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The main design determinant for small catchment hydraulic structures is rainfall intensity. Localised relationships between rainfall intensity, frequency and duration (IFD) are used to design each structure to a specified level of performance. However, limitations in rainfall observations introduce uncertainty in IFD values. Further, this uncertainty is compounded by changes in climatic conditions via anthropogenic forcing. Whether this is a cause for concern depends on the structure's sensitivity to deviations away from design rainfall values. Here, we investigate the ability of roof drainage systems to accommodate deviations in design rainfall. We assess the sensitivity of box gutter overflow designs across Australia to spatial variability in IFD values, and projections of IFD values due to climatic change. Different overflow designs were found to have markedly variable responses to rainfall intensity increases, from 13% to 406% before failure. Potential increases in rainfall intensity from spatial variability uncertainty varied from 2 to 54%. While rainfall projections for the 2090 decade ranged from -15 to + 59%. Rainfall intensity increases as high as 259% were noted when both sources of uncertainty were combined for a temperature rise scenario of 5 degrees C. At the majority of locations coupled increases in rainfall intensity were primarily driven by existing rather than future uncertainties for a 2 degrees C temperature rise scenario. Considering design rainfall uncertainties in terms of design sensitivity shows that adapting to present and future uncertainties can come at no additional cost for some design options while other options need to be altered to reduce the risk of failure.