Blue Roof – why it is a bad idea
[UPDATE: This post was written in 2013 and Blue Roof technology has moved on since then, though Ross still believes taking the water off a roof is the best underlying policy and a blue roof should remain a last resort, as this article states. Here’s an update from 2016 and another on Best Practice from 2018.]
Last week we delivered a CPD (our new RIBA certified one about roof design) to a group at a large London architects practice. One of the group asked us about Blue Roofs, here’s what we said.
What is a Blue Roof?
A Blue Roof is one that is designed to retain water above the waterproofing membrane. Rather than allowing water to drain off your roof as soon as possible, a blue roof deliberately retains some or all of the water, either temporarily or permanently.
“A roof designed to allow the build-up of water above the roof waterproofing membrane”- i.e. storm water attenuation on the roof itself.
Why people are thinking of Blue Roofs
- Urban Flooding – caused by storm water runoff is a significant problem in the UK, with the result that Sustainable Urban Drainage Systems (SuDS) are a prerequisite in most new developments.
- Storm water management – a blue roof can be used for stormwater attenuation, to temporarily mitigate the impacts of rainwater runoffs, both on water volume and quality. Indeed you may see blue roofs described as ‘rooftop attenuation’. The roof is usually completely flat without falls, and flow is controlled by outlets at roof level or flow control device at a lower level.
- Avoiding attenuation at ground level – space is at a premium in many new developments and the large expanse of a flat roof may be very attractive in contrast to providing attenuation at ground level in the landscape or within the building footprint.
- Rainwater harvesting – rainwater is a precious commodity, and harvesting may even be a requirement of the planning or building regulations, depending on the development. Storing the water on the roof seems like a good idea, after all, there’s plenty of space.
What’s just plain wrong with Blue Roof Design
Working in a company that provides a complete roofing service, from design right through to delivery and guaranteed installation, I have a different perspective. Flat roofs require regular, though not complicated, maintenance to ensure their performance is maintained. In the real world, keeping water on the roof is probably an accident waiting to happen.
1. Water is heavy
And structure costs money. Why pay to keep it up there? If you can get most of it onto the ground the structural costs will be reduced.
2. Membrane Failure would be catastrophic
Just imagine. A heavy rain storm, falling tree branch, mechanical damage… and the likely cost of damage from a blue roof failure.
3. There are better places to put the water
If your Rainwater Harvesting tank is elsewhere there will be nowhere near as much risk of damage. Indeed, a flat roof membrane, no matter how well detailed, isn’t the best place to keep a large quantity of water.
4. There are better things to put on the roof
Attenuation, if at a lower volume, can be achieved by green roof technologies, and green or brown roofs have other benefits.
Just Say No to Blue Roofs
So we recommend you just say no to Blue Roofs. In severely constrained situations where there is no other option for storm water attenuation, a blue roof solution may be the only option, but it needs to be the very last resort.
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A big concern for all our native protected bat species which use roof voids and tile/felt voids to roost within. Another destructive practice to push bats onto the brink. I thinks it’s fine on some new builds but not as a replacement for an already pitched roof.
I think that is ‘jumping to a conclusion’, there’s a lot more to blue roofing (which I think is a new term for an older technique) than is made apparent here.
Firstly, the retention of water is a benefit but shouldn’t be the primary design purpose.
Secondly, there is a lot of experience with these kind of techniques overseas, where hotter summers combined harsher winters have been the norm. Here in Britain we seem to think a dry roof is a good thing, but in those other countries, the water has been retained on purpose to help protect the building from the weather and to stabilise the temperature and fabric of the roof structure.
As an example, see this research paper on the matter http://www.building-physics.net/webfm_send/343
(Look for ‘inverted gravel roof’ for some ideas).
Please note that I neither agree nor disagree with the author of this blog, I simply want to point out that there are more ways to look at this issue and its not as straightforward as it first seems.
Hi, would any of you be interested in taking part in a short interview for my dissertation? The title: “How significant could the widespread adoption of blue roofs be in alleviating pluvial flooding withing the UK” discusses and examines the potential benefits and downfalls of blue roofing systems, and I am currently collecting data from industry professionals regarding this. Any extra help would be welcomed. Will.
Hi William, thanks for getting in touch. Ross is emailing you to discuss.
Hi William,
I would be really interested in reading your dissertation to get clued up on blue roofs if you would be willing to share it with me 🙂
Many thanks,
Rachel
Good evening,
I am about to write a dissertation on blue roofs myself, and would also appreciate if you could forward any useful references for academic pieces on the topic. I am finding it difficult to ascertain any reliable material, outside of grey literature and internet articles to support any findings.
If you could forward any useful information in this regard I would be very grateful.
Thanks in advance.
Hi William,
I am also very interested in you dissertation. Any change you can share it with me as well?
Thanks,
Niels
a copy of the blue paper would be very much appreciated, kind regards
I have, in the past, worked on many flat roofs (predominantly asphalt), where the outlets have been set in 75mm higher than the roof level. This allows water to sit on the roof and eventually reach the outlets. During the summer, this assists in keeping the asphalt at a lower temperature. Also, as the sun is not continually beating down on the asphalt it again assists with the protection of the roof covering. The retention of water on a roof is nigh on as old as the hills-albeit on the buildings I am talking about, is when construction in Britain was at its finest.