Blue roofs

Conceptual diagram of flow control drain cover type blue roof/ rooftop detention

Blue roofs can be a very economical and low maintenance way to manage undesirable rainwater on a flat roof. For rooftop retention on moderately sloped roofs, a green roof is going to be a better choice. On steeply sloped roofs, rainwater harvesting, or diverting roof leaders to a landscape integrated LID is a more appropriate alternative.

Overview

Preceded by green roof and flow control roof drain systems, blue roof systems are an innovative green technology for flood and drought resilience and adaptation. Blue roof systems 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. Any remaining water can be gradually released into the municipal stormwater system reducing peak flow rates. Additionally, during the summer season, rainwater ponded on a flat roof can cool the interior of a building and reduce air conditioning pressures through evaporative cooling. As such, smart blue roof systems are advantageous to both the private and public sector.

Blue roof systems regulate rooftop runoff by storing and controlling the release of rainwater, thereby reducing the potential of overflow in the municipal sewer system and thus, the potential for localized flooding. Combining a blue roof system with rainwater harvesting (RWH) allows for rainwater reuse and provides additional benefits. Blue roof systems are a popular stormwater management option for flat roof buildings, particularly within the industrial, commercial and institutional (ICI) sector (which make up 23 - 30% of most urbanized areas) .

Blue roofs are ideal for:

• Sites without significant space at ground level for infiltration practices
• Zero-lot line projects with outdoor amenity requirements

The fundamental components of a blue roof are:

• A flat roof
• Flow control device(s)

Planning

Things to consider when planning a rooftop detention system

Advantages	Limitations
Do not require ground level space, so particularly well suited where ground space is limited or sites with large flat roofs May not require additional sewer connections Easy to retrofit if structural support and waterproofing are adequate Relatively low cost compared to green roofs (no vegetation requirements) Potential for evaporative cooling and reduction of urban heat island effect	Must be regularly inspected and maintained Limited application on sloped roofs or sites with small building footprints Do not provide any co-benefits such as habitat creation or air quality improvements associated with vegetated practices Mechanical systems and other rooftop architectural features must not be adversely impacted by ponded water

Design

Water can be slowed in running from a flat roof (up to 2%) through the use of:

• drain covers (most common),
• small check dams or weirs across the roof deck, or
• modular tray systems.

All roofs in Canada must be designed to support peak snow loads, providing capacity for supporting an equivalent weight of ponded water. The Ontario building code permits designers to assume that snow and water loads are not cumulative, up to a maximum depth of 150 mm (water equivalent), if precautions are taken to prevent flooding by providing a sufficient number of roof drains with overflows. It is therefore uncommon for roof water detention systems to be designed to permit more than 150 mm of water to accumulate. This also means that these systems can be retrofitted onto existing roof structures that have been designed to support conventional snow loads.

The Ministry of Transportation does not currently permit blue roofs to form part of a SWM plan owing to concerns over long-term maintenance; this affects relatively few projects. The concerns arise from instances where building operators have found outlet control structures blocked with leaves or other debris. Without understanding the purpose of blue roof systems and flow control drains, these devices have been removed to prevent recurrence of the blockage. A study in New York City found that a modular system of trays outperformed an outlet flow control device in stormwater management. It might reasonably be expected that a modular system would present fewer opportunities for complete failure from a single action, either through clogging or breakage of a storage element.

Smart Design

Blue roofs can also be fitted with smart controls and electrically operated valves to regulate water discharging from the roof. The electrically operated valves can be tied to predictive weather algorithms to forecast incoming rain or snow events as well as extreme heat days. Based on the weather forecast, valves may be opened and closed to accommodate additional rain or snow loads. When coupled with a rainwater harvesting system, blue roofs increase rainwater storage capacity to meet non-potable water demand (i.e. toilet flushing or irrigation) without the need to oversize tanks. Rainwater harvesting tanks with smart controls are able to coordinate with blue roofs to draw water from the roof when the tank water level is low to meet demand and offset municipal potable water use.

Inspection and Maintenance

An inspection and maintenance plan is crucial to the success of a blue roof system and should be updated based on the results of the first-year post-construction monitoring. The plan should include a maintenance schedule developed for the entire life of the rooftop system. Aspects that need to be included:

1. Maintenance activities required to be completed per week/month/season/year;
2. Frequency or maintenance and inspection activities;
3. Timing of said activities; and,
4. Assigning the appropriate person/consultant (entity) responsible for completing the activity should be included (Credit Valley Conservation, 2018)

Inspections will need to occur within 24 hours of significant rain events (>25 mm) to ensure the specified ponding depths and drain times are being achieved as per design specifications, standing water does not persist for more than 24 hours, and there are no leaks occurring due to damage to the roof membrane or as a result of other underlying roof conditions (NYCDEP, 2012a)..

Recommended Inspection Activities for Blue Roofs (NYCDEP, 2012b)

Schedule	Activity
Semi-annually under dry conditions	Inspect roof drain inlets to ensure in good condition. Inspect drain inlet screens/covers and scuppers to ensure in good condition. Inspect roof membrane to check for signs of deterioration.
Quarterly and after rain events	Inspect roof to verify achievement of water depth and drain time requirements. Inspect secondary drainage inlets for blockage or debris.
After snow/icing events	Check roof drain inlets for blockage caused by buildup of snow or ice.

Maintenance activities for blue roofs should focus on preventing clogging of drainage inlets and deterioration of the roof membrane (NYCDEP, 2012b). In addition, adequate maintenance of the secondary drainage system is essential to ensure its performance in the event of a failure of the roofing system. The rooftop system and membrane should be evaluated every 20 years to assess the need for replacement. Maintenance access points should be clearly identified in the plan for the system. Special training required to perform specific tasks is required for the appropriate person/consultant (entity) responsible for completing the specific maintenance activity.

Recommended Maintenance Activities for Blue Roofs (NYCDEP, 2012b)

Schedule	Activity	Equipment Needed
During inspections or as needed to ensure performance	Remove debris from drainage inlets and inlet screens to prevent clogging. Remove debris from secondary drainage inlets/scuppers.	Shovel
Winter considerations	Break up ice formations around inlets	Ice pick, small pick-axe, or other equivalent/relevant tool

Take a look at the Inspection and Maintenance: Green Roofs page by clicking below for further details about proper inspection and maintenance practices:

Blue roofs in the treatment train

Blue roofs can be used in combination with green roofs by employing a scaffold system to keep the vegetation rooting layer out of the ponded water. Blue roofs could also be employed in combination with a rainwater harvesting system which has daily usage demands. Coupling blue roofs with rainwater harvesting systems could offset cistern capacity requirements and supplement tank capacity deficiencies.

For Review

https://www.state.nj.us/dep/stormwater/bmp_manual/NJ_SWBMP_9.8.pdf

Providers

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Flow control devices

• Wade Canada
• Watts Canada
• Zurn

Other

These products are designed to suspend green roof (or other) systems above a layer of free water for the detention period.

• Hydrotech
• Livegreen

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

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.

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

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 practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Rainwater harvesting.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Stormwater Management

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Water ponded on the ground surface.

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

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

Tank used to store rainwater (typically roof runoff) for later use.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.