DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 154

Vegetation
Growth media
Separation geotextile
Course-grained
drainage media
Retention detention panel
Protection layer
Waterproof membrane
Roof structure
Figure 7-26 Vegetated roof system: dual media.
• A separation geotextile
• An engineered growth medium
• A vegetation layer
Pitched-roof applications may require the addition of slope bars, rigid slope
stabilization panels, cribbing, reinforcing mesh, or a similar method of preventing
instability and sliding. Flat roof applications with mats as foundations typically
require a network of perforated internal drainage conduit to enhance drainage of
percolated rainfall to the deck drains or scuppers.
Dual-media assemblies (Figure 7-26) utilize two types of nonsoil media. In this
case a finer-grained medium with some organic content is placed over a base layer
of coarse lightweight mineral aggregate. They do not include a geocomposite
drain. The objective is to improve the drought resistance by replicating a natural
alpine growing environment in which sandy topsoil overlies gravelly subsoil.
These assemblies are typically 4 to 6 in. thick and include the following layers:
• A waterproofing membrane
• A protection layer
• A coarse-grained drainage medium
• A root-permeable nonwoven separation geotextile
• A fine-grained engineered growth medium layer
• A vegetation layer
These assemblies are suitable for roofs with pitches less than or equal to about
1.5 : 12 ( 7 . 1 ◦ ) . Large vegetated covers will generally incorporate a network
of perforated internal drainage conduit. Dual-media systems are ideal for base
irrigation methods.