Introduction of recycled plastic materials in structural applications such as bridges, retaining walls or railway sleepers requires a proper identification of necessary material properties. Given similarities in the microstructure of various structural elements we limit our attention to beams having a rectangular cross-section. Owing to the manufacturing process the cross-section is represented by a porous-core (inner section) surrounded by a homogeneous material (outer section). The influence of microstructural details on material parameters is examined here with a reference to the elastic Young’s modulus derived from nanoindentation measurements. To identify a gradual evolution of the stiffness of plastic material from the outer section into the core the grid indentation method based on the statistical evaluation of a large number of indentations was adopted. These tests were accompanied by standard static indentation measurements to address also the effect of temperature in the range of 20–40°C. When dealing with these types of recycled plastics, even a 5°C temperature variation leads to a significant change in the material stiffness. In addition, standard macroscopic material properties were measured by tensile tests of samples with and without the porous core and compared with microscopic parameters. The elastic modulus obtained from nanoindentation was found to be ~20 % higher than that provided by the tensile tests.