AbstractSewer pipe deterioration is driven by a finite number of root causes and processes. Thus, it should be both feasible and advantageous to have a uniform classification system that can be universally applied in sewer deterioration modeling and asset management. However, a literature review of existing classification systems revealed several problems and inconsistencies, and no widely adopted system. This work proposes a uniform classification system that can be used for different purposes in the fields of gravity pipe deterioration and asset management. This paper focuses on separated sewer systems, but the proposed system can be easily adapted for combined sewer and stormwater systems. The proposed system is based on three top-level categories of failures, defects, and factors with subcategories based on functional considerations. Each category is unambiguously defined and a classification flow diagram is proposed. This work demonstrates how existing parameters can be consistently classified, discusses the interactions between different defects and failures, and illustrates the application of the proposed system to the processes causing overflow failures due to sedimentation.