How can we tackle the heat in the city? Would a green roof work, or are white pebbles actually more effective? In the meantime, work is underway on an alternative: the tree roof.

The effects of climate change are felt more keenly in the city than in the countryside. Air quality is poorer there, extreme downpours are more likely to cause flooding, and on hot days the temperature in the city is on average several degrees higher than in the less densely built-up surrounding areas. More people also die from heat-related causes in the city during the summer than in the countryside.

On temperature maps, cities form ‘heat islands’. Between these islands, oases of blue and green provide cooling – trees provide shade and planted ground retains water. However, on hot days, the dark façades and streets absorb a great deal of heat, which they then release again at night.

Half a degree

Introducing more greenery into the city – ‘green adaptation’ – helps to combat the heat. This includes larger parks, extra trees, and green roofs and façades. The effect of this can be significant. For example, it has been calculated that for Dutch cities, a 10 per cent increase in green space reduces the average temperature by half a degree. At first glance, that might seem modest, but it isn’t. It is an average, and urban heat is not just a problem during the day. The real problems occur at night.

Anna Solcerova, a researcher at Amsterdam University of Applied Sciences (HvA) specialising in green roofs and urban heat, explains why. ‘During the day, it’s hot, both inside and outside the city. People seek out shade and a breeze, and that helps.’ But the night is a different story. Cities are warmer than the surrounding countryside: in extreme cases, sometimes by as much as five to six degrees locally. Cooling down at night after a hot day is important for people to recover. Lying awake because of the heat is harmful to health. It is precisely here that the average half-degree drop in temperature achieved through greening can make a meaningful difference.

Het Benno Premselahuis van de Hogeschool van Amsterdam is een van de eerste gebouwen met een blauw-groen dak. Bij de ontwikkeling daarvan zijn onder meer de gemeente Amsterdam, Waternet, Rooftop Revolution en diverse woningcorporaties betrokken. Foto: Anna Solcerova

Sedum

There are a number of reasons why more greenery provides extra cooling. Plants cool the air through evapotranspiration: water transported upwards via the roots evaporates at the leaf surface, thereby cooling the air. In addition, trees and shrubs provide shade and, to a limited extent, improve air quality by binding particulate matter, nitrogen dioxide and ozone. But which type of greenery is most effective?

Current green roofs are by no means always considered the most efficient when it comes to tackling the heat problem, says Solcerova. The vast majority of green roofs in the Netherlands consist of sedum roofs: a layer of succulents on a thin layer of growing medium. Sedum survives drought well, but absorbs very little water. ‘Green roofs evaporate water as soon as they have it. But many green roofs in the Netherlands are planted with sedum, which offers little water storage capacity. Three days after it rains, all the water has already gone,’ says Solcerova. After that, the roof no longer evaporates any water.

From the point of view of urban heat, it may be better to lay white gravel on the roof rather than green sedum

Anna SolcerovaExpert in climate-resilient cities, Amsterdam University of Applied Sciences (HvA)

White gravel roofs

One alternative to combat urban heat is a roof that is not green, but covered with white gravel. These pebbles reflect sunlight rather than absorbing it and thus have a demonstrable cooling effect on the immediate surroundings, even without evaporation. Canadian research shows that reflective surfaces in urban areas can lead to a temperature drop of up to 2.5 degrees Celsius, although the effect will be less pronounced in Dutch cities.

‘From the perspective of urban heat, it may therefore be better to lay white gravel on the roof rather than green sedum. But if you’re not just concerned with heat, but also with biodiversity or aesthetics, for example, then sedum remains a better option,’ says Solcerova.

Moreover, there are more options than just sedum for creating a green roof. Blue-green roofs appear to be the most effective: these are green roofs with a water buffer, ensuring water is available during dry spells and allowing for greater evaporation in hot weather. ‘Around 30 per cent of the city consists of unused space, and some of that is found on rooftops. These are relatively easy to adapt to help regulate the temperature in the city.’

Solcerova is not opposed to green roofs, but does wish to warn against overestimating their impact. ‘People sometimes think that green roofs can do everything, that they are a sort of miracle cure for both the heat and the biodiversity problem. But green roofs are, at best, a drop in the ocean’ – on an almost literally scorching surface.

