Blue Green Roof Rainwater Attenuation Systems

Creating Sustainable Urban Environments - Mitigating climate change

Watch our animation to learn more about the Patented ACO Blue Roof Attenuation system
Download our Blue/Green roof brochure below or visit our RoofBloxx page for roof build up drawings

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ACO RoofBloxx Brochure
ACO RoofBloxx Brochure

Read this guide about how Blue Green Roofs can be designed to drain slowly in normal use and quickly in extreme conditions, using ACO RoofBloxx

ACO RoofBloxx Design Guide
ACO RoofBloxx Design Guide

A Design Guide to the ACO RoofBloxx system showing the build ups for a variety of roof types. Don't forget we can help with calculations for any roof.




The Patented ACO Blue Roof System complies with Building Standards and Regulations and reduces risk

The Patented ACO Blue/Green Roof system complies with Building Standards and Regulations by ensuring the flat roof can drain quickly in extreme events and slowly in normal use.

The ACO solution separates the competing drainage requirements by engineering an elevated structural attenuation system that operates independently of the flat roof drainage system, allowing both storm eventualities and systems to be engineered to known standards and best practice. 

Separating the design issues ensures that compromise is not needed. The attenuation system can drain slowly in normal use and in an unforeseen eventuality the attenuation system can be bypassed if necessary and the flat roof drain quickly.

The system operates by creating a structural drainage void between the top of the roof waterproofing and the underside of the attenuation lining membrane that surrounds ACO RoofBloxx. As the blue roof tank sits inside the roof area a weir overflow is created at the perimeter of the tank and above the ACO roof outlets ensuring free flow of rainwater if the blue roof tank is full. The design of the system allows the blue roof tank to be controlled through as few ACO blue roof flow restrictors as needed and integrates with the ACO range of roofing outlets.

The ACO blue roof system ensures water is not encouraged to remain in contact with the waterproofing layer dramatically lowering the building risk.

The system can be incorporated under a wide range of roof finishes from soft landscaping to trafficked areas, and with a variety of roof construction types including inverted roofs and podium decks.

Our ACO RoofBloxx rainwater attenuation system is designed specifically as a green roof drainage layer for use on blue, green and blue/green roof construction applications. Fully compatible with other ACO drainage systems as well as ACO Pipe™ products, ACO RoofBloxx can store rainwater at source to control surface water run-off. It can also be used to create passive irrigation systems.

The Patented ACO design helps meet the requirements of the London Plan as it allows the Blue Roof to be effectively used for Rainwater Harvesting and irrigation which is the 1st point on the drainage hierarchy on the Sustainable Drainage policy of the London Plan..

What are the benefits of a blue roof and why are they important?

Blue, green and blue/green roofs have an increasingly important role to play in the creation of the sustainable urban environments which society needs and demands.

Blue green roofs are ideal flat roof drainage systems and provide numerous benefits including:

  • Climate Change - Flood control
  • Urban Heat Island Mitigation
  • Amenity space
  • Passive rainwater harvesting and mitigation
  • Energy efficiency through improved insulation
  • In addition, local planning guidance (e.g. The London Plan) increasingly recommends the use of blue green infrastructure as part of a roof build up wherever possible.
  • Reduced water bills if blue roof used to capture grey water for use in the building

How have blue roofs previously been designed?

Conventional blue roof design looks to provide the rainwater attenuation within the build-up of a normal roofing system as a single solution in a similar way to that of a green roof. Ultimately this has led to a degree of confusion and differing interpretations as to how a blue roof should be designed given that its purpose and function is very different from flat roof drainage design.

In simple terms a standard flat roof is designed to drain quickly in the worst case event – a short intense summer storm lasting minutes, whereas a blue/green roof is designed to drain slowly – to mitigate the effects of downstream flooding that can be a duration of hours. Designing a roof drainage system to cater for both extremes is a difficult challenge.

Current design dilemmas associated with blue roofs include, drain down times, roof falls, number of roof outlets, pipe gradients, roof construction - most notably with inverted designs, and not least a lack of standards, all of which lead to issues with design liability and warranties.


Read how the ACO system overcomes the design challenges for blue roofs in our article 'The Design Dilemma'

The ACO blue roof attenuation system

ACO RoofBloxx

ACO RoofBloxx offers architects, engineers and property owners an environmentally sustainable, efficient and cost-effective method to reduce rainwater run-off, enabling water storage and irrigation of blue/green roofs in urban environments.

