In urban areas, the classical exponential wash-off equation for modelling of stormwater suspended solids (SS) relates the washoff of solids to the overland flow rate, assuming that runoff is proportional to rainfall. However, in catchments where infiltration occurs through the pavement surface, this assumption may not be valid. Furthermore, many researchers have concluded that the effect of rain impaction, related to rain intensity, should be considered as a separate model parameter to runoff rate. To address this problem, a semi-deterministic stormwater quality model was developed which incorporated rain intensity directly in the exponential wash-off equation. The modified function was compared to the more classical wash-off equation using a dataset of 18 storm events sampled from a separate pipe and gully system in Dublin, Ireland. The modified wash-off equation showed a consistent improvement in the fit of observed SS fluxes at the outlet of the drainage system: the Nash-Sutcliffe index of modelled performance increased from 0.39 to 0.58 on average using the modified wash-off function. The form of the modified wash-off function allows for simple integration into many stormwater quality models currently in use, without the need for any additional model parameters.