CIRIA C817 (2024) — Blue Roofs: A Guide to Implementation

The definitive UK guidance on blue roofs

CIRIA C817 is the definitive UK guidance on blue roof design and implementation — the practice of using flat roof structures for stormwater attenuation. Vector Technical Director Ben Hickman was a Key Technical Author of this publication.

Why blue roofs are structurally different

Blue roofs introduce a deliberate design condition where water is retained on the roof membrane for extended periods. The waterproofing membrane must perform as a permanent containment layer — not as a layer that will see occasional weather. The consequence of failure, and the difficulty of investigating it once the system is operational, are both substantially higher than for a conventional flat roof.

What C817 says about leak detection and testing

C817 is guidance, and its recommendations are framed as things that should be done or considered rather than mandated. On monitoring and testing it recommends:

• Independent third-party integrity testing of the waterproofing before the roof is handed over and before the attenuation build-up goes on (Section 4.5)
• Independent, competent third-party quality assurance of the waterproofing, including electronic leak detection where the substrate is conductive (Section 3.26.6)
• That consideration be given to installing a permanent leak detection system on a warm blue roof, because locating a leak once the roof is operational is very difficult (Section 3.2.3)
• That consideration be given to permanent leak detection wherever alternative waterproofing systems or build-ups make a defect harder to locate and repair (Section 3.26, Repairability)

Once a blue roof is operational, finding a leak by conventional means is very difficult. The time to verify the membrane is before water sits on it.

How Vector applies C817 on live projects

Two services apply C817 guidance directly: electronic integrity testing prior to overburden placement, and buried leak location using electrical field mapping for blue roofs that have already failed.

Publication: CIRIA C817 — Blue Roofs: A Guide to Implementation (2024)
Checked against current edition: 11 September 2026
Publisher: CIRIA

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