Blue roof design considerations

Development and expansion of towns and cities has seen exponential use of impervious surfaces causing artificially high rates of rainwater runoff. With the trend of urban growth coinciding with an ever increasing number of ‘extreme’ weather events, flooding is becoming a common and expensive problem.

The key sustainable urban drainage solution (SuDS) design is the principle of dealing with rain water as close as possible to where it falls. This means that blue roofs are now fundamental to many drainage plans. A blue roof controls and limits the rainwater flow from the roof ensuring that the drainage system below is not overwhelmed.

The Bauder Approach

Bauder will work with the client, architect and drainage engineer to provide a single point solution for waterproofing, blue roof and green roof layers, and system guarantee.

Bauder will produce a specialist report and calculations to determine the most effective scheme for the project using the following key data from site:

• Geographical and climate data for the project address.
• Allowable discharge for the roof.
• Maximum attenuation volume on the roof.

The Bauder report will provide the following information:

• 1:100 year storm profile for the roof (+40% factor for climate change).
• Number of outlets required, complete with an assigned number of control holes, restricting the flow of water in line with the discharge rate for site.
• The depth of void required on the roof on to which any landscaping finish can be installed (referred to as the H-Max).

This information will then be drawn up into a specification for the project complete with compatible waterproofing layers, blue roof system and landscaping/green roof layers.

Learn about:

Legislation and drivers - Sustainable Urban Drainage is now part of legislation and is a critical part of planning. Blue roofs form part of the options available for SuDS.

Design of a blue roof - The incorporation of a blue roof into a project can be at rooftop or podium level. A blue roof is designed to attenuate storm water within a void which sits directly above the waterproofing layer and beneath a surface finish such as a vegetated green roof or hard landscaping.

Drainage and roof deck construction - The discharge rate for the site is set by the local planning authority (LPA). Many LPA's are setting limits that equal greenfield run-off rates, 5 – 10 litres per second per hectare. 

Waterproofing system selection and outlets - Consideration must be given to the appropriate form of waterproofing so that it can meet the demands placed on it by the blue roof. 

Blue roof components and surface finishes - The waterproofing and insulation must have the correct structural capacity, along with the void-forming components of the blue roof. In unison they can resist the permanent load of the required finish and any imposed loading.

Waterproofing installation, detailing and inspection - British Standards and Systems Codes of Practice for waterproofing detailing should be followed and will provide a guideline to detailing principles, even if they are not specific to a blue roof application.

The maintenance of a blue roof - Maintenance of a blue roof is critical to the viability to its function.