Construction and oil projects, due to their technical complexity and high-risk nature, represent one of the most challenging contexts for applying the general theory of civil liability, as the occurrence of damage does not necessarily imply blameworthiness or normative attribution. The core issue in technical disputes is not the principle of liability itself, but the identification of the relevant custom and the manner in which specialized standards operate within the normative structure of liability. Exclusive reliance on general custom may lead to misidentification of fault and a shift toward a quasi-strict liability model. Adopting a descriptive-analytical approach grounded in Imami jurisprudence, legal doctrine, and judicial practice, this study examines the position of “engineering custom” as a specialized professional norm governing technical conduct. The findings demonstrate that, provided it satisfies the requirements of Sharia legitimacy, technical rationality, and professional acceptance, engineering custom operates at three structural levels: first, as the benchmark of the “reasonable specialist in similar circumstances” for establishing or negating professional fault; second, as a mechanism for limiting the normative attribution of harm in cases involving multiple causes and inherent project risks; and third, as a criterion for assessing justificatory grounds, including benevolent intervention, necessity, and adequate warning. Accordingly, the legal effect of engineering custom manifests in the negation of fault, limitation of attribution, or recognition of a justificatory ground. The article concludes by proposing a three-stage judicial framework to enhance coherence, transparency, and predictability in technical liability adjudication.