Over two years ago SIG Design & Technology published a blog post by Ross Finnie called ‘Blue Roof: Why it is a Bad Idea’. It created a lot of interest and some controversy. Ross’ message was that there are better places to keep rainwater than on the roof – for a number of good reasons. Two years on SIG are revisiting the topic to see if things have changed. SIG spoke to Steven Vincent of BBS Green Roofing and their green roofing partners Verdico, to get his view and an update on the situation.
Were SIG right about Blue Roofs?
“Ross’ point of view is absolutely valid,” says Steve. “It remains the case that keeping large quantities of water on a roof for long periods of time is not a great idea, and if it can be avoided, it should be.”
“At the same time, pressures on designers are getting harder, and for good reason” says Steve. “Government policy encourages all developers to incorporate sustainable urban drainage systems (SuDs). Part H of the Building Regulations in particular requires a sustainable approach with a hierarchy of options for the disposal of rainwater, with the preferred option being discharge to a soakaway or infiltration system.”
CIRIA also advises that that rainwater is controlled as close to the point it lands as possible. “Previously this would be determined as just the local catchment (permeable pavements and tanks). Together with the improvement of quality control and waterproof membranes the roof begins to look like a cost effective solution” says Steve.
The definition of a Blue Roof has evolved
As regulation and an increased understanding of the effects of urbanisation on the environment drive demand, the design of blue roofs has become more sophisticated and intelligent.
The first Blue Roofs were called “Reservoir Roofs” which were designed to hold water with an overflow, keeping significant quantities of water on the roof. This type of blue roof was very common in the US. “They have fallen out of favour and are rarely used now”, says Steve, “for the very reasons that Ross described in his post.”
The New Blue Roof
The concept of a Blue Roof has changed to emphasise the role of attenuation – slowing down the discharge of water from a roof into the drainage system by restricting the flow. In this kind of blue roof the roof fills up during heavy rain, but then drains completely over time. Today we are looking to get all the water off the roof in a manageable time rather than waiting for the held water to reach the discharge point. This reduces wear and fatigue in the building structure as well as risk of leaks.
A Green Roof is a Blue Roof
“One could say that all Green Roofs are a type of Blue Roof, because they do retain water” says Steve. “Green Roofs only retain a small amount of water, but it can have a dramatic effect on the environment. Several studies have saught to quantify the effect of green roofs on surface water runoff [Mentens, J., D. Raes, et al. (2006)] and in cities like Chicago stormwater runoff is being used as an argument for green infrastructure projects.”
The general case for green roofs has been very well made, although there is not yet a sufficient standards structure. For this reason it is even more important to obtain good quality advice.
Hybrid Blue Roof /Green Roof
Today a third type of blue roof has evolved; the hybrid blue/green roof. It encompasses the benefits of a green roof with the additional water attenuation of a blue roof. Here is an example from Branch Place in London: a Biodiverse Hybrid BBS Blue Roof/Green Roof System, installed by BBS Green Roofing Ltd with waterproofing by Cambridge Polymer Roofing.
So what if you need to Attenuate Water?
Here are some key steps to take to help identify what type of roof is right for your project.
• Firstly determine what SuDs criteria you have to meet. This will be provided by guidance at county or unity authority level, plus you can consult Part H and the CIRIA guide.
• Secondly look at the options for sustainable drainage that do not require significant water retention on the roof. Can you carry out the attenuation off the roof?
• Thirdly look at the other criteria for the roof and project design. Is it possible to incorporate the SuDs system to satisfy other criteria such as water recycling? This can be very cost effective.
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