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Public health Rainwater

Blue roof vs traditional method

Traditional roof

Roof type: flat roof office building, 5,000 sq m

Outflow: 15 litres per second for a one in 100 year storm

Scenario one: Category two storm intensity for a two minute storm = 0.062 litres per second per sq m (BS 12056 Part 3) Total flow rate = 310 l/s

Conclusion: Using a traditional approach, the entire system, from the outlets through to the underground drainage, would need to be sized to remove this peak flow (although time of concentration allowances can be made on larger systems).

Blue roof

Scenario two: If we look at the same location and storm, but choose to install a truly flat roof with outlets (or other means) restricting the flow rate from the roof to the required 15 l/s:

Temporary available ‘pond’ space: 3,750 sq m (assuming 25% volume of the roofscape is taken up by plant bases or other structures)

Conclusion: Looking at a range of storm durations from one minute to several hours and an outflow rate of 15 l/s, the maximum depth of water on the roof would only reach 40mm! (Most rainwater outlet manufacturers assume 35mm head of water over their outlets to achieve the stated flow rates.)

As the storm intensity decreases and the outlet capacities begin to exceed the rainfall rate, the roof will slowly clear of water. In this example, after 3.5 hours the water will have cleared, with the exception of some minor ponding retained in any depressions in the flat slab caused by building tolerances.

A void can be used to both attenuate and store water for harvesting >

book as it will be important that the calculated volume on the roof is not reduced during the installation of tenant plant on the roof, for example. The roof waterproofing must, of course, be carefully

designed, and the suppliers/contractors happy to provide the necessary guarantees, which should be little different from those required for a green roof. So, what could the next step be? In the example described in the panel on the left, we use only 40mm of the potential 180mm structural capacity for water storage. Could the next step be to install a shallow crate or paving pedestal system, across the flat roof to provide a storage zone beneath a pedestrian accessible area and re-use the collected rainwater? Currently, a typical rainwater harvesting system may

consist of the following elements: • Conveyance of the water from the roof as quickly as possible in large pipework;

• Routing roof water to an underground tank in its own system of underground drainage;

• Pumping water back up to a ground-level plant room; and

• Pumping water again to distribute it through the building. In this example, if we were to construct the rainwater

outlets as standpipes, creating a storage zone of 40mm beneath it as rainwater harvesting, it would yield a storage volume in excess of 120,000 litres. The rainwater harvesting could then consist of: • Storage at source with small bore pipework delivering water by gravity to the treatment plant;

• Pumped distribution to fittings; and • No associated underground drainage or tanks To use water stored in this manner for rainwater

harvesting may require some additional treatment because of the higher storage temperatures and potentially less effective silt removal. However, when the energy, carbon and cost of such a simplified system are taken into account, this becomes insignificant. (Roughly speaking, the use of the pumps will be reduced by 50%, so energy consumption may be reduced by about 40%. Embodied energy/carbon would be reduced in line with the reduced materials as given in the box, top right.) Every building is different, but the public health

engineer is well placed to become the person to advise the design team on what solution, or combination of solutions, best delivers the environmental aspirations

46 CIBSE Journal September 2010 Attenuation for free?

Using a 5,000 sq m, six-storey building, the traditional approach to rainwater disposal may consist of:

Roof membrane Screed laid to falls 45 rainwater outlets

550m of 100-150mm diameter rainwater pipe 470m of 150-450mm dia

underground drainage Total traditional system without attenuation

£225,000 £90,000 £6,000 £16,000 £35,000


During the planning process a requirement for 218 cu m of attenuation is identified.

underground attenuation tank Total cost

£50,000 £422,000

Blue roof approach: Total traditional system without attenuation £372,000

Add: enhanced roof membrane Add: higher waterproofing detailing Add: above ground flow attenuators

Omit: screed laid to falls Omit: 39 rainwater outlets

Omit: reduction in rainwater pipework Omit: reduced underground drainage

Total blue roof attenuated system

+£60,000 +£17,000 +£3,000

-£90,000 -£5,000 -£14,000 -£28,000


and value for the client. If we can reconsider the need to drain flat roofs quickly, with no residual water, then it will give us, as designers, significant scope for innovation, cost reduction and real sustainability benefits. Just as the building services engineer is now an

essential member of the conceptual design team, advising on building form in relation to energy and ventilation, perhaps it is time for the public health engineer to forge a role in the early design stages to add value and shape the way in which a development manages its impact on the water environment. l

Carl Harrop IEng FCIPHE MCIWEM MSoPHE is an associate director with multi-disciplinary consultancy, WSP

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