Cities are very efficient in usage of space, but surprisingly rooftops are often left unused. This study is on the use of this forgotten place. One possible use is roof gardens, as a park and/or growing food. It can become a place of meditation in the middle of the busy city and even provide for local food-production. Roof gardens provide various other benefits: they retain water to mitigate peak water overflow during heavy showers; they reduce air pollution, and; they give a cooling effect against the urban heat-island effect. A second usage of rooftops are to create additional building space. Many post-war residential towers were built from concrete and in need of a renovation. Topping up additional floors can finance the renovation. Concrete structures are durable and the continues curing of concrete strengthens the structure over time. Moreover, buildings before 1980’s were manually calculated which required higher safety factors. This could result in a redundant structural capacity to top up several additional floors. Waterarchitect inspected and calculated the redundant structural capacity of the Lijnbaanflats in Rotterdam and concluded and additional three residential floors could be added. The proposal ‘Concrete for eternity’ was selected as finalist for ‘In de Kiem gescoord’.