Water and sewerage companies (WaSC) in the UK are under increasing pressures to improve customer satisfaction. The biggest cause for customers dissatisfaction in the wastewater sector is a service failure caused by a blockage. There is therefore a need to understand the factors which influence blockage processes in order to prevent them. This work demonstrates how preceding rainfall impacts the sewer system of two highly populated regions within south wales that have differing gradients. The total rainfall, number of dry days and consecutive number of dry days prior to a blockage were investigated using statistical analysis in order to determine the impact that rainfall has on blockages. The results obtained demonstrate the importance that dry weather has on blockage rates in both steep and flat catchments. Future work will incorporate predicted rainfall impact into a proactive maintenance scheduling model.