Green Roof: Life Cycle Costs

Green roof located in Toronto, ON. To read more about the life cycle costs and savings associated with building and owning a green roof in the GTA, check out the STEP report "An Economic Analysis of Green Roofs", here. (Photo Source: STEP, 2007).

Overview

Green Roofs consist of a layer of vegetation and planting medium installed on top of a conventional flat or sloped roof. Green roofs are touted for their environmental benefits as they improve energy efficiency, reduce urban heat island effects and create greenspace for passive recreation and aesthetic enjoyment. They are also desirable for their water quality, water balance and peak flow control benefits. The green roof acts like a lawn or meadow by temporarily storing rainwater in the planting medium and ponding areas. Excess rainfall enters underdrains and overflow points and is conveyed to a storm sewer or other stormwater practice in the building drainage system. After a storm, a large portion of the stored water is evapo-transpired by the plants, evaporated or slowly drains away. STEP has prepared a life cycle cost estimate for an extensive, 4 inch deep pre-grown sedum mat, green roof design configuration, based on a 2,000 m² roof drainage area, runoff control target of 25 mm depth, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) here.

Design Assumptions

Green roofs are an ideal technology for sites without significant space at ground level for infiltration and zero-lot line projects with outdoor amenity requirements. Green roofs are typically sized based on the available roof area, as opposed to treatment volume requirements. Green roofs are feasible in most greenfield development situations, but should be planned at the time of building design.

Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:

• Building height between 1 and 5 stories.
• Roof length and width of 46.05 metres each, and slope of 2%.
• Includes an EPDM waterproofing membrane.
• Irrigation and leak detection systems are not included.
• Growing medium depth of 4 inches.
• Plants consist of pre-grown sedum mats.
• Operation and maintenance (O&M) cost estimates assume watering six times a year, annual inspections, removal of trash and debris twice a year, and weeding every five years. Verification inspections are included every 5 years to confirm adequate maintenance.

Notes

• Operation and maintenance cost estimates assume no rehabilitation of the green roof plantings nor waterproofing membrane is required to maintain acceptable drainage performance over the 50 year timeframe.
• The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
  • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
  • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
  • The tool adds 10% contingency and additional overhead as default.
• All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
• Unit costs are based on 2018 RSMeans standard union pricing.
• Additional costs associated with retrofit or redevelopment contexts is assumed to be 16% of the cost estimate for new (greenfield) construction contexts.
  • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.
    

Construction Costs

Construction Costs Per Unit Drainage Area (CAD$/m²) - No Infiltration Design

Note: Please click on each image to enlarge to view associated construction cost estimates.

Above you can find a breakdown of construction costs by expense type for the 2,000 m2, 4 inch deep pre-grown sedum mat green roof design configuration.

Material & Installation expenses represent the largest portion of total construction costs (85%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the extensive, 4 inch deep sedum mat green roof design configuration over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. Operation and maintenance costs are predicted to represent 12% of total life cycle costs over the 25-year evaluation period, and increase to 18% of total life cycle costs over the 50-year period.

25-Year life cycle cost break down

Green Roof: Extensive 4 inch deep sedum mat

Note: Click on each image to enlarge to view associated life cycle cost estimate.

50-Year life cycle cost break down

Green Roof: Extensive 4 inch deep sedum mat

Note: Click on each image to enlarge to view associated life cycle cost estimate.

Cost Summary Tables

Total life cycle cost estimates for the extensive 4 inch deep sedum mat green roof design configuration is $927,010 making it one of the most expensive LID BMP options. This is due to the costs of expensive components such as waterproofing membrane and flashing, roof insulation required, drainage layer and filter cloth, growing medium, the plants themselves and crane requirements for construction.

Extensive 4 inch deep sedum mats

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

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.

Sustainable Technologies Evaluation Program

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

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

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

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

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.

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.

Toronto and Region Conservation Authority

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.