An experimental study was conducted to investigate the impact of distributed surface roughness on the linear amplification of Tollmien-Schlichting instabilities in a boundary layer without pressure gradient. Two distinct types of roughness were considered: evenly distributed and stochastic. A preliminary \(\Delta N\) model for predicting the onset of transition based on average roughness heights \(S_a/\overline{\delta }_1 \le 0.16\) ( \(S_a \le 67 \upmu \) m) is proposed. Furthermore, the effect of roughness extent in the streamwise direction on boundary layer transition is also studied, proving its impact on the linear amplification of modal instabilities.