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DESCRIPTION: Green roof systems of this type are drained by waffled plastic sheets called “drainage plates”. Water is retained within pockets on the upper sides of the drainage plates; excess water spills over the edges of the plates and is carried off the roof.

APPLICATION: Since drainage plates are lightweight and easy to install, they have become the most popular drainage system for green roofs. Drainage plates are recommended for roofs with slopes less than 1:12.

25mm drainage plate

25mm drainage plate

COMPONENTS: A synthetic protection/water-storage fabric weighing 500g/m2 (15 oz/yd2) is applied over the waterproofing or root-barrier membrane. Drainage plates are laid over the fabric with the joints staggered. Three thicknesses of drainage plates are available to meet the drainage and water storage requirements of almost any green roof: 25mm (1” thickness) is standard for thin extensive roofs, 40mm (1.5”) is recommended for enhanced water storage and drainage on thicker extensive roofs and semi-intensive roofs, and 60mm (2.5”) is available for maximum water retention on intensive roofs. A 200g/m2 (6 oz/yd2) separation fabric is applied over the drainage plates and soil is spread over the fabric. See the following pages for installation details.

  DP1 DP2 DP3 DP4

system designation



P3 P4

typical plants



extensive soil mix 3" 5" - -
intensive soil mix - - 8" 12"
separation fabric 1/8" 1/8" 1/8" 1/8"
drainage plate 1" 1-1/2" 1-1/2" 1-1/2"
protection fabric 1/4" 1/4" 1/4" 1/4"
nominal thickness 4" 7" 10" 14"
dry weight 13 lbs/ft2 21 lbs/ft2 34 lbs/ft2 51 lbs/ft2
saturated weight 21 lbs/ft2 34 lbs/ft2 53 lbs/ft2 78 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 Optigreen 500g/m2 Standard Protection Mat, overlapping adjacent sheets at least six inches, and cut openings for roof drains. For greater mechanical protection or enhanced water storage capacity, substitute Optigreen 900g/m2 Heavy-Duty Protection Mat.


install drain boxes

2. Position Optigreen Drain Access Boxes over roof drains, adding sidewall elements as needed to match the system thickness.


install 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.


lay drain plates

4. Lay Optigreen Drain Plates in a staggered pattern. Multiple layers of plastic plates can be easily cut with a circular saw, or half-sheets can be purchased to speed installation. The plates should fit tightly but are not overlapped. Three thicknesses are available based on the water storage requirements: 25mm (1”), 40mm (1.5”), and 60mm (2.5”).


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 drain plates. Unroll Optigreen Separation Fabric over the drain plates, overlapping adjacent sheets at least six inches. Trim the fabric carefully at the edges.


spread gravel

6. SPREAD GRAVEL PERIMETER Spread well-washed gravel screened to 3/8” minimum particle size. Whenever possible, 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 100 square feet (3”). 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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