Optimizing Green Roof Substrates for Coastal Resilience: Thermal Performance and Energy Implications under Extreme Weather in Auckland

• 410-day Auckland (New Zealand) field monitoring benchmarks extensive green roofs in maritime climate. • Dual baselines reduce overestimation when comparing green roofs to realistic roofs. • Mean cooling is 2.78-3.62°C vs asphalt; drought-day peak reaches 19.65°C. • Solar dominates cooling ( r = 0.61-0.72); wind/rain shrink roof-to-roof differences. • Soil-based systems cut 50-year life-cycle cost 27-28% vs asphalt. Extensive green roofs are widely advocated for peak-heat mitigation, yet evidence from temperate maritime climates under extreme weather remains limited. We report a 410‑day, instrumented rooftop experiment in Auckland that compared three extensive systems, Daltons Living Roof Mix (DLR), Daltons PineGro Mix (DPG), and Eco-pillows (ECP), against two controls, an asphalt membrane (TAS) and a stone‑ballast roof with bark mulch (SBB). A dual baseline and a radiation‑weighted degree‑hour (RwDH) framework were used to isolate solar‑driven benefits from nocturnal insulation; RwDH integrates real‑time solar irradiance as a weighting factor into degree‑hour calculations, yielding a site-tested metric for radiatively driven cooling. Under clear, high insolation, green roofs strongly reduced daytime surface temperatures relative to TAS; on drought‑representative days, cooling effect reached 19.65 °C for DLR. Over the full record, the three green roofs sustained whole‑day mean surface‑temperature reductions of 2.78–3.62 °C relative to TAS, with smaller but positive advantages over SBB. Over a 50‑year service life, DLR and DPG reduced total life‑cycle cost by approximately 28 % and 27% relative to asphalt, while ECP achieved a modest 2.7 % saving due to its lightweight modular design. Wind and rainfall episodes collapsed inter‑roof differences, whereas radiative forcing dominated diurnal cooling and ambient humidity modulated latent heat fluxes. Systems with greater moisture retention and effective heat capacity, DLR ≥ DPG > ECP, delivered the most robust daytime moderation while providing modest nocturnal heat conservation. The findings provide measurement-based evidence for optimizing green-roof substrates and for benchmarking against reflective or ballasted alternatives within this maritime rooftop setting.