ACO Blue Roof Flow Restrictor

The ACO blue roofing outlet flow restrictor and overflow enables designers to control the rate of water runoff from a blue roof. The restrictor has been designed for use with the ACO range roof outlets and accessories.

ACO RoofBloxx Reservoir Tray

The ACO RoofBloxx Reservoir Tray is a lightweight interlocking modular green roof tray which offers complete design flexibility

ACO RoofBloxx Accessories

The ACO RoofBloxx system includes a complete range of accessories including capillary wicks for passive irrigation systems, a protection fleece which provides additional protection for the roof membrane, and a filter geotextile.

What is a Blue Roof?

A blue roof is a flat roof design that is explicitly intended to temporarily store rainfall. The purpose of the design is to detain rainwater for a specified period at a given flow rate to prevent downstream flooding, particularly in an urban environment.

What is a Green Roof?

A green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. They have many benefits for the environment from reducing stormwater run-off, climate change adaptation, to improvements in building lifespan.

What is a Blue/Green Roof?

A blue/green roof is a roof design that combines the benefits of both a blue roof and a green roof. It can have the added benefit of providing passive irrigation to the green element of the roof.

Why use a Blue/Green Roof?

In urban environments where space is at a premium, conventional sustainable drainage system tend to be unviable, as blue roofs require no additional space, they offer a logical way of controlling and re-using rainwater at source.

Blue/Green roofs have numerous environmental benefits:

• Climate change – flood control
• Urban heat island mitigation
• Amenity space
• Passive rainwater harvesting and irrigation
• Energy efficiency through improved insulation
• Prolongs life of roofing membrane
• Reduces noise and air pollution
• Biodiversity
• Carbon sequestration

What structural load does a blue roof impose?

The usual depth of the stored water on a blue roof is unlikely to be much more than 100-150mm in depth, as such the increased load imposed  by a blue roof is minimal (1.0-1.5kN/M2 load), however a detailed structural analysis should be carried out by a suitably qualified engineer. As most blue roofs are designed in conjunction with a green roof build up, they will tend to require a concrete slab to be specified.

How do I design the blue roof drainage?

The roof drainage should be designed to BSEN12056-3:2000 and if draining a podium deck or a landscaped roof then consideration should also be given to the surface water drainage. The design of this should be carried out to BS EN 752:2008. Consideration should also be given to:

• Waterproofing details
• Penetrations
• Roof geometry
• Fire and vegetation
• Roof outlets
• Thermal insulation
• Wind Uplift

The purpose of a blue roof is to detain water rather drain quickly and whilst design should be undertaken to the relevant standards where applicable. The hydraulic design of the system is often site specific and we would recommend early consultation with ACO.

What types of roofs can a blue roof be used on?

A Blue Roof can be used on a variety of flat roof designs, including warm, inverted roofs as well as podium decks. They can be used under green roofs, landscaped decks, parking areas and beneath photovoltaic panels. Due to the loadings imposed by blue and blue/green roofs a concrete roof slab tends to be specified.

How do I control the rate off flow off a blue roof?

A flow restrictor fitted to a flat roof outlet is used to control the rate of run-off, the design of this restrictor is critical to the performance of the system.

What is an achievable flow rate?

The flow rate is determined by site specific conditions such as roof area, threshold heights, and storm return period. In general terms the larger the roof area the lower the achievable rate.

Over what period of time should the blue roof drain?

Accepted practice is that the blue roof should fully drain down in 24 hours, this differs from attenuation drainage below ground that is normally required to ½ drain down in 24 hours.

Why choose ACO?

ACO Building Drainage is the established leader in the provision of building drainage and surface water management systems and provides blue/green roof drainage systems designed and manufactured to support the functionality of commercial buildings both large and small. We have more than 60 years’ experience of designing and manufacturing high performance drainage solutions, and our in-house design team provides tailored solutions to suit individual client requirements.  

Technical support

Hydraulic Calculations & In-House Technical Support

Hydraulic calculations and advice regarding system design are available free of charge from our in-house technical design team. Our team is also available to deliver a free CPD regarding the specification of rainwater attenuation systems on blue, green and blue/green roofs.

RoofBloxx Products

Blue/Green Roofs



ACO RoofBloxx
ACO RoofBloxx

Load Class A 15

ACO RoofBloxx Cell
ACO RoofBloxx Cell

Load Class A 15

RoofBloxx Accessories
RoofBloxx Accessories

Load Class A 15

RoofBloxx Reservoir Tray
RoofBloxx Reservoir Tray

Load Class A 15

ACO RoofBloxx Cell
ACO RoofBloxx Cell

Load Class A 15