Structural defects in the form of relict joints are frequently found in residual soils derived from the decomposition of igneous and metamorphic rocks. Previous studies indicate that the shear strength along relict joints is generally less than that of the intact material and therefore such joints form planes of weakness in the jointed soil. Slope designs in jointed soils, based on the strength of the intact material without considering the presence of adversely oriented relict joints, may be unsafe. The distribution of mass strength of jointed soils in the field is bimodal, with the intact strength as an upper limit and the joint strength as a lower limit; it varies with direction and with the geometry of the potential slip surface. In order to describe this behaviour, a probabilistic method to estimate the mass strength is presented and its practical application is illustrated by a case study.