Light-gauge steel sheeted diaphragms on wood frames may be used to transfer in-plane shear forces to the end walls in post frame structures. The amount of shear force transferred is dependent upon the in-plane stiffness of the diaphragm and the frame stiffness;The results of 31 full-scale cantilever diaphragm tests are presented. Two different sheet profiles and three fastener patterns were used. The results are compared with the predicted stiffness from a plane truss computer analog. Test variables include openings in the sheeting, recessing of purlins in from the sheet ends, seams in the length of the sheet and placing the purlins flat;An analytical method and an empirical equation to be used in conjunction with testing are presented. The analytical method predicts the stiffness of the control diaphragms within 1.5% of that predicted by the plane truss computer analog. The empirical equation was fitted to steel diaphragms on steel frames and can be used to adjust for different diaphragm lengths, purlin spacings, and sheet thicknesses;It was found that the location and size of an opening does influence diaphragm stiffness. Openings with sheeting on two sides only will reduce diaphragm stiffness approximately twice as much as openings with sheeting on three or four sides. Fastener stiffness and location have the largest impact on diaphragm stiffness. Increasing the number of fasteners at seams in the length of the diaphragm will off-set the effect of the discontinuous sheet length. Fasteners near the edge of the sheet have a much larger impact on stiffness than those near the center of the sheet. The plane truss computer analog predicted the test diaphragm stiffness reasonably well;Several methods of modeling the diaphragm frame interaction are reviewed. It was found that the plane frame/truss model will model frame-diaphragm interaction more accurately for complex structural systems if the diaphragms are represented by two spring elements. Also varying frame and diaphragm stiffness can be included easily.