Inspection and Maintenance: Green Roofs

Inspection & Maintenance Guidance of Green roofs best management practices. These specialized roofs are specially engineered and designed to support the growth of vegetation while protecting the structural integrity of the roof, aiding in helping to cool the building and effectively infiltrate, absorb and retain precipitation (TRCA, 2018).

Overview

Green roofs are engineered rooftop design features that allow the growth of vegetation on rooftops and have numerous benefits. These LID BMPs can also be referred to as vegetated roofs, rooftop gardens or eco-roofs. A green roof acts like a lawn, meadow or garden by intercepting and absorbing a portion of the rainwater or snowmelt that falls on it. The typical layers of a green roof (in ascending order from the roof surface) include a water-proofing membrane, drainage layer, lightweight growing media layer and the vegetation. Excess water that is not absorbed by the growing media or vegetation is collected by the underlying drainage layer, directed to outlet structures and conveyed via the roof drainage system to another BMP or the municipal storm sewer system. A portion of the water absorbed by green roofs is returned to the atmosphere by evaporation and transpiration by plants. Green roofs are typically designed to retain precipitation from small to medium-sized (e.g., 5 to 25 mm rainfall depth) storm events. Overflow outlets are necessary to safely convey flows from major storm events.

Generalized cross-section view of a green roof showing key components and common layers that make up this LID BMP (TRCA, 2018)

Some of the benefits of green roofs include:

• The ability to reduce the quantity of pollutants and runoff being discharged to municipal storm sewers and receiving waters (i.e., rivers, lakes and wetlands);
• Growing media and plants retain pollutants deposited from the atmosphere and reduce metals and other pollutants from conventional roof materials transported by runoff;
• Improve the energy efficiency of the building due to their insulating properties;
• Reduce the urban heat island effect;
• Can provide food and shelter for pollinators;
• Can provide aesthetic value as attractive landscaped features.
  

Key components of Underground Infiltration Systems to pay close attention to are the:

• Filter Bed / Growing Medium
• Vegetation
• Protective layers
• Irrigation System
• Overflow outlets

Associated Practices

• Intensive green roofs contain greater than 15 cm depth of growing media, can be planted with deeply rooted plants (e.g., shrubs and trees) and can be designed to handle pedestrian traffic.
• Extensive green roofs consist of a thinner growing media layer (15 cm depth or less) and are typically planted with.
• Blue roofs are systems that temporarily capture rainwater using the roof as storage and allow it to evaporate and/or to be used for non-potable requirements (i.e. irrigation, toilet flushing, truck washing) and ultimately offset potable water demands.

Inspection and Testing Framework

Maintneance of the Green roofs filter bed and vegetation composition/condition being performed by staff (TRCA, 2018).

Visual Indicators Framework - Green Roofs

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Perimeter
	BMP dimensions	x	x		x
Growing medium/filter bed
	Green roof structural integrity		x	x	x
	Standing water		x	x	x
	Filter bed erosion		x	x
Vegetation
	Vegetation cover	x	x	x	x
	Vegetation condition		x	x
	Vegetation composition	x	x	x
Overflow outlets
	Overflow outlet obstruction	x	x	x	x

Testing Indicators Framework - Green Roofs

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Testing Indicators
	Soil characterization testing	x	x		(x)
	Green roof irrigation system testing	x	x	x
	Green roof leak detection testing		x		x
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)

Construction Inspection Tasks

Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:

1. During site preparation, prior to BMP installation to ensure the roof structure is ready for green roof construction work and confirm that BMP layout area matches approved design drawings and that construction materials meet design specifications
2. After installation of leak detection system (if applicable) to ensure it was done properly
3. At installation of water-proofing membrane, prior to installation of root barrier, drainage layer and overflow outlets to ensure it was done properly and to confirm that slopes are acceptable
4. After installation of root barrier, drainage layer (including filter fabric/layer) and overflow outlets, prior to installation of growing medium and plants to ensure it was done properly and confirm that depth and slopes are acceptable
5. After installation of growing medium layer and plants to ensure it was done properly and to confirm depth, slopes and elevations at overflow outlets are acceptable; After installation of irrigation system to confirm system is functioning
6. Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the building and green roof installation contractors)
7. After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure roof drainage or flow diversion devices are functioning and adequately maintained. You can also download and print the table here
   

A technician using a non-invasive locate method to determine the source of a leak in a Green roof membrane (Construction Canada, 2012.)

An example of a conceptual planting design for a green roof project in Mill Valley, California (O’Connell Landscape, 2010).

