维也纳第三区凯格尔街34—38号是这座城市的一处建筑奇观。佛登斯列·汉德瓦萨（Friedensreich Hundertwasser）在20世纪80年代建造的这座大楼有着不同寻常的色彩和形状，具有生态意识的几代建筑师都在这里找到了灵感。这座特立独行的建筑紧邻一间玻璃屋，整栋大楼就是一片绿洲，生长在屋顶和阳台上的各种树木和灌木不少于250株。
“百水公寓”（Hundertwasserhaus）或许是最具代表性的维也纳屋顶花园建筑，但它绝不是唯一的。近几十年来，这座城市的空中花园越来越多。市政府在2011年开展了一项调查，确定适合建造空中花园的屋顶空间约有1070公顷，其中将近四分之一在调查之时已经栽种了植物。相关项目层出不穷。前不久，一家瑞典家具店在位于市中心的分店开设了一个公共屋顶花园。与此同时，一家高档商场的新楼即将竣工，屋顶建有一座900平方米的花园。
© 保罗·鲍尔（Paul Bauer） 20 世纪 80 年代，佛登斯列·汉德瓦萨在维也纳建造了这座“百水公寓”，屋顶上生长的各种树木和灌木不少于 250 株。
打造这些悬浮空间不是转瞬即逝的心血来潮，它们其实可以满足实际需求。绿色空间的裨益众所周知，并且有据可查；在夏季，绿色空间可以形成凉爽的绿岛，还有助于减少二氧化碳排放。但大城市往往没有足够的空间来建造新的公园和花园。为容纳不断增长的人口，同时避免占用更多空地，城市建筑不得不越来越密集。
“打造绿色屋顶不是一时的心血来潮，它们可以满足实际需求”
现代建筑大多使用吸热和保温的材料（例如，混凝土），而且这些建筑的高度使其得不到树木的自然荫蔽。每当气温升高时，空调、交通工具和工业散发出的热量都会将城市变成名副其实的火炉。
因此，面对气候变暖问题，绿色屋顶这种替代方案越来越受到欢迎。2018年在维也纳开展的一项研究发现，假如在所有平屋顶遍植绿色植物，所有坡屋顶均覆盖反光材料，热浪侵袭市中心的天数可以减少将近30%。
“绿手指行动”协会十多年来一直鼓励人们建设城市园林，特别是在维也纳人口最稠密的玛丽亚希尔夫区，当地的一些屋顶如今已经开辟为菜园。协会创始人迈克·格拉纳（Mike Graner）坚信，从长远来看，这些举措有助于转变人们的观念，他是这样解释的：“居民自己动手种植蔬菜水果时，他们每天都要面对园艺生产的挑战和气候变化的影响。”
这些悬浮在空中的绿色表层对建筑物也是有好处的——植物有助于稳定温度，可以保护屋顶，延长其使用寿命。维也纳市政府开展的一项研究发现，绿色屋顶如能符合现行技术标准，其最终造价会低于砂砾屋顶。
昆虫栖息地
在屋顶上安装太阳能电池板之后，绿色屋顶的好处就更加明显了。供职于维也纳GrünStattGrau研究和创新实验室的土壤栽培专业博士艾琳·兹鲁瓦（Irene Zluwa）解释说：“不需要在安装太阳能电池板或是种植屋顶花园之间作出选择。将这二者结合起来，既可以得到低碳能源，又能享受到植物带来的冷却效果。”
安装了太阳能电池板的屋顶花园可以产生能源，起到蓄水池的作用，改善空气质量，还能减少噪音。如果维护得当，屋顶花园甚至可以为某些昆虫创造出一片栖息地，楼里的居民也会因此受益。艾琳·兹鲁瓦说：“在屋顶上安装太阳能电池板，可以保护花园免受阳光和风雨的侵袭。环境会更加怡人，人们在一年当中可以欣赏花木的时间也将延长。”
“安装了太阳能电池板的屋顶花园可以产生能源，改善空气质量，还能减少噪声”
不过，要大范围建造这种空中花园，会遇到许多障碍。个人可以随意使用自家房屋的屋顶，但要使用属于第三方的建筑物的屋顶区域，情况就会比较复杂。迈克·格拉纳的协会在维也纳市所属的一处停车场安装了屋顶花园，他感叹道：“市政府对我们的这次行动表示欢迎。但私人业主对待这类项目则谨慎得多，因为这些项目在技术上很复杂，利润也不高。”艾琳·兹鲁瓦也证实说：“大多数房地产开发商更愿意在顶层修建住房，这样比较有利可图。”
尽管存在以上种种障碍，屋顶花园还是在世界各地逐渐普及开来。逐一列出最新项目的“绿色屋顶”等网站*就证明了这一点。艾琳·兹鲁瓦看好屋顶花园的发展前景，她说：“项目数目正在不断增加，这让分享经验变得更加容易，也鼓励其他人勇敢尝试。”
拉伊现在已是一名资深培训师，每月可以挣到30000卢比（合105 美元），过上了体面的生活。他在接受培训后建造的第一栋房子就是他自己的住房，他原来的家已经被洪水冲毁了。拉伊已经建造了将近60栋房屋，他坚信这些房子即便遭遇最强烈的狂风暴雨也能安然如山。
*“绿色屋顶”：https://www.greenroofs.com/projects/
Vienna’s rooftop gardens
In the Austrian capital, green roofs have been flourishing on the tops of buildings for years, acting as islands of coolness on hot days and helping to reduce carbon emissions.
