Green roof construction

Description

Green roofs are composed of several layers (waterproof membrane or existing membrane, root barrier, drainage layer, filter fabric, growing medium) that support the growth of plants and other vegetation on top of a roof.

Types

There are two main types of green roof systems: extensive and intensive.

1. Extensive green roofs have growing medium depths of 10 cm to 15 cm (0.1 m to 0.15 m), maximum structural weight loads of 250 kg/m², and vegetation is primarily limited to sedum spp or commonly known as stone crop.
2. Intensive green roofs have growing medium depths greater than 15 cm (0.15 m), no maximum structural weight load and various plant types can be incorporated including native flowers, trees and shrubs.

Within these two types of green roofs are two installation type systems:

1. Modular system: green roof component layers are fused together as a moveable modular block system that can be linked together with other green roof blocks to form the green roof. This type of system typically forms an extensive green roof type and are used where roofs are sloped and flat.
2. Layered system: all components' layers are loose laid as individual layers. This type of system can form both extensive and intensive green roofs and used typically on flat roofs.

Component description

• Vegetation free zone: separates the green roof perimeter from the roof perimeter and other structures on the roof (e.g., vents), that is kept devoid of vegetation and natural debris.
• Root barrier: often a clear plastic sheet, the root barrier layer protects the water-proofing membrane from root penetration and degradation by microbial activity.
• Drainage layer: provides void space for excess water to drain from the growing media that is either stored for later uptake by plants or allowed to flow to roof drain.
• Filter fabric or geotextiles: are used to retain growing media and prevent migration out of green roof system
• Growing media will vary according to the green roof system/product installed but is generally designed to be porous and lightweight with adequate fertility and drainage capacity to support plant growth and allow for absorption and infiltration of water.
• Vegetation: Green roofs rely on vegetation (i.e., sedums/succulents, grasses, herbs, wildflowers and for intensive green roofs, shrubs and trees) to intercept, absorb and evapo-transpire stormwater. Vegetation can be installed as mats or as plugs or pots based upon the green roof design.
• Overflow outlets: Flows exceeding the storage capacity of the BMP are conveyed to an adjacent drainage system via an overflow outlet structure and the roof drainage system.
• Irrigation System (if applicable): Most green roofs will require periodic irrigation, especially during the first 2 months of the establishment period.
• Leak detection system (if applicable): leak detection systems should be used to periodically check for the presence of leaks in the water-proofing membrane.

Key construction steps

Modular system

1. Sweep and blow the roof membrane clear and free of dirt, debris, and any sharp objects (such as flash clippings)
2. Inspect the waterproof membrane for any potential damage prior to installing root barrier
3. Install green roof perimeter or vegetation free zones (if applicable) a minimum 2 feet from parapet wall and around other critical infrastructure such as vents or roof drains
4. Install the root barrier (if applicable) on top of the roof membrane.
   1. Ensure sheet is goes up the parapet wall the full height of the green roof system (if applicable).
   2. Where additional root barrier sheets are laid side by side, a minimum of 30 cm overlap between sheets is needed.
   3. Make appropriate slits in root barrier with a retractable knife (away from existing waterproof membrane) to allow installation around roof infrastructure such as drains, air vents and stacks
   4. Please note that modular systems often include a root barrier built into the system but some systems recommend doubling up
5. Install the modular system according to manufacturer specifications or green roof design
6. If modular system is installed on a slope roof, the system will need to be anchored according to manufacturer specifications or design
7. Install irrigation system installation (if applicable) according to manufacturer specifications or design
8. Backfill vegetation free zones with clear stone (if applicable)

Layered system

1. Sweep and blow the roof membrane clear and free of dirt, debris, and any sharp objects (such as metal flashing clippings)
2. Inspect the waterproof membrane for any potential damage prior to installing the root barrier
3. Install green roof perimeter or vegetation free zones (if applicable) a minimum 2 feet from parapet wall and around other critical infrastructure such as vents or roof drains root free zone. Green roof perimeter may consist of metal edging which will be installed according to design and/or manufacturer’s specifications
4. Install the root barrier (if applicable) on top of the roof membrane according to design or manufacturer specifications.
   1. Ensure sheet goes up the parapet wall the full height of the green roof system (if applicable).
   2. Where additional root barrier sheets are laid side by side, a minimum of 30 cm overlap between sheets is needed.
   3. Make appropriate slits in root barrier with a retractable knife (away from waterproof membrane) to allow installation around roof infrastructure such as drains, air vents and stacks
   4. Install drainage layers on top of the root barrier according to design or by manufacturers specifications
5. Install filter layer according to design and manufacturer’s specifications. Ensure 30 cm overlap between adjacent rows and it runs up green roof edging (perimeter) the full thickness of the green roof
6. Place green roof growing medium to specified depth within the design or manufacturer’s specifications. Growing media is typically blown onto the filter fabric layer by blower truck.
7. Install plants and vegetation according to design
8. If rolled vegetated mat system, take sedum clippings and infill sparsely vegetated areas
9. Install irrigation system according to design and/or manufacturer’s specifications

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.