Green Roofs: Construction Inspections

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - During site preparation, prior to BMP installation	Ensure the roof structure is ready for green roof construction work
	BMP layout area and dimensions match approved design drawings
	CDA is stabilized or runoff is diverted around BMP layout area
	Construction materials have been confirmed to meet design specifications
Leak detection System - After installation of leak detection system (if applicable), prior to installation of water-proofing membrane	Quality control check leak detection system installation
Water Proofing Membrane – After installation of waterproofing membrane, prior to installation of root barrier, drainage layer and overflow outlets	Quality control check membrane installation
	Confirm that slopes conform with approved design drawings
Root barrier / Drainage layer and Overflow outlets – After installation of root barrier, drainage layer (including filter fabric) and overflow outlets, prior to installation of growing medium layer and plants	Quality control check root barrier and drainage layer installations
	Installation of drainage layer (e.g., depth and slope) is acceptable
	Installations of overflow outlets (e.g., elevation and slope) are acceptable
Filter bed – After installation of filter bed (growing medium layer and plants)	Quality control check installation of any structural components of growing medium layer (if applicable)
	Installation of growing medium (e.g., depth, elevations at overflow outlets) is acceptable
	Growing medium is free of ruts, local depressions
	Planting material meets approved planting plan specifications (plant types and quantities)
	Quality control check installation of erosion matting/protection (if applicable)
Irrigation System - After installation of irrigation system	Confirm installation is acceptable and system is functioning (through testing)

Routine Maintenance - Key Components and I&M Tasks

Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.

The table below describes routine maintenance tasks for green roofs, organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible or high pedestrian traffic locations or intensive green roofs featuring shrubs, trees and a wider variety of vegetation types. Tasks involving removal of trash, debris and weeding/trimming or replacement of dead plants may need to be done more frequently in such contexts. For further guidance on maintenance of vegetation cover on green roofs, refer to ASTM D2400/E2400M-06 Standard Guide for Selection, Installation and Maintenance of Plants for Green Roof Systems (ASTM International, 2015)

Individuals conducting vegetation maintenance and in particular, weeding (i.e., removal of undesirable vegetation), should be familiar with the species of plants specified in the planting plan and experienced in plant identification and methods of removing/controlling noxious weeds. Key resources on these topics are provided below at the links provided:

• Agriculture and Agri-food Canada’s Weed Info database
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Ontario Weed Gallery
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Noxious Weeds In Ontario list
• Ontario Invasive Plant Council’s Quick Reference Guide to Invasive Plant Species
• Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
• Plants of Southern Ontario (book), 2014, by Richard Dickinson and France Royer, Lone Pine Publishing, 528 pgs.
• Weeds of North America (book), 2014, by Richard Dickinson and France Royer, University of Chicago Press, 656 pgs.

