Fouling layers developing on membrane filters can display significant spatial heterogeneity (roughness), yet the effect of fouling layer spatial distribution on hydraulic resistance has not been studied systematically. We used simple flux models, based on Darcy's law, to assess the impact of spatial distribution on resistance and permeate flux. Our results show that, for a given mean fouling layer thickness, greater spatial heterogeneity could increase fluxes by over an order of magnitude. The degree of error depended on the membrane resistance, fouling layer permeability, and the characteristics of the spatial distribution. We developed 1-dimensional and 2-dimensional flow models, both considering spatial heterogeneity, to improve the accuracy of the conventional model. The proposed models were validated with experimental data from a gravity driven membrane filter with a fouling layer having a high degree of spatial heterogeneity. Accounting for spatial heterogeneity significantly improved the prediction of permeate flux. (C) 2014 Elsevier B.V. All rights reserved