Green (Vegetated) Roofs in the Chesapeake Bay Watershed.

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1 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.

2 Green (Vegetated) Roofs in the Chesapeake Bay Watershed Outline: Chesapeake Bay Stormwater Training Partnership 2

3 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 3

4 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. Chesapeake Bay Stormwater Training Partnership 4

5 Green Roof: Basics Intensive Media depth greater than 8 Weight varies $$$$$ Level 2 Extensive Media depth 2 8 Weight lbs/sf $$ Level 1 or Level K Street, Washington, DC Image courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership 5

6 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 LID manual (draft updates) Energy Independence and Security Act Section 438 guidance document Chesapeake Bay Stormwater Training Partnership 6

7 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 Rainwater is acidic and low in dissolved minerals - thus aggressive on roof materials (zinc, lead) Chesapeake Bay Stormwater Training Partnership 7

8 Green Roof: Benefits 1050K Street, Washington, DC Economic Heating and cooling energy use Extended roof life anecdotal evidence from Germany shows 2-3 X roof life, from 20 to years. Social Habitat Living Space Aesthetics view-scape Environmental Urban heat island reduction Temperature Stormwater Image courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership 8

9 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 (RR) 45% 60% Total Phosphorus (TP) Removal Total Nitrogen (TN) Removal Channel Protection & Flood Mitigation 3 Use the following Curve Numbers (CN) for Design Storm events: 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 9

10 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.) Effective RCN Additional treatment is needed to compensate for the loss of recharge volume (Re v ) from rooftop areas. Chesapeake Bay Stormwater Training Partnership 10

11 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 11

12 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) Chesapeake Bay Stormwater Training Partnership 12

13 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) Chesapeake Bay Stormwater Training Partnership 13

14 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 , E ) 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. Chesapeake Bay Stormwater Training Partnership 14

15 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 15

16 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); Chesapeake Bay Stormwater Training Partnership 16

17 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.) Chesapeake Bay Stormwater Training Partnership 17

18 Vegetated (Green) Roof Level 1 Design (RR:45; TP:0; TN:0) Tv = 1.0 (Rv) 1 (A)/12 Depth of media up to 4 inches Drainage mats No more than 20% organic matter in media Level 2 Design (RR: 60; TP:0; TN:0) Tv = 1.1 (Rv) 1 (A)/12 Media depth 4 to 8 inches 2-inch stone drainage layer No more than 10% organic matter in media All Designs: Must be in conformance to ASTM (2005) International Green (Vegetated) Roof Stds. 1 Rv represents the runoff coefficient for a conventional roof, which will usually be 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(ft 3 ); RA=green roof area(ft 2 ); D=media depth(in.); P=media porosity (0.3 or as specified) Chesapeake Bay Stormwater Training Partnership 18

19 Green Roof: Elements Created; or Manufactured System Images courtesy of Timmons Group Chesapeake Bay Stormwater Training Partnership 19

20 Green Roofs in the Chesapeake Bay Watershed: Comparison of Green Roof in Different Water Resource Settings Practice Vegetated Roofs Spec No. Karst Terrain 1 Coastal Plain 2 Trout Waters 3 Ultra- Urban 4 Hotspots 5 5 Preferred Accepted Accepted Preferred Accepted Preferred Practice: widely feasible and recommended Accepted Practice: can work depending on site conditions KEY 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) Chesapeake Bay Stormwater Training Partnership 20

21 Chesapeake Bay Stormwater Training Partnership 21

22 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 22

23 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 23

24 Green Roof: key to success Collaboration! Architect Landscape architect Structural engineer Civil engineer Plumbing engineer Mechanical engineer Chesapeake Bay Stormwater Training Partnership Images courtesy of Timmons Group 24

25 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 25

26 Images courtesy of Timmons Group T IMMONS GROUP 1'0UII I :110'"... IU I a 1!1 D'wR Chesapeake Bay Stormwater Training Partnership 26