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DESCRIPTION: Green roof systems of this type are drained by a base layer of lightweight, inorganic, granular media. To maximize water storage and minimize weight, the granular media should contain a significant percentage of porous material such as heat-expanded rock. To minimize separation, a well-graded blend of particles with sizes ranging from 1/8” to 1/2” is preferred. A slotted plastic drainage conduit with a triangular cross-section is embedded within the media to transport excess water.

drainage triangle

trianglular drainage conduit

APPLICATION: Although granular media is heavy and labor-intensive to install when compared with other drainage systems, it provides an optimal environment for plant root growth. Granular drainage is recommended for roofs with slopes less than 1:12.

COMPONENTS: A synthetic protection/water-storage fabric weighing 900 g/m2 (27 oz/yd2) is applied over the waterproofing or root-barrier membrane. For extensive roofs, a 900 g/m2 fabric with capillary water-equalization properties is recommended for superior performance. A network of slotted plastic triangular drainage conduit is assembled over the fabric and connects to roof drain access boxes. Then 2” of granular media is spread over the fabric for intensive roofs and 4” to 6” for semi-intensive and intensive roofs. A 200 g/m2 (6 oz/yd2) separation fabric is applied over the granular media and soil is spread over the fabric. See the following pages for installation details.

  G1 G2 G3 G4

system designation



G3 G4

typical plants



extensive soil mix 2" 4" - -
intensive soil mix - - 6" 9"
separation fabric 1/8" 1/8" 1/8" 1/8"
granular media 2" 2" 4" 5"
protection fabric 1/4" 1/4" 1/4" 1/4"
nominal thickness 4" 6" 10" 14"
dry weight 16 lbs/ft2 24 lbs/ft2 40 lbs/ft2 56 lbs/ft2
saturated weight 23 lbs/ft2 36 lbs/ft2 58 lbs/ft2 82 lbs/ft2
minimum slope 0:12 0:12 0:12 0:12
maximum slope 1:12 1:12 1:12 1:12
water retention 50% 60% 70% 80%

The following illustrations describe the basic installation procedure. We often customize the procedure for each roof.

unroll protection fabric

1. If the primary roof waterproofing is not root resistant, first apply a root barrier membrane. Then unroll a protection mat with water-storage capacity, overlapping adjacent sheets at least six inches, and cut openings for roof drains. Use Optigreen Standard Protection Mat, or for optimal performance use Optigreen Capillary Protection Mat for extensive roofs and Optigreen 900 g/m2 Heavy-Duty Protection Mat for intensive roofs.


install triangles

2. Position Optigreen Drain Access Boxes over roof drains, adding sidewall elements as needed to match the system thickness. Construct drainage “trees” using Optigreen triangular drainage channels and tees. For optimum performance, any point on the roof should be within two channel lengths of the closest point of the drainage tree: this will be achieved if the “branches” of each drainage tree are laid four channel lengths (4 meters or 13 feet) apart.


install retaining edge

3. Optigreen Retaining Edge should be installed on extensive green roofs approximately 30” from roof edges with roof drains and 18” from other edges. If there will be a gravel perimeter, the retaining edge will separate the soil and gravel and does not need to be attached. If there will be no gravel perimeter, the retaining edge should be attached to the waterproofing membrane or the protection fabric so it will stay in place.


spread granular media

4. Spread Optigreen Granular Drainage Media or other free-draining inorganic media with well-graded particle size and neutral pH. Apply a minimum of one cubic yard per 150 square feet (2”) for extensive or semi-intensive roofs and one cubic yard per 75 square feet (4”) for intensive roofs. If feasible, the media should be dispensed from supersacks suspended from cranes to minimize the potential for damage.


unroll separation fabric

5. Optigreen Separation Fabric is engineered to retain soil without clogging while allowing plant roots to easily penetrate to reach water in the granular media and protection fabric. Unroll Optigreen Separation Fabric over the granular media, overlapping adjacent sheets at least six inches. Trim the fabric carefully at the edges.


spread gravel

6. Spread well-washed gravel screened to 3/8” minimum particle size. If feasible, the gravel should be dispensed from supersacks suspended from cranes to minimize the potential for damage.


spread soil

7. For extensive roofs, spread Optigreen Extensive Greenroof Growing Media at a minimum rate of one cubic yard per 150 square feet (2”). For intensive roofs, spread Optigreen Intensive Greenroof Growing Media as needed to obtain the required system thickness. Where possible, the media should be dispensed from supersacks suspended from cranes to minimize the potential for damage.



8. Insert green roof plants in a random pattern, two per square foot. Water thoroughly after installation, and during extended dry periods for the first two years. Broadcast Optigreen Slow Release Fertilizer yearly on extensive roofs.


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