Granular substrates and their hydrological properties are of critical interest in the design and performance of green roofs. Therefore, it is necessary to model the substrate’s Hydraulic Conductivity (HC) which is directly dependent on water content retained by the substrate. Thus it is essential to model Water Retention (WR) too. Since capillary water is held within pores, how much of this water can be retained by the substrate at different suction pressures is therefore dependent upon the Pore Size Distribution (PSD). Since pores are just spaces where grains are absent, their size distribution too is related to the substrate’s Grain Size Distribution (GSD) which provides the probability of finding grains that are smaller than some diameter dg,s. While earlier studies have attempted to model PSD, WR and HC, they use simplifying mono-fractal approximations. This study overcomes this limitation by proposing a generalized multifractal-based approach. Furthermore, even though it is quite usual to incorporate pore tortuosity via some indirect parameter in the HC model, a related ink-bottle effect which although capable of affecting WR behaviour is usually ignored. Therefore, the use of a new parameter pie in the WR model is suggested here to physically represent ink-bottle effect. The proposed models are validated using measurements from four different commercial green roof substrates.