Root cause analysis (RCA) in DevOps systems is difficult because user-visible failures are often downstream symptoms of longer workflows. The actual fault may occur during event generation, forwarding, permission checking, scheduling, artifact synchronization, or event consumption. Methods based on global log analysis, anomaly detection, or unconstrained semantic reasoning therefore miss workflow-stage constraints and explicit evidence checks. This paper presents WERCA, a DevOps RCA framework guided by an expert-maintained Workflow/DAG repository. Given a failure ticket, WERCA selects a likely execution path, retrieves logs only from nodes on that path, and checks each stage against its responsibility. A candidate root cause is retained only when supported by log evidence, workflow position, and stage semantics. When later evidence contradicts the current path, WERCA rolls back and revises the diagnosis. Historical incidents provide retrieval-based priors for ranking, confidence estimation, and remediation generation. Our experimental results on real cases from DevOps systems show that WERCA can improve the ranking results compared with the baseline using only logs, despite a slight increase in time overhead. The results also indicate that Workflow/DAG can guide the RCA process of DevOps systems by tracing concrete execution paths for failure as evidence reasoning.