Green Roof: Life Cycle Costs

Enhanced swale with rocky check dams and a metal overflow grate in Northgate Mall parking lot, Seattle. Photo credit: MLSmith.

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 life cycle costs estimates for each design configuration, based on a 2,000 m² road drainage area, runoff control target of 25 mm depth and 72 hour drainage period, 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 physically feasible in most 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:

• Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 (where applicable) was applied to calculate the design infiltration rate.
• Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, watering, weeding, removal of litter, plant replacement and removal of weeds (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable)
• Load bearing capacity of building structure/roof deck need to be sufficient to support the saturated weight of the soil, vegetation and accumulated water.
• Green roofs may be installed on roofs with slopes up to 10%.
• Designed to capture precipitation falling directly onto the roof surface, not to receive runoff diverted from other source areas.

Notes

• 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) - Full Infiltration Design, 25 mm Treatment

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

Above you can find a cost breakdown of a 1000m² LID configuration:

1. Enhanced Swales: Full Infiltration

Material & Installation expenses represent the largest portion of total construction costs (50%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the Enhanced swales Full Infiltration 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 45% of total life cycle costs over the 25-year evaluation period, and increase to 59% of total life cycle costs over the 50-year period, due to increased levels of litter removal, watering (establishment period - years 1 & 2 / annually and / drought watering) cleaning out and vacuuming of the catchbasin, mowing, replacement of sod, weeding and pruning of the BMP's planted vegetation.

25-Year life cycle cost break down

Enhances Swales: Full Infiltration

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

50-Year life cycle cost break down

Enhances Swales: Full Infiltration

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

Cost Summary Tables

Total life cycle cost estimates for the Enhanced swale Full Infiltration design configuration is ($23,451.36), makign it one of the cheapest LID BMP options.

It is notable that a sensitivity analysis was conducted in 2019 to compare construction cost estimates generated by the tool to actual costs of implemented projects. The analysis found that tool estimates were typically within ±14% of actual construction costs

Full Infiltration

Enhanced Grass swale in CVC headquarters parking lot, Mississauga ON.

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

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

Sustainable Technologies Evaluation Program

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.

The natural ground material characteristic of or existing by virtue of geographic origin.

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

Any form of rain or snow.

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.

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

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Credit Valley Conservation