Barbara Hallmann
Journalist in Havelberg, Germany
At 34-38 Kegelgasse, in Vienna’s 3rd district, stands one of the city’s architectural curiosities. Built in the 1980s by Friedensreich Hundertwasser and adorned with odd, coloured shapes, this building has become a source of inspiration for generations of ecologically-minded architects. Standing right next to a glass building, this quirky house is an oasis of green, with no fewer than 250 trees and shrubs growing on its roof and balconies.
Hundertwasserhaus may be the most emblematic of Vienna’s roof-garden buildings, but it is by no means the only one. Hanging gardens have been on the increase here for several decades now. A 2011 survey by the city authorities identified almost 1,070 hectares of roof space suitable for a garden. At the time, almost one in four of these spaces had already been planted, and projects continue to flourish. A Swedish furniture shop recently opened a public roof garden at its city centre branch. Meanwhile, a luxury department store is soon to inaugurate a new building with a 900-square-metre garden on its roof.
Hanging vegetable garden
These suspended spaces are more than just a passing fad – they also meet a genuine need. The benefits of green spaces are well known and documented, providing important islands of coolness in summer while helping to reduce carbon dioxide emissions. But in major cities there is not enough space to create new parks and gardens. In order to absorb a growing population and avoid covering over any more open surfaces, cities are effectively forced to become more dense.
“More than just a fad, green roofs meet a genuine need”
Today’s buildings are almost always made of materials that absorb and retain heat, such as concrete, while their height means that they cannot benefit from the natural shade of trees. The heat given off by air conditioners, traffic, and industry all help to turn cities into veritable furnaces when temperatures peak.
That’s why roofs are becoming increasingly popular in the face of climate warming. A study carried out in Vienna in 2018 showed that if all flat roofs were planted and sloping roofs covered with reflective materials, the number of heatwave days in the city centre could be reduced by almost 30 per cent.
The Operation Grüner Daumen association has been working for over ten years to encourage urban gardening, particularly in the Mariahilf district, one of the most densely populated in the capital, where some rooftops now have vegetable gardens. Mike Graner, its founder, is convinced that, in the long term, these initiatives can help to change attitudes. “When residents grow their own fruit and vegetables, they are confronted daily with the challenges of horticultural production and the effects of climate change,” he explains.
These suspended green surfaces also have benefits for the buildings – the plants help to stabilize temperatures, thus protecting the roof and extending its lifespan. A study carried out by the city of Vienna authorities shows that a green roof, if it meets current technical standards, is ultimately less expensive than a roof covered in gravel.
Habitat for insects
These benefits are even greater when solar panels are installed on the roofs of buildings. “It’s not a matter of whether to install solar panels or plant a roof garden. By combining the two, you produce low-carbon energy and benefit from the cooling effects of the plants,” explains Irene Zluwa, a member of staff at the GrünStattGrau Research and Innovation laboratory in Vienna, with a PhD in soil cultivation.
The roof garden generates energy, acts as a water reservoir, improves air quality and reduces noise. Properly maintained, it can even create a habitat for certain species of insect. The building’s residents also benefit. “A solar-panelled roof protects the garden from the sun, wind and rain. It’s more pleasant and the garden can be enjoyed for longer in the year,” says Irene Zluwa.
“The solar roof-garden produces energy, improves air quality and reduces noise”
But there are a number of obstacles to the widespread use of these hanging gardens. While private individuals can do as they please with the roof of their house, the situation is more complex when it comes to using an area on top of a building belonging to a third party. Mike Graner’s association has installed its garden on the roof of a car park owned by the city of Vienna. “In this case, the council welcomed the initiative. But private landlords are much more cautious about projects of this kind, which are technically complex and not very lucrative,” he laments. “Most property developers prefer to reserve the top floor for housing, to make it more profitable,” confirms Irene Zluwa.
Despite these obstacles, roof gardens are gaining ground around the world. Sites such as Greenroofs*, which list new projects, bear witness to this. Irene Zluwa remains optimistic about their development. “The number of projects is increasing, which makes it easier to share experiences and encourages others to take the plunge.”
*Greenroofs Project: https://www.greenroofs.com/projects/
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