UML class diagrams are widely used for conceptual modelling but provide only limited built-in reasoning support, whereas OWL offers well-defined logical inference but is not typically represented in a diagrammatic, UML-like style. We propose endogenous OWL overlays as a lightweight bridge between the two. An overlay is a rule-derived OWL/RDFS-aligned view computed on top of a triple-based representation of a UML-style model. Since the overlay is recomputed on demand and can be consumed by standard OWL tools, the UML-style model remains authoritative. A key feature is that overlays do not merely encode UML meta-elements, but also capture the intended semantics of common modelling patterns. Pattern declarations such as subsetted role ends, role classes, and powertype/partition intent are represented as facts and compiled by a small, domain-independent rule kernel into OWL assertions (e.g., subproperty axioms, domain/range constraints, property chains, and disjointness constraints). This enables OWL-style services such as type propagation, subproperty closure, derived associations, and the detection of logically unsatisfiable classes to be obtained directly from familiar UML idioms, without hand-coding OCL or maintaining a separately synchronised OWL artefact. For closure-based tasks, the resulting overlay supports OWL 2 RL-style materialisation; when satisfiability-style diagnostics are required, the same overlay can be analysed with a DL-capable reasoner while preserving traceability to the original model.