It's a bold vision for Australia's future. The grey roofs of city buildings transformed into a green canopy. City councils — including Sydney and Melbourne — have been nurturing the green roof concept for about a decade. But progress has been slow. Researchers have now published new data further measuring the benefits of these gardens in the sky in the hope that more developers and property owners will take up the idea. The University of Melbourne's Dr Claire Farrell said her team found a 10-centimetre-deep substrate could absorb up to 95 per cent of annual rainfall in Melbourne. "They act like a sponge," she said. "They keep that rainfall from going into creeks and rivers where they cause a lot of damage and carry pollutants." She said Melbourne was helped by its "four seasons in one day" because light showers gave roof gardens the best chance of soaking up moisture. The benefits of capturing run-off also extend to flood prevention. "As cities become more dense, we're getting more and more rooftops and more impervious surfaces," Dr Farrell said. "[It] can lead to flash flooding when we have big events in a short period of time." Green roofs for our cities are a 'no brainer', but not much progress has been made Ya, these people are putting them on roofs. I can't manage to keep the ones on my porch alive.