Conceptual hydrological models are practical tools for estimating the performance of green roofs, with respect to stormwater management. Such models require calibration to obtain parameter values, which limits their use in cases when measured data are not available. One approach that has been thought to be useful is to transfer parameters from a gauged roof calibrated locally (i.e., single-site calibration) to a similar ungauged roof located in a different location. This study tested this approach by transferring calibrated parameters of a conceptual hydrological model between sixteen extensive green roofs located in four Norwegian cities with different climatic conditions. The approach was compared with a multi-site calibration scheme that explores trade-offs of model performances between the different sites. The results showed that single site calibration could yield optimal parameters for one site and perform poorly in other sites. In contrast, obtaining a common parameter set that yields satisfactory results (Kling Gupta Efficiency >0.5) for different sites, and roof properties could be achieved by multi-site calibration. The practical implications of multi-site calibration have been discussed in the context of stormwater management. The multi-site calibration scheme is recommended not only for transferability amongst roofs in different sites but also when applying conceptual models for evaluating climate change scenarios in which the climatic variables are significantly different from the ones used for calibration.