This interview is an excerpt from the Rooftop Catalogue, which was produced by MVRDV and published by Rotterdam Rooftop Days on behalf of the municipality of Rotterdam. Interview by Irene Start.

Rotterdam has 18.5 km2 of flat rooftop space. Why does the city still only use a limited number of rooftops, when there is a whole new world to be explored? Head of Urban Planning for the Municipality of Rotterdam Mattijs van Ruijven, MVRDV’s co-founder Winy Maas and Rotterdamse Dakendagen director Léon van Geest discuss how rooftop programming can make the city more liveable and inclusive. What will it take to get major players like real estate developers and housing corporations as excited about rooftop use as they are? And to what extent do laws and regulations need to be changed? "The crux of any new development is: if you don’t ask for it, you won’t get it."

Why was it important to create a Rooftop Catalogue?

Mattijs van Ruijven: Because rooftops can be beautiful. Space in the city is becoming increasingly scarce, we want more greenery and housing, and there is a lot of demand for more space. Rooftops are still not taken into account enough, even though Rotterdam has 18.5 km2 of flat rooftop space available. It’s really still unexplored territory. The Rooftop Catalogue shows how rooftop use can be an answer to current issues such as the climate challenge, the housing challenge and the energy transition.

Winy Maas: Rooftop use could make a huge contribution to the densification of the city – and it can also prevent us from building more on the outskirts of our cities. This catalogue shows the dynamics: you can start and your neighbour will follow. It would be great if that collection could be expanded into the third dimension. So that means not only adding a layer of greenery on rooftops, but also turrets, an elderly neighbourhood or a skate park.

Léon van Geest: We do see that some individuals are already getting started with their rooftops, but this needs to become a bigger and more structural movement. With this catalogue, we want to inspire not only individuals but also the big players such as developers and housing corporations to invest in rooftops.

Maas: A cemetery may sound like a bit of a joke, but at this moment MVRDV in Monaco is actually creating a cemetery on top of an office due to lack of space on the ground floor. That one will definitely be ‘heavenly’. If you want to get serious about rooftops, you will also have to think about a new kind of urban development, as you do want to keep decent sight lines on bridges and historic buildings, for example. I find it exciting to take on such a composition. How are we going to put that together in the future?

Van Ruijven: Indeed, that’s very important. A build-up of individual types of rooftop use is possible, but it is important to ask what you will do where. How can different initiatives contribute to an interesting rooftop landscape?

Van Geest: The misconception is that the highest rooftop is always the most beautiful or the best option. But, in fact, a rooftop on the fourth or fifth floor is even better, as that would allow you to still feel a connection with what’s happening in the city below so you’re not completely disconnected.

A practical question for those who don’t deal with rooftops on a daily basis: what is and what isn’t possible on a rooftop?

Van Geest: At the moment, rooftops are usually built to be able to take the weight of a layer of snow, and of someone doing work on them. What we propose in the Rooftop Catalogue goes far beyond that. It would be able to hold multiple people, it would have greenery on it, it would have solar panels on it, it would be able to hold water. That asks a lot more of a rooftop. New buildings will need to be built more sturdily and existing buildings will need to be reinforced.

Is that the main reason why the available 18.5 km2 of flat rooftop space in Rotterdam is not being used yet?

Maas: Cost is an important consideration: people are always looking for the lowest price. If you want to turn that around, the first thing you have to do is agree with each other to ‘lock’ certain outskirts and find space within the city itself. Then the interest in rooftops will follow naturally.

Van Ruijven: The municipality wants nothing more than to use the rooftops, for example for water storage, as that would relieve the sewer systems. But residents are not going to solve the municipality’s water problem on their own, which is why grants are now available. And it’s not just about the costs either – the legal side must be taken care of as well. For existing buildings, all members of the Owners Association must agree, for example. An individual can change their own home, but it’s more difficult for large buildings with multiple owners.

Van Geest: The idea that a rooftop can be a public space or a place to stay has not yet fully registered with people. We also notice that when we are organising events on rooftops during the Rotterdamse Dakendagen.

Van Geest: Certainly! There is a lot of experimentation going on right now: we are located on the roof of De Heuvel near St. Lawrence Church, as part of the LIFE@Urban Roofs project and the European LIFE programme. It has water storage, greenery, solar panels and residential space. Then there’s the DakAkker. It is relatively small in terms of surface area, but it is considered a good example worldwide. Erasmus Hospital has an orchard with 38 fruit trees on the eighth floor. And the DakPark is amazing just because it’s so normal. It’s located nine metres off the ground and people don’t even realize they’re walking on a rooftop. We also have a rooftop forest that’s located 34 metres off the ground at Depot Boijmans Van Beuningen, designed by MVRDV.

The rooftop forest is very heavy. How did MVRDV go about building it?

