In order to provide accident free timber rafting on small and medium-sized rivers, it is necessary to consider in detail the features of ensuring the raft flexibility with all the necessary calculations. The research aims at developing an improved methodology for calculating the parameters that ensure the flexibility of a raft made of flat raft units. The raft flexibility is formed through the optimal interval between flat raft units, which directly affects the flexibility index. The minimum allowable interval between raft units depends on the length of guard rails in the section line: the guard rail laying along the section line convex side and the guard rail laying along the section line concave side. Length of the guard rails in the timber transportation unit will be determined by the minimum turning radius of the rafting course, the section line width, the length of flat raft units and the distance from the raft board to the guard rail. When determining the optimal interval between the flat raft units and the raft flexibility, it is assumed that the section line of flat raft units, regardless of a strong piling current, passes within the raft course dimensions, where the raft course axis coincides with the raft axis, and the flat raft units located between the 1st and the last flat raft units can move freely in the longitudinal direction. We have studied the dependences of the interval between flat raft solid units on the minimum turning radius of the raft course, the section line width and the length of flat raft units, using the proposed methodology for calculating raft flexibility. We have found that with increasing turning radius of the rafting course, the interval between the flat raft units decreases, and the raft flexibility index increases. The interval between flat raft units becomes larger as the width of flat raft units increases and the coefficient of raft section density decreases in this case. As the length of the flat raft unit increases, the interval between flat raft units increases; the coefficient of raft section density decreases.