How to: from grey to green and blue


The first time I was personally confronted with the effects of climate change was when I woke up from a weird banging sound above my head. My neighbour was walking on my roof, sweeping water over the edge, because he was experiencing a leakage (by doing this he caused a leakage in our roof, but that is another story). The intensity of rain showers increases due to climate change. Add this increase to the surface cover in cities, and you have a big problem. Most cities are ‘’concrete jungles’’: the largest part of the surface consists of concrete or buildings made of stone. Rain cannot infiltrate into the ground due to all this stone, and the showers are too intense to processed by the sewage system, which can result in flooding (or a leakage in my case).

Big cities all over the world have to adapt to the (future) climate. This climate will pose some challenges on cities like Amsterdam, Rotterdam and Leiden: the intensity of rain showers will increase, droughts will occur and higher temperatures will occur more often and for prolonged periods. Heat becomes an enlarged problem in city centres. To make it even more complicated, the city itself is enhancing these high temperatures. Due to the large amount of concrete and stone cities are made of, they expose the Urban Heat Island Effect: the temperature in the city is often higher than the temperature of the surrounding area. For example, in Rotterdam, the difference in temperature can rise to 8 °C.  (source:

Urban Heat Island Profile. Source image:


Green and blue

Luckily, part of the solution to cope with the effects of climate change is already present and ready to be used: green and blue. Why these colours? If more water (blue) and vegetation (green) are present in a city, rain showers can be retained and infiltrate more easily, so the amount of rainwater that has to be processed by the sewage system is reduced, which decrease the risk of flooding. In addition, green and blue both cool their surroundings. This cooling effect is much needed and wanted in periods of heat in the city.

However, another part of the problem is that cities are densely populated, so little or no space is available for more water or vegetation. So, we need more blue and green but there is little space available. How do we still implement it?

Smart use of space and combining of functions is necessary when we are talking about big, densely populated cities. One of the great examples of combining social and water retaining functions is present in Rotterdam: ‘water plaza Benthemplein’. To see a video about the water plaza, press the following link: For more information about the water plaza in Rotterdam, visit:

Water plaza Benthemplein in Rotterdam. Source image:

Public squares in Leiden can also be converted into these kind of water plazas. These squares will not only contribute to a climate adapted Leiden, but will also improve the ‘face’ of the city and social connections between its citizens.

One of the easiest solutions to prevent flooding and heat is replacing grey by green: stimulating citizens to replace stone in their gardens by plants. Leiden already is participating in ‘Operatie Steenbreek’, which is an organization with exactly this goal. Another smart solution is using already existing buildings, especially their roofs. Roofs can be converted to so-called ‘blue roofs’, which can be used for rainwater retention. An additional benefit of water roof is its cooling effect in the summer.

Intensive green roof. Source image:

The other option is a green roof: a roof covered with vegetation. Roofs can be covered with various kind of vegetation: flowers, moss or even trees. Apart from the retaining of rainwater, green roofs offer more benefits: they can increase biodiversity, cool the surrounding area and cool the house itself. For Leiden, green roofs have an additional benefit: seagulls don’t like green roofs to breed or rest on!

For Leiden, many options are available to become a climate-proof city. It will be challenge because of its densely populated, stone covered city centre. However, a lot of possible solutions are available. Green roofs and watersquares are just a few of them. If you are interested in implementing blue and green in cities, visit:

If me and my neighbor both convert our roof to a water roof or a green roof, we will hopefully prevent other leakages, reduce the risk of flooding of our street, cool our surrounding areas and can enjoy a cool house during hot summer days. We can make our cities climate-proof and livable. Let’s turn grey into blue and green!






4 thoughts on “How to: from grey to green and blue”

  1. So many great ideas! What do you think is a reason why the municipality hasn’t done green or water roofs on houses? Lack of knowledge? Costs? Feasibility? It seems like a relatively easy implementation. How high of an impact could this be to creating climate-adaptive houses?


    1. @erinshattuck Yeah, green roofs are fairly easy. The municipality subsidizes 25 euros per m^2, which means that you have to pay the (on average) 30 till 45 remaining euros yourself. The municipality tries to stimulate citizens to make their roof green, using this subsidy. Costs are not that high.

      Green roofs offer their owners various benefits: the lifespan of your roof can be doubled, due to the protection the vegetation offers. Furthermore, your house remains cooler in the summer (this benefit will only get more convenient with the increased predicted frequency of heat waves), and seagulls don’t like the roof anymore, which is very nice if you live in Leiden! If you are also thinking about solar panels on your roof: a green roof and solar panels are a great combination! Solar panels generate more energy on a green roof.

      Green and blue roofs both offer great opportunities in relation to climate adaptation: they cool the building itself and its surroundings, so overall the surrounding area will be cooler in summer, which is great for everyone! Furthermore, they reduce the risks of flooding, because of the water retention, which is also beneficial for the surrounding area.

      A great start would be to make the roofs of public buildings like the municipality building, LUMC and ambulance post etc. green, to show citizens what it looks like and to be a guidance for your city. In Rotterdam, a green roof was made on the roof of a café: (sadly, only in Dutch) The café has their terrace on this green roof. Every first Sunday of the month (for half a year), information days took place: citizens could ask anything about green roofs and could get help for requesting the subsidy and acquiring the materials. On 13-02-2015, Rotterdam counted 20.000 m2 green roofs! This is a great example for the municipality of Leiden, and how they can inform and stimulate people.


      1. Thanks for all the extra info! That’s so cool that Rotterdam is already doing great with this. It seems like a great option!!


  2. I believe creating greener cities is a great way of both tackling effects of climate change and at the same time improving the quality of life in cities. Also, more and more cities have incentives to make their centres car-free and if this were to be successful, streets and parking lots could be replaced by gardens and green (or blue) areas. Particularly because cities are expanding and more and more people will live in cities, making them greener and more connected to nature will become central in future city planning.

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