We present a metric for quantifying the roughening observed during the in situ laser reflectance monitoring of GaN metal organic vapor-phase epitaxy. A roughness factor, F-rs, is derived by comparing the absolute reflectance at the turning points (peaks and troughs) in the interferogram, to that which would be expected from consideration of the equations of thin film interference for 2-D epitaxy. This factor is seen to be sensitive to the recrystallisation period prior to epilayer deposition, and ammonia flow during epilayer growth. This is the first report in the literature of a quantitative analysis of the roughening observed during the early stages of gallium nitride growth.