Green roofs are part of larger and more complex solutions: more trees in public spaces, reducing hard surfaces on streets, reflective façades. Every little helps. ‘If everyone does this, if every shed, bus stop or cycle shed is green, then it certainly makes sense,’ says Solcerova, a researcher affiliated with the Amsterdam University of Applied Sciences (HvA) Climate-Resilient City research group.

Moreover, research consistently shows that nature in our immediate surroundings has a positive effect on our mental and physical health. Green spaces encourage us to get active, socialise and relax.

Swinging trees

For those who want to think bigger and cover their roof with trees rather than sedum plants, research carried out at TU Delft’s The Green Village may offer some inspiration. The Green Village site is home to all sorts of green start-ups and experimental construction projects. One of these is a row of deciduous trees, which do not stand upright but are suspended by their round root balls in frames – referred to as ‘tree springs’ by their inventor and researcher, Stef Janssen.

These ‘tree springs’ have been suspended here for more than nine years in a framework that allows them to sway in the wind, with a root ball measuring eighty centimetres in diameter. The idea for this arose from a specific problem: planting trees on a roof is useful and desirable, but trees with their root balls are extremely heavy, weighing around 1,600 kilograms. By lifting the root ball off the ground, the tree weighs just 350 kilograms.

Uitvinder Stef Janssen bij een aantal van zijn boomveren in The Green Village van de TU Delft. Foto: De Ingenieur

Living windmills

Another problem is that a tree suffocates itself when planted in a container. Trees continue to produce roots until there is no space left. In the tree spring, this process is reversed. The roots grow within a suspended root ball. As soon as they encounter air and light, root growth stops naturally.

As a result, the tree channels its energy into the crown: just as it does in nature. The root ball within the frame ensures that the tree can move freely in the wind, which prevents it from being pulled off the roof by a gust of wind or large branches breaking off and falling to the ground. An additional benefit is that much more oxygen reaches the root ball, which improves soil life and allows for greater water storage.

As well as the structure that allows the tree to move freely, the root ball is packed with other features, such as sensors that measure movement, moisture and evapotranspiration. In the future, Janssen hopes to apply a technique inspired by termites. Termites use tunnels and air currents to regulate the temperature of their nest and maintain humidity levels. Janssen wants to apply this principle by creating a double layer around the root ball, with a system layer in between that can retain mist. This keeps the microclimate around the roots stable, even during dry periods. ‘The root ball using the termite technique also has a cooling effect,’ says Janssen.

Janssen is also conducting research to investigate whether the trees in the structure can generate energy, acting as living ‘windmills’. When the wind blows through the tree’s crown, the tree sways back and forth, much like a pendulum. That movement can be converted into electricity. In this way, the trees on the roof would continue to water themselves and the other plants using minimal energy. ‘We used to think we were above nature. But we are part of it,’ says Janssen. ‘In The Green Village, we are demonstrating that nature and technology can go hand in hand.’

The difference in surface temperature between a paved garden and a garden with vegetation can be as much as 10 degrees Celsius.

Flipping tiles

There are also practical steps that can be taken at an individual level. Solcerova offers some practical advice: effective external sun protection is the most effective way to keep the house cool, as it blocks the sun before the sunlight hits the glass. Night-time ventilation – that is, opening windows as soon as the outside temperature drops below the inside temperature – helps to cool the house down.

And if you have a front or back garden, it’s best to replace the paving with plants. ‘Every paving stone that’s removed makes a difference,’ says Solcerova. The difference in surface temperature between a paved and a planted garden can reach up to 10 degrees Celsius on hot summer days.

The message from both Solcerova and Janssen is the same: green roofs work, but only if they are implemented seriously. ‘Allocate space on the roof of new buildings for greenery,’ says Solcerova. ‘It should become the norm to incorporate greenery into urban planning; it always adds value.’ Janssen hopes that his tree spring will eventually become more affordable so that it can be widely used on every roof and in every street. ‘Every green roof, every tree, every paving stone that makes way for plants: they are drops. But enough drops together fill an ocean.’

Photo above: Anna Solcerova