Blue Green Roof Rainwater Attenuation Systems Creating Sustainable Urban Environments

ACO Blue Green Roof Attenuation System (patent pending)

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 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. This means that the roof waterproofing is not subject to increased and prolonged hydraulic heads of water, so its primary function of keeping the building dry is not compromised.

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 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.

De-risking roof design

Fire Prevention  - The incorporation of a blue roof into a conventional flat roof and or green roof adds an other level of complexity, in particular to the compartmentalisation of the roof to reflect the internal layout below and the potential for any fire to spread across the roof to other areas of the development. We would stress that a suitably qualified fire engineer should assess these risks. Where the ACO patent pending system can assist is by providing passive irrigation to the whole green roof to prevent drying out – unlike a reservoir tray which only allows 2 dimensional flow of water, the RoofBloxx tank allows 3 dimensional flow of water to anywhere on the roof it is needed. The patent pending system also allows fire breaks to be built into the tank whilst also allowing water to fill and bypass the tank as needed.

Hydraulics – The fact that the ACO System is a separate system to the roof waterproofing means that we are not limited to the time the rainwater can sit  on the roof, conventional blue roof systems limit this to a total of 24 hours -this potentially means storage volumes will not agree with the planning conditions – if this is not picked up at the design stage the developer could breach these conditions and have to rectify it during or after construction which is all additional cost (possibly very costly) and inconvenience. In addition from an exceedance perspective, the fact that the ACO system has a complete bypass system for extreme events means virtually zero risk of water entering the building, or it building up on the roof where it might cause structural overload of the roof deck

Preview and Download our Blue/Green roof brochure

ACO RoofBloxx Brochure

ACO RoofBloxx overview brochure - Blue Roof Attenuation Systems for a Sustainable Urban Environment

ACO RoofBloxx Design Guide

A Design Guide to the ACO RoofBloxx system

The Benefits of Blue Green Roofs

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
  • Deliver on the four pillars of SUDS: A blue green roof can achieve the objectives of water quantity, water quality, amenity, and biodiversity
  • Passive rainwater harvesting and mitigation
  • Energy efficiency through improved insulation
  • In addition, local planning guidance increasingly recommends the use of blue green infrastructure as part of a roof build up wherever possible.

Overview of current design practice

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 our Design Dilemma Article here

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 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.

Blue/Green Roofs FAQs

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?

Although many blue roof system manufacturers state that a blue should fully drain down in 24 hours, this is often at odds with the stated principle in the site Surface Water Strategy document of half empty in 24 hours. The ACO Patent Pending Blue Roof attenuation System operates under ½ empty principle ensuring that planning conditions are not compromised

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.  

In regards to number of outlets for a blue roof wouldn't there need to be a safety system on top of the secondary overflows? i.e. there is a system in place for the total storage volume draining away from the blue roof if the primary outlets and secondary overflows block / fail etc?

The ACO Patent Pending system creates a separate attenuation system that sits on the roof, as such the attenuation system sits within the footprint of the roof area and allows the open tank to overflow at the perimeter of the tank as well through internal overflows in the tank positioned above the normal roof outlets. From here the water can overflow the sides of the tank and underneath to the rainwater outlets or parapet overflow, even if the overflows and restrictor are blocked.

What would prevent the apertures from blocking?

Both the diffuser unit and extension rings that surround the restrictor have drainage slots less than 10mm which is the smallest aperture for a restrictor Additionally the placement ACOTex geotextile, together with the required gravel surround around either the diffuser unit or extension rings prevent particles greater than 100 micron from reaching the restrictor aperture, The access to the restrictor is via a solid cover on the outlet or diffuser unit ensuring no debris can enter from the finished surface.

Will the reduction of flow from outlet affect rainwater pipework offsets within building?

The ACO RoofBloxx blue roof system is designed to work with as few flow restrictors as possible. This means a single, or a smaller number of larger aperture restrictors resulting in a higher flow rates within the downpipe(s). This will significantly reduce or eliminate the need for greater offsets.

Is a blue roof system compatible with a siphonic rainwater system?

No, the design requirements of a siphonic rainwater drainage system and blue roof attenuation systems are not compatible.

What is the impact on depth of roof build up compared to other systems?

Blue Roofs require a depth of water to be stored on the roof area for in excess of 24 hours, this clearly has an impact on both structural load and build depths compared to both a conventional flat roof or green roof. If the roof area is capturing and storing only the rainfall that falls directly on it and controlled to greenfield run off rates then typical storage depths (dependant on FSR/ FEH) range between 85-125mm. This excludes the 30mm structural drainage layer.

Is there any issues with bolting down street furniture with the roof build up / surface finishes?

Any street furniture, planter, PV Panels, ballustrades etc should be fixed appropriately to the roof structure as may be required by the relevant engineer. The ACO RoofBloxx attenuation system can be designed to fit around such fixings as required. In certain circumstances it may be possible to fix items e.g PV Panels to through the RoofBloxx system using a rail system. Please consult ACO Design Services Team for further advice.

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