Maas: First of all, we made sure we had a strong structure. Other than that, you need a different kind of greenery than you would plant on the ground level: the earth dries out faster up there, the temperatures are higher and whatever you plant up there must be more wind-resistant than down at street level. The 75 birches that now make up the rooftop forest were specially grown to be able to live at a height of 34 metres. When they were being transported, their roots were gathered up the same way people gather up their hair. When we placed them in the soil, one root was placed over the other, which is how we managed to create a solid root system. We added 20 pine trees to prevent diseases and to create a nice winter look, so you get a kind of Veluwe vibe in the heart of Rotterdam. We’re going to expand on that experiment even further, with different species of grass and inter-vegetation that people can sit and lie on.

Van Geest: In Amsterdam, there have also been experiments with a dune landscape on a rooftop.

Maas: I am in favour of testing and interweaving as many different biotopes as possible – that’s how you create an actual National Park in Rotterdam!

Van Ruijven: That’s definitely a great goal. By the way, there is more nature in the city than we think: oystercatchers are fleeing from the monotonous polders to nest in the city on rooftops with pebbles. So are we then going to make that rooftop green? We also need to think about biodiversity.

We are now talking about new buildings, but the catalogue primarily addresses how to use rooftops of existing buildings. Can you explain to what extent that would improve the quality of life in the city?

Maas: If you were to make all of Rotterdam’s rooftops green – not just with small plants, but with substantial greenery – that would contribute to lowering the temperature by 1 degree Celsius. That definitely means something. If we were to use the entire empty flat rooftop space that Rotterdam now has, we could accommodate greenery nine times the size of Kralingen Forest.

Van Ruijven: It is much warmer in the city centre than along the edges of the city. If we can reduce heat exhaustion, it would be a lot more pleasant for everyone in the city. We could also prevent health problems with that, especially for the elderly. On top of that, water storage is also extremely important: if water could be stored on rooftops and gradually carried to the sewers, the streets would no longer be flooded after every heavy rainfall.

Van Geest: Greenery expansion and water storage are often seen as two separate things, but they can also be combined: if you collect water on a rooftop, it would be very nice to add greenery to that. If there is greenery, it would also be pleasant for people to stay there. This is one example of how to use rooftops for different purposes at the same time.

Van Ruijven: The new Environment and Planning Act offers a little more leeway – so the answer is now ‘yes, if…’. But if you then run into all kinds of other issues, for example with an Owners Association, which would then still make it impossible, that’s of course a problem.

Van Geest: Perhaps the municipalities do have more room within the new Environment and Planning Act than before?

Van Ruijven: Well, the biggest discussion for zoning plans, certainly within the province, is about function: how much retail or offices do we add?

Maas: Still?

Van Ruijven: Still. In zoning plans, we as a city have already provided more room to allow, for example, a rooftop terrace. But when it comes to increasing building heights, it’s a different story. We don’t just want to think about that carefully, but we also want to involve the community.

Maas: I think we need a new Building Code, or rather a Rooftop Code. You should be able to stack the four elements – water, greenery, energy and population – on top of each other, like a sandwich. It should be defined in some kind of regulation that rooftops should be able to support that weight. I think we should be able to convince the ministry of that.

Van Geest: That means buildings should be built stronger by default to allow for rooftop use.

Maas: Indeed, stronger structures are more sustainable.

Now back to the costs of these plans: who will pay the bill?

Van Ruijven: That’s the million-dollar question. Rooftop use should be the standard, so we should be able to say ‘let’s just do it’. The social added value of storing water or introducing greenery in this way is many times greater than any bill. But it would take a smart approach. If you’re talking about combining greenery, water and energy, then in the current situation you’re talking about various government grants.

Van Geest: I think we have to be careful to not only talk about earning back the investment. When you buy clothes or a new kitchen for your house, you don’t just convert it to ‘return on investment’ either. Not everything can be expressed in money – you can also express profit in value.

Maas: With regard to energy saving and water storage, of course, we are talking about billion-dollar items. You would have to provide those grants for a long time before someone thinks: “Hey, I’m just going to put an additional residential layer on that building without compensating for it.” I’m in favour of also making rooftop use a task for the government and not just leaving it up to a select few.

Can water storage, more greenery and more social cohesion be arguments for housing corporations and real estate developers to invest more in rooftops?

Van Geest: For housing corporations, a building’s major repairs could be a perfect moment to not only think about insulating the façade, but also to think about a better use for the rooftop. And to meet the sustainability requirements that we have all agreed to and that the municipality also sets. The costs can then simply be part of the major repairs of the property.

