The growth of disturbances in the pre-transitional region of a boundary layer in presence of surface roughness is of relevance to many engineering situations and has been a topic of research for several studies in the past. In this work, we present velocity measurements in a boundary layer developing on a flat plate (mounted in a wind tunnel) downstream of a 2-D strip of distributed surface roughness of limited streamwise extent. Two different grades of sandpaper roughness are used toward this purpose. The power spectra of the streamwise velocity fluctuations show that the roughness introduces distinct frequencies as well as a broad range of disturbances into the boundary layer, with the latter being primarily responsible for the onset of transition. The disturbances in the pre-transitional region exhibit amplification which can be attributed to the transient growth mechanism. For the finer grade of roughness, the disturbance kinetic energy grows approximately linearly with streamwise distance from the mean roughness position, which is broadly consistent with the linearized disturbance theories. The wall-normal location of the peak disturbance energy shifts away from the wall with increasing streamwise distance, and the fluctuating velocity profiles exhibit a non-similar distribution in the streamwise direction. These results are qualitatively similar to the theoretical predictions of Fransson et al. (Phys Fluids 16:3627–3638, 2004) but in contrast to the optimal-disturbance theory of Luchini (J Fluid Mech 404:289–309, 2000), both of which deal with steady disturbances. The results for the coarser roughness, however, depart considerably from these theoretical trends.