There is increasing interest worldwide in the decentralized treatment of gray water. Since ground-level areas are generally costly, gray-water treatment can also be carried out using helophyte mats on the roofs of buildings as an alternative. Alongside the water treatment itself, this process also has a positive effect on the indoor climate in the building. In a test carried out under realistic conditions, it was shown that a helophyte mat with a root-layer depth of 0.1 m and with hydraulic loads per unit up to 15 L m(-2)d(-1) is suitable for treating typical gray water from a residential building.