DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 152

Vegetated roof covers that are 10 in. deep, or deeper, are referred to as intensive
vegetated roof covers. These are more familiar in the United States and include
many above-structure urban landscaped plazas. Intensive assemblies can also
provide substantial environmental benefits, but are intended primarily to achieve
aesthetic and architectural objectives. These types of systems are considered roof
gardens and are not to be confused with the simple extensive design. This LID
summary focuses on extensive vegetated roofs, which are more cost-effective as
a stormwater management measure.

Design Elements of a Vegetated Roof System

• Engineered media should have a high mineral content; media for extensive
vegetated roof covers are typically 85 to 97% nonorganic.
• Two to six inches of non-soil-engineered medium; assemblies that are 4 in.
and deeper may include more than one type of engineered media.
• Vegetated roof covers intended to achieve water quality benefits generally
should not be fertilized.
• Irrigation may be required for plant survival and optimal functioning of the
vegetated roof for stormwater management in southern California.
• Internal building drainage, including provisions to cover and protect deck
drains or scuppers, must anticipate the need to manage large rainfall events
without inundating the cover.
• Assemblies planned for roofs with pitches steeper than 2 : 12 5 ◦ ) ( 9 . must

incorporate supplemental measures to ensure stability against sliding.
• The roof structure must be evaluated for compatibility with the maximum
predicted dead and live loads. Typical dead loads for wet extensive vegetated
covers range from 8 to 36 pounds per square foot (lb/ft 2 ). Live load is a
function of rainfall retention. For example, 2 in. of rain equals 10.4 lb/ft 2
of live load.
• The waterproofing must be resistant to biological and root attack. In many
instances a supplemental root-fast layer is installed to protect the primary
waterproofing membrane from plant roots.
• Standards and guidelines (in English) for the design of green roofs are
available from FLL, a European nonprofit trade organization. In the United
States, new standards and guidelines are in development by the American
Society for Testing and Materials.

Types of Vegetated Roof Systems

Most extensive vegetated roof covers fall into one of three categories:

1. Single medium with synthetic underdrain layer
2. Dual media
3. Dual media with synthetic retention/detention layer