Rainfall Data
Rainfall depths for the site are calculated according to location, storm duration and return period (the number of times in a set period that a storm of that magnitude is likely to occur (with 1-in-30 year and 1-in-100 year storms usually considered). An allowance is also made for future climatic change.
In the UK, values are taken from either the Flood Studies Report (FSR) or Flood Estimation Handbook (FEH 2013), depending on which is key to determining the exact storm design.
Rainfall and run-off should be considered simultaneously to give an actual representation of the behaviour of the ABG blueroof system under storm conditions.
Waterproofing Design
The ABG blueroof is compatible with both warm and inverted roof constructions and compatible with all modern waterproofing materials. However the selection of the waterproofing depends on the type of flat roof construction and a BBA certificate for zero falls is recommended.
As with traditional roof types, the waterproofing should be detailed to standard height above the final floor levels, with the recommended test methods for integrity adhered to. ABG partner with leading manufacturers and installers of waterproofing systems and can offer project specific advice and guidance to ensure the optimum solution is selected.
Rainwater Outlets and ABG Restrictor Chambers
The BS EN 12056-2:2000 standard for gravity drainage systems sets out a conservative approach to determining the number of rainwater outlets required, based on the principle of discharging water from the roof as quickly as possible. This typically results in more outlets being specified than are actually required.
More current CIRIA SuDS best practice guidance is designed to control the rate of rainwater discharge, and since the ABG blueroof system calculates the exact number of restrictor chambers required to attenuate the flow rate at each outlet, fewer outlets and penetrations are typically required. This has a positive knock on effect on the waterproofing and insulation detailing, and in turn helps to reduce the associated programme time and overall cost of the building.
Overflow Outlet
As a precautionary measure in the event that the restrictor chamber becomes blocked, or a storm occurs that exceeds the designed capacity; an overflow outlet is positioned level with the top of the restrictor chamber to drain from above the nearest parapet wall. This provides a visual indicator from ground level that a blockage or exceedance has occurred and that maintenance may be required.
Structural and Loading Considerations
The introduction of a blue roof but may have loading implications and a structural engineer should be consulted at an early stage, especially when designing for a SuDS solution where water will be temporarily attenuated within the roof structure. This will enable any constraints to be determined, although these are often not as onerous as might be expected. The ABG blueroof attenuates the collected water across the entire area of the roof at a shallow depth, typically less than 100mm and at full capacity this would only exert a maximum additional load of 1.0kN/m2.
The ABG blueroof components can be designed to take differing loads and to accommodate nearly all eventualities; including landscaped areas, podium decks for emergency fire and HGV vehicle access and large HVAC plant and PV installations.
Water Quality
Installing the ABG blueroof system has a positive impact on the quality of the water discharged. Before the rainwater reaches the roof outlet, it has passed through several filtration layers that remove particulates and pollutants. Even if the surface finish isn’t green, the water has passed through at least two layers of filtration.
The water is treated to such a degree that it reaches the level required in treatment train stage one of the SuDS process, allowing the water to be released from the roof directly into the river or sewer systems.
Installation Process, Access and Maintenance
The ABG blueroof has a mandatory provision for maintenance and therefore future access should be considered to suit the final finish, as with all roofing applications.
The ABG blueroof restrictor chambers contain integral geotextile filters that are inspected and serviced twice a year (consistent with the BS EN 12056-2 provision that rainwater outlets on flat roofs should receive bi-annual maintenance).
Paved podium decks and ballasted roofs are relatively low maintenance, whereas extensive green roofs usually require an annual maintenance visit to remove weeds and trim vegetation. Intensive green roofs require maintenance like any garden or landscaped area, and this is a service that ABG provide on most projects.
Geography
Geographical location and orientation are an important part of designing a blue roof. The region, average amount of rainfall in that location and the prevalent wind direction all affect the design and must be considered. This can affect the calculation depending on the data from either the FSR, FEH 2013, or other global rainfall data standards.
The specific design guidance will reflect the local climatic conditions, for example to store water for plant irrigation in the Middle East, or to attenuate and slowly release water as part of a flood management strategy in Central Europe. The geographic location also impacts the growing media and plant selection for vegetated surface finishes, with green roof species chosen to suit the region.
Thermal Performance
Currently the components of a blue or green roof are not considered as part of the roof build-up when calculating thermal performance, so the insulation design and specification is the same as for a traditional roof.
Despite no allowance being made in the building regulations at present, research shows that the introduction of a green roof surface finish with growing media and vegetation has a positive impact on the thermal performance of a roof, with the most significant benefit being a reduction in solar gain.
Surface Finishes
The ABG blueroof system must include a surface finish above the water attenuation layers. This surface finish can be constructed from any permeable surface that is in-keeping with the development. This protects the blue roof system, and is in accordance with wind uplift allowances. An impermeable surface can be used, but adequate measures should be taken to ensure the water can filter into the blue roof system.
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