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.

Want to find a better solution? We won’t only sell you one product, we’ll recommend the best system for your needs. Speak to one of our team, ring us up or fill out the contact form for us to call you.