The turbulent wake behind a truck is responsible for a considerable proportion of the total aerodynamic drag. There is evidence to suggest that the underbody flow affects the wake topology, although this interaction is not well understood. This study investigates the impact of underbody roughness, which simulates the standard blockage caused by typical underbody components, on the wake structure. This work uses a simple 1/10th scale model with the geometry and aspect ratio of a generic HGV. A roughness pattern is attached to the underbody of the model and compared to a smooth configuration. Testing is conducted in a water towing tank, which establishes correct ground conditions. The facility operates at Reynolds Numbers of approximately Re = 690000. Optical access into the underbody and the near wake is possible through the clear working section of the facility. Stereoscopic Particle Image Velocimetry is used to analyse the flowfield and a load cell is employed to measure drag differences between the configurations tested. The results are compared with those of a full, 1/10th scale European truck model with a detailed and fully-reconfigurable chassis and rotating wheels. It is found that underbody roughness considerably affects the wake topology and leads to a greater interaction between the low momentum flow in the wake and the higher momentum flow around it. Underbody roughness also increases the size of the region where the low momentum flow is concentrated. This expands laterally away from the model centreplane to a greater extent than for the smooth configuration. It is also noted that the wake flow moves up and down as it flows downstream. This suggests that there may be some rotation of the flow in a vertical plane perpendicular to the direction of motion.