For convenience of experiment or manufacture many workers have measured the heat transfer and frictional properties of passages containing rough and smooth surfaces, although the properties of the roughened surface has been of primary interest It is therefore necessary to be able to separate the properties of the component surfaces, particularly if the results are to be applied to passages of different configurations. Various methods have been proposed. Their principles are discussed and the assumptions they invoke are examined and tested against experimental measurements. The effects of errors in the assumptions are estimated and the overall accuracy of the methods is considered.