Low Impact Development Manual for Michigan - OSEH - University ...

Dual media assemblies
Dual media (Figure 7.58) assemblies utilize two types
of non-soil growth media. In this case a finer-grained
media 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 drought resistance by replicating
a natural alpine growing environment in which
sandy topsoil overlies gravelly subsoil. These assemblies
are typically 4 to 6 inches thick and include the
following layers:
1. Waterproofing membrane
2. Root barrier/ protection layer
3. Coarse-grained drainage media
4. Filter layer
5. Growth media
6. Vegetation
These assemblies are suitable for roofs with pitches
less than, or equal to about 1.5:12 (7.1 degrees). Large
vegetated covers will generally incorporate a network
of perforated internal drainage conduit located within
the coarse grained drainage layer.
Dual media with synthetic retention/detention layer
These assemblies introduce impervious plastic panels
with cup-like receptacles on their upper surface (i.e.,
Figure 7.58
Dual media assembly
a modified geocomposite drain sheet). The panels are
in-filled with coarse lightweight mineral aggregate. The
cups trap and retain water. They also introduce an air
layer at the bottom of the assembly. A typical profile
would include:
1. Waterproof membrane
2. Protection layer
3. Retention/detention panel
4. Coarse-grained drainage media
5. Filter layer
6. Growth media
7. Vegetation
These assemblies are suitable on roof with pitches
less than or equal to 1:12 (4.8 degrees). Due to their
complexity, these systems are usually a minimum of
five inches deep. If required, irrigation can be provided
via surface spray or mid-level drip.
Treatment Train
Vegetated roof covers are frequently combined with
ground infiltration measures. This combination can be
extremely effective for stormwater management and is
one of the best ways to replicate the natural hydrologic
cycle. Vegetated roofs evapotranspirate a significant
fraction of annual rainfall and typically discharge larger
Vegetation
growth Media
Filter layer
course-grained
drainage Media
retention/detention panel
protection layer
waterprooF MeMbrane
rooF structure
LID Manual for Michigan – Chapter 7 Page 306