An excel-based tool to support decisions on the selection of outflow devices for blue roofs using historical maximum rain events.

Distributed infrastructures play a key role in urban stormwater management by reducing flood risks. Over the past decade, green and blue roofs have emerged as effective distributed solutions, especially as roofs cover a large share of urban land and climate change intensifies storm events that centralized systems often struggle to manage. Designing these infrastructures poses challenges, particularly in selecting an appropriate design hyetograph based on rainfall duration and return period. Simulating water storage and release dynamics enables the optimal selection of outflow devices, ensuring compliance with maximum water levels and flow rates to prevent flooding and structural issues. To support this process, an Excel-based tool has been developed to simulate and select outflow devices for multiple blue roofs contributing to decentralized stormwater systems. The tool identifies which outflow devices meet performance requirements for different rainfall durations. A design case study in Norway demonstrated its application, illustrating how construction sector operators can use it to improve design practices and customer communication in the Norwegian context. Future advancements might consider different add-ons,including (a) green roofs and rain harvesting systems models, (b) datasets from different nations, (c) multiple hyetograph shapes, (d) different shapes and outflow devices curves, (e) future climatic scenarios.