If heavy rains are predicted, a smart valve system empties the stored rainwater into the municipal storm drains and sewers in advance of the weather, allowing the roof to soak up water and reduce flooding in the city.

In Amsterdam, 45,000 square meters of flat metropolitan rooftops have already been fitted with these systems, and the contracting firms behind the technology say they make sense in dry climates like Spain just as much as wet climates like Amsterdam.