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GREEN ROOF SYSTEMS

It is useful to categorize green roofs by drainage type and nominal thickness. The combination of these factors determines the structural load, the allowable slope, the type of vegetation supported, and the rainwater retention characteristics.

DRAINAGE TYPE: Drainage Type P utilizes drainage plates, waffled plastic sheets that store water above and drain water below. Drainage plates are lightweight, are easy to install, and are available in three sizes to meet the drainage and water storage requirements of almost any green roof. Drainage Type G utilizes a lightweight, porous inorganic granular media embedded with slotted plastic triangular drainage conduit. Granular media is heavier and is more labor-intensive to install than drainage plates, but provides a superior environment for plant root growth. Drainage Type M utilizes “drainage mat” a multi-layer fabric mat that combines soil separation, drainage, and protection functions into one product. This system is the fastest to install and creates the thinnest and lightest green roof assembly. However, its water storage and drainage capacity is limited, so it is primarily used for sloped roofs not suitable for Drainage Type P or Type G.

DP1

Drainage Type P
drainage plates

GD1

Drainage Type G
granular media

 

DM1

Drainage Type M
drainage mats

NOMINAL THICKNESS: The “nominal thickness” of a green roof is the approximate total height of the soil and drainage components that constitute the green roof system, excluding the roof structure, insulation, waterproofing, and plants. It is common practice to use the term “extensive” to describe green roofs that are very thin and will only support hardy, drought-resistant vegetation such as sedums, herbs, and perennials; “intensive” to describe green roofs that are thick enough to support a wider range of vegetation including grasses, shrubs, and small trees; and “semi-intensive” for roofs that are somewhere in between. These distinctions can be more precisely quantified by the four thickness ranges shown below:

DP1

Thickness 1
3” to 4”
sedums
herbs

DP2

Thickness 2
5” to 7”
sedums
herbs
perennials

 

DP3

Thickness 3
8” to 11”
perennials
grasses
shrubs

DP4

Thickness 4
12” +
grasses
shrubs
trees

SYSTEMS: The three types of drainage and four system thicknesses are combined to yield ten green roof systems designated P1, P2, P3, P4, G1, G2, G3, G4, M1, M2.

 

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