Most real-world materials are composed of multiple layers , whose physical properties impact the appearance of objects. The accurate reproduction of layered material properties is thus an important part of physically-based rendering applications. Since no exact analytical model exists for arbitrary configurations of layer stacks, available models make a number of approximations. In this technical report, we propose to evaluate these approximations with a numerical approach: we simulate BRDFs and BTDFs for layered materials in order to compare existing models against a common reference. More specifically, we consider 60 layered material configurations organized in three categories: plastics, metals and transparent slabs. Our results show that: (1) no single model systematically outperforms the others on all categories; and (2) significant discrepancies remain between simulated and modeled materials. We analyse the reasons for these discrepancies and introduce immediate corrections that improve models accuracy with little effort. Finally, we provide a few challenging cases for future layered material models.