutrient reduction (due to leaching from the media) in early stages of vegetated roof development. 3 See Miller (2008), NVRC (2007) and MDE (2008) Chesapeake Bay Stormwater Training Partnership

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Vegetated (Green) Roof Performance in Maryland 

Green roofs are considered an alternative surface: A1 (MD SWM Manual Supp.1, Ch 5)

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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.

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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.) Effective RCN

2

3

4

6

8

94

92

88

85

77

Additional treatment is needed to compensate for the loss of recharge volume (Rev) from rooftop areas. Chesapeake Bay Stormwater Training Partnership

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Pollutant Concentrations In Green Rooftops Don’t Always Decline

Green roof, increasing media depth

The media can add nutrients and turbidity Chesapeake Bay Stormwater Training Partnership

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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 redevelopment)

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Green Roofs in the Chesapeake Bay Watershed: Maryland Design 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: Maryland Design 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: Virginia Design Specifications:  Conveyance: • The drainage layer below the growth media should be designed to convey the 10-year storm without 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). Chesapeake Bay Stormwater Training Partnership

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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  Roof

capacity of 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)/12

Depth of media up to 4 inches

Media depth 4 to 8 inches

Drainage 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)/12 Tv=Treatment Volume(ft3); RA=green roof area(ft2); D=media depth(in.); P=media porosity (0.3 or as specified) Chesapeake Bay Stormwater Training Partnership

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Green Roof: Elements Created; or • Manufactured System •

Images courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership

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Green Roofs in the Chesapeake Bay Watershed: Comparison of Green Roof in Different Water Resource Settings Practice Vegetated Roofs

Spec No. 5

Karst Terrain 1

Coastal Plain 2

Trout Waters 3

UltraUrban 4

Preferred

Accepted

Accepted

Preferred

Hotspots 5 Accepted

Preferred Practice: widely feasible and recommended KEY

Accepted Practice: can work depending on site conditions

Restricted Practice: extremely limited feasibility Prohibited Practice: do not use due to environmental risk NOTES: SS = small scale applications LS = large scale applications 1 CSN Tech Bulletin No. 1 2 CSN Tech Bulletin No. 2 3 CSN Tech Bulletin No. 6 4 CSN Tech Bulletin No. 5 5 CWP (2004)

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Chesapeake Bay Stormwater Training Partnership

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Construction Costs on a Per Acre Treated Basis

$14K -$44K

$650K - $1.1M

$800-$7K

Need to consider the life cycle costs of the Building and roof to assess total cost/benefit Chesapeake Bay Stormwater Training Partnership

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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? Chesapeake Bay Stormwater Training Partnership

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Green Roof: key to success Collaboration! • • • • • •

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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

LiveRoof - Holgren Residence, MN Images courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership

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Images courtesy of Timmons Group T IMMONS GROUP a • 1'0UII I

Chesapeake Bay Stormwater Training Partnership

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