A genealogy of the roof garden — from idea to technique, and on to the department-store roof

The history of a technology is usually told as a single river: someone invents, someone improves, and in time it becomes the standard. The history of the green roof does not run that way. Putting soil on a roof was begun at least three separate times, out of wholly different motives, by people who knew nothing of one another. One began for fire safety, one raised it as an idea, and one did it to draw customers. For a long while the three did not meet; each grew up in its own country. The load calculations and root-barrier standards examined in yesterday's second instalment descend from only one of these three lines — the postwar German one.

Why trace the lineage? Because what reaches anyone setting out to start a rooftop farm today is weighted heavily towards just one strand. Say rooftop greening in Japan and the conversation becomes one about environmental performance, about standards for load and waterproofing. That is the German line. Yet Japan's roofs carry an entirely separate history of their own, more than a century long — a history of amusement parks, of zoos, of beer gardens. The two are, as things stand, barely connected at all. When we speak of rooftop farming as a new undertaking, we are forgetting our own rooftop culture wholesale.

This article arranges the three lineages not by date but by motive. First, nineteenth-century Germany, setting out from fire safety and durability. Then the modern architecture of the 1920s, which raised the roof garden as an idea. Then early-twentieth-century Japan, which invented the roof as commerce. Against that, we set the process by which the three were bound into one technical system in postwar Germany alongside the process by which Japan's roofs closed for quite different reasons. Finally we watch those supposedly closed Japanese roofs open again, in another form, in the 2000s.

Carl Rabitz — born 22 December 1823 in Halle, died 10 April 1891 in Berlin — is usually known in architectural history as the inventor of the Rabitz system: wire mesh stretched beneath timber beams and rendered with plaster to make a fire-resistant ceiling, patented in 1864. His preoccupation throughout was fire. The nineteenth-century city was an accumulation of timber and combustibles, and the roof was the surface most exposed to flying sparks. Rabitz proposed covering that roof with earth and plants. In 1867 he published a pamphlet titled Naturdächer von vulkanischem Cement — Natural Roofs of Volcanic Cement — and in the same year exhibited a greened flat roof at the Paris World Exposition.

The order of the motives is worth noting. For Rabitz the greenery was not the end but the means. A layer of earth and plants protects the roof from sparks, shields the waterproofing from ultraviolet light and temperature swings, and extends the roof's life. Landscape, wildlife and cooling were, if present at all, secondary arguments. Flat-roof greening spread in nineteenth-century Germany for this practical reason rather than out of environmental awareness. The tar and cement layers then used for waterproofing were vulnerable to sun and thermal movement, and laying gravel or soil over them to protect them was, by the standards of the day, an entirely reasonable construction method. Rooftop greening began not as greening but as roof protection.

That starting point still shadows roof design today. As we saw yesterday, the most urgent layers in a green-roof section are the waterproofing and the root barrier, and the plants are merely the last sheet laid on top. It is not that the greenery is primary and the building secondary; the greenery sits on a system built to protect the building. Rabitz's ordering has not been reversed in more than 150 years. The reason any rooftop-farm proposal has to begin from the building's requirements comes back, in the end, to precisely this.