Suitable proportions of total cementing agent (cement and lime) in mixes for rendered finsihes are governed partly by the requirement that the amount of such material shall be just sufficient to fill the voids between the coarser sand particles, and partly by the working properties necessary. The function of the cement in a rendering mix is to provide strength and durability. For most work the strength is far greater than necessary, and the use of such a mix, particularly on rigid backing materials of lower strength, is likely to give rise to cracking. Such a stong mix cannot yield locally to the stresses set up by drying shrinkage, and these stresses tend to build up until relieved by a crack at some weak point. If the backing material is not strong, the rendering will come away form the backing. Once this happens rainwater may penetrate through the cracks, and together with frost may cause further loss of adhesion. For all other work, mixes of lower cement content should be used; satisfactory working qualities can be retained most readily by using lime, which contributes practically no strength at early ages, in place of a portion of the cement.