Engineered design and specification

Fundamentally, a blue roof is designed to attenuate water and should not be considered as a water storage solution.

The Bauder Blue Roof is designed for use with either the Bauder Total Green Roof System as a warm roof construction or the Bauder Hot Melt cold roof construction. The design of the blue roof requires:

  • Specific flow restrictor outlets to meet the requirements of planning for total allowable water discharge.
  • Emergency overflows included in the design, unconnected to the blue roof outlets, to discharge the rainwater should the maximum height of rainwater attenuated be reached.
  • Free-flowing multi-directional water movement within the void space to the specifically engineered outlets.
  • Minimal or elimination of penetrations in the area where water is to be attenuated, other than the rainwater outlets or emergency overflows that are required for drainage functionality.  In some instances, it may be possible to individually isolate the roof penetration from the blue roof SuDS to prevent any possible water ingress from contaminating the insulation.
  • Roof design must adhere to the British Standards and Systems Codes of Practice for waterproofing and roof detailing.

Flow Restrictor Outlet Design and Specification

The baseplate of the blue roof system sits within a standard Bauder outlet and slows water from leaving the roof via a calculated number of restrictive flow holes. The number of flow holes, up to a maximum of 12, is calculated to reflect the SuDS calculation for permitted discharge rate for the site.

If, in the event of a storm of greater magnitude than 1 in 100 years plus 40% for climate change, then water will evacuate the roof through the central overflow.

The design of every Bauder blue roof is individual to the project and geographical location. We use details of the roof area and the drainage requirements for the site to produce a roof specific discharge report to show the following:

  • Storm profile for the roof during a 1 in 100-year storm event + 40% for climate change.
  • Maximum attenuation volume on the roof.
  • 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.

The report and series of calculations allows us to produce the most effective scheme for your project.

Installation of a BauderBLUE Roof

The Bauder blue roof outlet overflow is inserted into the baseplate and measured, marked and cut to the specified Hmax for the roof. The entire blue roof outlet is protected with the Bauder ALU250 Inspection Chamber which allows for ease of access during regular maintenance.


Blue roofs require regular maintenance and inspection of the outlets to ensure the drainage holes are free of debris or blockages. Inspections should take place following any significant storm event, any notable traffic or remedial works that take place on or around the roof, and following leaf fall during the autumn months to ensure leaf litter is not causing obstruction.