The difficulty in the measurement of the strain at the joints of members of wooden trusses has made the evaluation of theoretical strengths uncertain. This is particularly important if one desires to economically assure adequate roof strength for installation of solar collectors and other similar roof loading. This measurement problem has been solved by using interferometric techniques to determine small strains at truss joints. A modified Michelson interferometer has been adapted for this purpose. The interferometer, including He-Ne laser source and related optics, has been fabricated into two parts: One of the interferometer mirrors is floated independently on one truss member (two-point bearing), and the remainder (including source) in one piece on a two-point bearing on the other joint member. Using photographic techniques, both angular and lateral relative displacements of the two members have been measured. Design and details of the interferometer use are outlined.