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Green (Vegetated) Roofs in the Chesapeake Bay Watershed

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Chesapeake Bay Stormwater Training Partnership 2

Green (Vegetated) Roofs in the Chesapeake Bay Watershed

Outline:


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.


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.


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


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


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)


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


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)


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.


Pollutant ConcentrationsIn Green RooftopsDon’t Always Decline

The media can add nutrients and turbidity

Green roof, increasing media depth


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)


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)


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.


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).


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);


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.)


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)


Green Roof: Elements• Created;or• Manufactured System

Images courtesy of Timmons Group


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)



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


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?


Green Roof: key to successCollaboration!

• Architect• Landscape architect• Structural engineer• Civil engineer• Plumbing engineer• Mechanical engineer



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



••••• • • T IMMONS GROUP 1'0UII I :110'" ....... IU I a • 1!1 D'wR

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Green (Vegetated) Roofs in the Chesapeake Bay …chesapeakestormwater.net/wp-content/uploads/downloads/...Green roofs capture and temporarily store stormwater runoff in the growing