With water stress already affecting parts of the UK and urban drainage networks under increasing pressure from storms and downpours, blue roofs have emerged as a critical tool in sustainable drainage strategies, particularly in dense urban environments. But besides just managing water, developers, designers and building owners need more from their rooftop spaces now.
Neill Robinson-Welsh, Consultant at ACO Building Drainage, explained in the January/February 2026 issue of Flood Industry magazine how planners and flood risk managers can view blue roofs not simply as a sustainable drainage system (SuDS) solution, but as integrated components that deliver multifunctional benefits across biodiversity, energy, and urban design.
To be (a roof), or not to be (a roof) – that is the question. Today, rather than just shielding a building from the elements, increasingly, roofs need to help manage deluges of rainwater, particularly in densely built-up areas where other SuDS solutions are not feasible.
Large cities such as London are already looking at how they can better manage flood risks, with the capital’s emergency services having spent over £11m responding to flooding incidents since 2018[1]. In the summer of 2025, the Greater London Authority (GLA) identified flash flooding as the main environmental risk to residents. The importance of blue roofs has arguably never been greater.
But that’s not all - beyond flooding, we also want roofs to help with biodiversity, amenity, and mitigating climate change effects. Hence, an increasing number of green roofs are popping up across cityscapes to support these desires, as well as managing the ‘heat island’ phenomenon. Meanwhile, with the need to lower energy bills and increase renewables use, including solar, roofs are now expected to house PV arrays alongside heat pumps, air conditioning units and more.
Given these rising demands, how can designers and building managers ensure that modern roofs meet multifunctional requirements?
Understanding blue roofs
Unlike conventional flat roofs designed to drain water rapidly, blue roofs temporarily store rainfall and release it in a controlled manner. This reduces downstream surges and aligns with planning policy obligations for peak flow reduction. For both retrofit and new-build schemes, integrated blue roof systems form part of a layered flood mitigation approach, complementing permeable paving, rain gardens, and other SuDS measures.
When water is stored directly on the waterproofing layer, competing demands arise: planting media, paved finishes and photovoltaic arrays each impose different structural, access and moisture regimes. As a consequence, storing water directly on the roof waterproofing can compromise ecological performance and accelerate membrane deterioration — particularly at joints, penetrations and perimeter details where movement and thermal cycling are concentrated.
By contrast, off-membrane storage creates a clear functional separation: water is retained in purpose-designed tanks above the build-up and distributed as needed, enabling planted zones, paved areas and biosolar edges to operate as coordinated components rather than as competing loads. For planners, this translates into longer-term resilience, lower risk, and reduced lifecycle costs.
[1] https://www.bbc.co.uk/news/articles/cdrzzjmy73eo
Supporting green roofs - passive irrigation and wicking
The off-membrane attenuation model better supports policy-driven requirements such as biodiversity net gain (BNG) and urban greening factors.
Capillary distribution systems connected to off-membrane tanks deliver moisture to vegetation without leaving standing water. This type of passive irrigation, in turn, reduces energy demand and mechanical maintenance. Predictable moisture availability supports planting adjacent to PV arrays and access routes.
For flood risk managers, this means roof-based SuDS can sustain vegetation during long dry periods, which we are seeing more of in recent years.
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Structural, planning, and compliance considerations
On inverted or hard-landscaped roofs, conventional ballast-dependent designs can introduce substantial weight and complicate insulation stability. Modular, off-membrane tanks placed above insulation remove the need for heavy ballast in many applications; this approach reduces the risk of insulation displacement and simplifies install sequencing.
Best practice is to position tanks to maintain clear access for PV maintenance and for irrigation distribution runs, and coordinate routes so that tanks and capillary networks do not obstruct service corridors. Where possible, stage tank placement to permit later access to critical interface points without requiring removal of planted or paved finishes.
Blue roofs also contribute directly to planning metrics such as the London Plan’s Urban Greening Factor. By integrating storage volumes, distribution patterns, and planting density at the concept stage, designers can optimise ecological performance while avoiding late-stage conflicts. This positions blue roofs as policy-compliant SuDS features that deliver measurable outcomes in flood risk reduction, biodiversity, and amenity.
Managing sequencing and blue roof projects in practice
Delivering multifunctional roofscapes involves treating blue roofs as system-integration exercises. However, with so many different trades on the roof at once, complications can easily arise during the construction phase. Therefore, early engagement between architects, engineers, and landscape designers ensures tank locations, penetrations, and access routes are coordinated.
To allow for a smooth design and build process for the blue roof, and ensure PV and irrigation services will work long after installation, it’s important to follow these steps:
- Prior to the blue roof installation, conduct a site survey to confirm the flat roof has been constructed as designed
- Select the appropriate off-membrane tank, considering its type, volume and connection details and position it so it integrates with the landscape design and photovoltaic layout
- Specify the capillary or wicking elements and the routing strategy for distribution
- Coordinate and agree maintenance regimes that gives safe, practical access for future PV and irrigation servicing requirements
Strategic SuDS assets
When designed as interdependent systems, blue roofs deliver optimised drainage performance through controlled attenuation, biodiversity benefits via sustained planting, and maintainable access for PV and landscaping.
ACO’s RoofBloxx system exemplifies this approach, using modular tanks above the roof build-up to separate water storage from waterproofing, reducing risk while enabling multifunctional use.
Blue roofs are no longer niche roofing products; they are strategic SuDS assets. By integrating them early in design, planners can secure compliance with policy, reduce flood risk, and unlock multifunctional benefits across ecology, energy, and amenity.
To support better blue roof designs, ACO has compiled a comprehensive Blue Roof guide containing key considerations and approaches for blue roof schemes, which can be downloaded here.
This feature appeared in Issue 10 of Flood Industry magazine (Jan/Feb 2026). You can view the article here.