Flat roofs, terraces and balconies

7.1.13Blue roofs

Blue roofs shall be suitable for their intended use

Blue roofs should:

  • be designed in accordance with relevant parts of BS 6229: 2018 and the NFRC Technical Guidance Note for Construction and Design of Blue Roofs. Roofs and podiums with controlled temporary water attenuation.
  • have a certified waterproofing system that is endorsed by the manufacturer as fully suitable to be specified for a Blue sustainable roof design.
  • have supporting data to demonstrate compliance with relevant material standards and codes of practice.
  • include waterproofing layer suitable for use in the blue roof system and subject to independent third-party assessment.
  • have specific flow restrictor outlets to meet the required water discharge rate and which are accessible for inspection and maintenance.
  • fully drain over the designed retention period. Permanent retention of water is not accepted on the roof waterproofing layer.
  • include overflows independent of the rainwater drainage system, to avoid water ingress into the building should the water attenuation level be exceeded. Operation of an overflow should be visible to warn of a potential blockage in the drainage system.
  • have minimal penetrations of the waterproofing layer other than rainwater outlets and overflows.
  • Be designed as a warm roof or inverted warm roof.

If designed as an inverted warm roof the design should take into account the additional thickness of insulation, over and above the dry state ‘U’ value calculation, that is required to compensate for the cooling effect of water penetrating the insulation and reaching the waterproofing layer, in accordance with the guidance in BS 6229.

The design should also include sufficient topping of ballast, paving or green roof to avoid flotation of the insulation, Flotation forces can be quite significant and occur before the attenuation system reaches full capacity.

Care should also be taken in the design and installation of the WFRL so that it performs as the principal drainage layer and reduces water penetration to the insulation layer below.

The WFRL should be:

  • lapped and sealed to avoid/minimise rainwater getting below the insulation layer.
  • lapped and taped onto the lip of the water attenuation chamber.
  • finish a minimum of 50mm above the top of the blue roof attenuation level at parapets/upstands.
  • taken up all protrusions/penetrations and sealed. This includes between the top of the insulation layer and the bottom of the recessed lip of the water attenuation chamber, and behind parapet chambers.