A theory dealing with the strength of porous brittle bodies which has been shown previously to apply to neat plasters is extended to cover sand bearing plasters. The sand grains have to be regarded as solid pores. The strength of sanded plasters is equal to that of the plaster matrix cementing the sand grains together. In spite of the increase of the characteristic parameters with sand content plasters are in practice the weaker the more sand they bear. This is because more water has to be added with respect to the plaster to obtain the desired consistency. The results are derived graphically from a special representation of the experimental results. Griffith's crack theory of brittle strength is shown to apply to the quality factor.