5.4.10
5.4.10.1 BMP Description
Vegetated roofs (BMP T5.17 in Volume V, Chapter 11 of the Ecology Manual) are areas of living vegetation installed on top of buildings, or other above-grade impervious surfaces. Vegetated roofs are also known as ecoroofs, green roofs, and roof gardens.
A vegetated roof consists of a system in which several materials are layered to achieve the desired vegetative cover and stormwater management function. Design components vary depending on the vegetated roof type and site constraints, but may include a waterproofing material, a root barrier, a drainage layer, a separation fabric, a growth media (soil), and vegetation. Vegetated roof systems are categorized by the depth and the types of courses used in their construction.
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Intensive Roofs – Intensive roofs are deeper installations, comprised of at least 6 inches of growth media and planted with ground covers, grasses, shrubs and sometimes trees.
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Extensive roofs – Extensive roofs are shallower installations, comprised of less than 6 inches of growth media and planted with a palette of drought-tolerant, low maintenance ground covers. Extensive vegetated roofs have the lowest weight and are typically the most suitable for placement on existing structures. Extensive systems are further divided into two types:
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Single-course systems consist of a single growth media designed to be freely draining and support plant growth.
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Multi-course systems include both a growth media layer and a separate, underlying drainage layer.
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5.4.10.2 Performance Mechanism
Vegetated roof systems can provide flow control via attenuation, soil storage, and losses to interception, evaporation, and transpiration.
5.4.10.3 Application and Limitations
Vegetated roof systems can be designed to meet the LID Performance Standard but cannot be used in the List approach to meet on-site stormwater management requirements. They can also be used to meet flow control requirements (Table II-5.14). The degree of flow control provided by vegetated roofs can vary greatly depending on the growth media (soil) depth, growth media composition, drainage layer characteristics, vegetation type, roof slope, and other design considerations. This BMP can be applied to meet the requirements listed below.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Vegetated Roofs |
X |
|
X | ||||
5.4.10.4 Site Considerations
Vegetated roof systems for stormwater management are accepted for roof slopes between 1 and 22 degrees (0.2:12 and 5:12) but require additional analysis at slopes exceeding 10 degrees (2:12).
A primary consideration for the feasibility of vegetated roofs is the structural capability of the roof and building structure. Related factors, including design load, slipping and shear issues, and wind load, are outside the scope of this manual.
5.4.10.5 Design Information
See the LID Technical Guidance Manual (Hinman and Wulkan 2012) for a more detailed description of the components of and design criteria for vegetated roofs. It also includes references to other sources of information and design guidance.
Note that the LID Technical Guidance Manual is for additional informational purposes only. Follow the guidance within this manual if there are any discrepancies between this manual and the LID Technical Guidance Manual. See BMP T5.17 in Volume V, Chapter 11 of the Ecology Manual for guidance on Runoff Model Representation.
5.4.10.6 Minimum Construction Requirements
The growth media shall be protected from over-compaction during construction. See Section 6.5 of the LID Technical Guidance Manual (Hinman and Wulkan 2012) for additional requirements.
5.4.10.7 Operations and Maintenance
See BMP T5.17 in Volume V, Chapter 11 of the Ecology Manual for general O&M guidelines and procedures for vegetated roofs.