This paper introduces a theoretically-based method to compute the behaviour of a planing hull in a steady yawed motion when it is free to roll. This method was developed by using 2D+T theory and the oblique-water entry of an asymmetric wedge. Sectional forces were determined using added mass theory. The forces and moments acting on the boat were computed by extending the sectional forces in the longitudinal direction. Trim angle and centre of gravity (CG) rise were found by solving the motion equations for the rigid body of the vessel. The results were compared against experimental data, and suggests that the current method has reasonable prediction accuracy for these parameters. Moreover, cases of free-to-roll and fixed-in-roll were determined and compared against each other, indicating that the trim angle of a boat is reduced, while CG rise increased in free roll.