Green Roofs: Key Components, Descriptions and Routine I&M Requirements

Comnponent	Description	Inspection & Maintenance Tasks	(Pass) Photo Example	(Fail) Photo Example
Perimeter	Separates the green roof from the roof edges and other structures; kept clear of vegetation and natural debris as a fire prevention measure. Parapets or other wind break structures may also be present around the perimeter to help prevent wind scour of growing media.	Check for damage or vegetation; Remove any vegetation or natural debris annually to biannually.	CDA has not changed in size or land cover. Sediment, trash or debris is not accumulating and point sources of contaminants are not visible.	Size of the CDA has changed from design assumptions (i.e. large asphalt area drains to a small portion of the permeable pavement). Evidence of surface ponding is visible.
Growing medium/filter bed	Devices or features that retain trash, debris and sediment; help to extend the operating life cycle; examples are eavestrough screens, catch basin inserts and sumps, oil and grit separators, geotextile-lined inlets, gravel trenches, grass filter strips and forebays.	Remove trash, debris and sediment annually to biannually or when the device sump is half full; Measure sediment depth or volume during each cleaning, or annually to estimate accumulation rate and optimize frequency of maintenance.	Although permeable pavers are generally considered pretreatment for other BMPs in a treatment train system, using eavestrough screens can act as pretreatment as they don't add to sediment build up and accumulation on the paver surface. (Photo Source: Guertin, 2010)	Build up of leads, sediment and leaf detritus in an eavestrough downspout disconnection leading to a permeable pavement surface. If not cleaned regularly this can lead to clogged pores between the pavers reducing the infiltration rate where the downspout deposits water onto the feature. (Photo Source: My Gutter Pro, 2021)
Inlets	Structures that deliver water to the BMP (e.g., impermeable pavement edges, pipes from roof downspouts or catchbasins).	Keep free of obstructions; Remove trash, debris and sediment biannually to quarterly; Make note of sediment depth or volume during each cleaning or annually to estimate accumulation rate and optimize frequency of maintenance	Impermeable pavement edges along the edge of the pervious concrete and impermeable asphalt graded so excess sheet flow is infiltrated down through the pores of the infiltration BMP. (Photo Source: Fairfax County, 2014)	Accumulated sediment, poor grading and vegetation is preventing stormwater from entering the swale. Sediment on the curb cut surface behind of the inlet indicates ponding is also occurring and is depositing water towards the permeable paver surface.
Pavement Surface	The surface of the pavement, including pavers/pavement, joints and edge restraints (e.g., curbs, edging); should not allow water to pond on the surface so any observation or evidence of surface ponding (e.g., sediment caking on the pavement) indicates a drainage problem.	Check for sediment, surface ponding, deformation, sink holes, damage and loss of joint fill; Remove trash regularly (biannually-quarterly); Grid systems with soil and grass fill are maintained like lawns; Sweep and vacuum interlocking and rigid pavements regularly (annually-biannually) with regenerative air/pure vacuum sweeper; Plow snow as needed and spread deicers sparingly during winter; Repair ruts or local sinking of 15 mm or greater over a 3 metre length, adjacent pavers or cracks in pervious concrete or porous asphalt are vertically offset by 5 mm or greater and paver joint fill is missing or below 15 mm from the paver surface.	No damage, displacement or sinking of the permeable surface is visible and there are no weeds growing between paver joints.	The pavement surface has sunk in local areas, creating a trip hazard and the potential for further damage from snow plowing and parked vehicles nearby.
Vegetation	Applies to grid paver systems only; a mixture of deep rooting perennial grasses or low growing ground covers, tolerant to both wet and dry conditions and salt (if receiving impermeable pavement runoff); roots uptake water and return it to the atmosphere, provide habitat for soil organisms that break down pollutants trapped in the soil and help maintain soil structure and permeability.	Grid systems with soil and grass fill are maintained like lawns; In the first 2 months water frequently (biweekly in the absence or rain) and as needed (e.g., bimonthly) over the remainder of the first growing season; Remove weeds and undesirable plants biannually to quarterly; Replace/reseed dead plantings annually to achieve 80% cover by the third growing season; Do not apply chemical fertilizers.	The vegetation cover on the permeable pavement is turf grass as specified in the final design and contains very few weeds. (Photo source: WEF).	The turf grass cover on the permeable walkway is not thriving in some areas and needs cutting in others. It is also impairing the aesthetic value of the BMP.
Overflow Outlet	Structures (e.g., catchbasin, curb-cut, swale) that convey flow that exceeds the storage capacity of the BMP to another drainage system (e.g., municipal storm sewer or other BMP).	Keep free of obstructions; Remove trash, debris and sediment biannually to quarterly.	The overflow outlet is free of damage and obstruction and functions as designed to safely convey excess water from the BMP.	The overflow outlet is obstructed with sediment which impairs its function to convey excess water from the BMP.
Sub-drain	Optional component; perforated pipe(s) surrounded by gravel and may be wrapped in geotextile filter fabric; installed in the base or sub-base gravel layer to collect and convey treated water to an adjacent drainage system or other BMP; may also include a flow restrictor.	Keep pipe and flow restrictor free of obstructions by flushing annually; Inspect flow restrictor regularly (e.g., biannually to quarterly).	The solid section of the sub-drain pipe is not obstructed by sediment, debris or roots and shows no signs of damage.	A section of the sub-drain pipe has been crushed which substantially reduces its conveyance capacity
Monitoring well	Perforated standpipe that extends from the bottom of the excavation to just below the pavement surface and contains perforations or slots to allow measurement of subsurface water level; used to track drainage performance over the operating life cycle of the BMP.	Standpipes should be securely capped on both ends and protected from damage by vehicular traffic by a casing.	The well is undamaged and accessible and the cap is in place and secured to prevent unauthorized access	The well cap is missing and the casing is clogged by sediment, preventing access for monitoring and allowing sediment to flow into the sub-drain system.
Control Structure	Manhole or catchbasin to which the subdrain outlets that provides access to the underdrain and flow restrictor. Inspect for accessibility, damage and sediment.	Remove trash, debris and sediment regularly (biannually/quarterly)..	TThere is some sediment accumulated in the manhole but it is not impairing the flow of stormwater into or out of the BMP. (Photo source: SWC Canada)	The manhole sump is full of sediment and debris and it is beginning to impair flow of stormwater into a perforated pipe of the exfiltration storm sewer system.

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.

Any form of rain or snow.

Toronto and Region Conservation Authority

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

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.

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

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.

Abiotic transfer of water vapour to the atmosphere.

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

Water ponded on the ground surface.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

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

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A body of water smaller than a lake, often artificially formed.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

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.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.