The application on existing Reinforced Concrete (RC) slabs of cast-on site Ultra-High Performance Fiber Reinforced cement-based Composite UHPFRC) layers is a very efficient reinforcement technique, currently spreading. For the design of these tensile reinforcement UHPFRC layers with thicknesses between 25 and 50 mm, a representative in-plane tensile response of the UHPFRC has to be determined considering the effects of fiber orientation. To do so, theoretical analysis tools are derived to (1) analyze the relation between fiber orientations in perpendicular directions and (2) determine the average effect of fiber orientation on fiber efficiency at pull-out. A comprehensive material testing campaign of a strainhardening UHPFRC is then carried out on specimens with various thicknesses and casting processes and a meso-mechanical model is calibrated on the results. The results of this campaign as well as results taken from the literature are used to validate the theoretical developments. Finally, the most likely fiber orientation in a layer of UHPFRC for the casting method considered herein is estimated based on the results of the theoretical developments and the material characterization and a representative tensile response is estimated using the calibrated meso-mechanical model.