Maas: But there are still too little of the improvements I’d like to see in those general major repairs. They usually only involve some ecological modifications to a building. If you also want to make it suitable for a few extra homes on the roof, with a garden and a staircase on the outside, then such a corporation must also be legally allowed to apply all that. However, in recent years, they have been limited in this by the state. By creating multifunctional rooftop use like that, you are taking a next step, and I think we have to push for that.

Maas: Sometimes the image of Gotham City comes to mind, in which the upper layer is only for the wealthy who have immediate access to the sky and the sun, while the rest rots in the slums below. This is why cooperation from the housing corporations is so important. Rooftops should also become accessible to mid-range renters and social housing, but that costs money and so we need a bigger budget.

Van Ruijven: Accessibility is an important issue. Not everything has to be the same: one rooftop can be just for the owners, the other publicly accessible. The Groene Kaap in Katendrecht is a good example of this – anyone can walk up there, but at night it’s closed.

Van Geest: I think the same applies here as with nature: some high-rises are for rich people – they have paid for them and in that case I don’t think it’s a bad thing that they have a nice roof terrace. But we have to make sure we have a mixture. Rooftops are not the solution, but they are part of the solution. If the street level isn’t sufficient and the plinth courses don’t work, then we have to start fixing that before we start working on the rooftops.

Van Ruijven: Exactly. If you start lifting the public space up from the street to the rooftop and you have nothing left at the street level, it doesn’t work. You also shouldn’t do the same thing everywhere in the city – in a district like IJsselmonde, a green rooftop might be a better idea than a publicly accessible rooftop, which in turn fits better in the city centre.

So we need to change the way we think. Who could be our leaders for that shift? Architects, urban planners, the municipality or that individual enthusiast who has already started working on their rooftop?

Maas: I foresee a big role for the municipality, which can mediate and can allow and determine how many square metres can be built on. The municipality is the party that can permit something under certain conditions. We do need a larger municipal apparatus to handle this.

Van Geest: The big contractors and the institutional investors also have to cooperate, and there is a certain inertia there. I think these big institutions know what’s possible, but they still think too much in terms of existing revenue models.

Maas: I would like to call for a different way of planning things. A spreadsheet would clarify the development costs. Real estate developers usually look primarily at profit maximisation, and that can sometimes lead to crazy residential towers. I would suggest a new kind of spreadsheet thinking in which profits are set at, for example, a maximum of 8%. That way, money can be set aside for the more vulnerable components such as social housing, incubators, greenery, water, energy, public space.

Van Ruijven: The crux of any development is: if you don’t ask for it, you won’t get it. We want affordable housing, greenery – we would have to figure that out together. The challenge at this stage is too big to keep doing it the way we used to.

Van Geest: I don’t think it’s that they’re unwilling, by the way. Real estate developers also want sustainable buildings that will stand the test of time. It’s really not like the three of us want to change things and the rest don’t – no, everyone wants to, but it’s a complex process and it takes a shift in the way we think.

Maas: To realise that shift, we would first of all need good urban development plans, and we need to invest in research. The Why Factory, the think tank I founded at TU Delft ten years ago, has been working on the Green Dip for years. We calculate what is needed for each plant and climate: what kind of vegetation is there, what kind of soil does it need and how much soil, and what about the wind, what do they contribute to the water challenge, the energy challenge, the cooling challenge? Within MVRDV, we are developing an interactive tool to make the complexity of all factors visible – software we have, for now, christened the ‘Air-maker’. Our dream is to test it out in Rotterdam. The software should actually be created on a national level. We have a coronavirus app, so why not quickly put together an ‘air app’ too? Universities play a big role in this, and small contributions from agencies like ours can help us move in the right direction.

Van Ruijven: I hope the catalogue will be used as a guide and that we will inspire others with it. Rotterdam’s high-rise policy is a great example for other cities – we have a pioneering role, and, as an innovative and modern city, we owe it to ourselves to do something with our rooftops.

Van Geest: I hope that the Rooftop Catalogue will not only be seen as something by and for Rotterdam, but that it will be applied more broadly. The Rotterdamse Dakendagen team is part of a European network with cities that look at rooftops in the same way as us, and they are very interested in what’s happening in Rotterdam. We do not build rooftops ourselves, but we do share knowledge and want to inspire everyone in any position – from the mayor to a school teacher who thinks “Hey, we could do something on our rooftop.” This catalogue provides the tools for that.

Maas: It’s definitely a great start that we’re all part of here, but of course there’s still a lot to determine and discuss. How do we arrange it legally and cost-wise, how do we help the Owners Associations, how do we make a do-it-yourself kit and a catalogue for creating, how do we make rooftops accessible, how do we provide a new style of zoning plans that intertwines diversity and density on rooftops? This book is the first in a series, I would say.

Learn more about the Rooftop Catalogue and purchase the book here. Alternatively, a pdf of the rooftop catalogue can be viewed online here.