At low contact pressures, conduction across the gas gap is the predominant mode of heat transfer in a joint. Experimental results are presented for the solid spot and the gap conductance for a range of surface finishes and several interstitial gases and gas mixtures. The mean separation distance is then estimated as the difference between the effective gap thickness and the temperature jump distance. It is seen that a simple relation exists between the mean separation distance and surface roughness for all the gases and gas mixtures. This correlation satisfies 85% of data points to within +/-4%.