National Building Studies Bulletin no. 10 is a comprehensive guide to the design, application, performance and repair of external renderings and a British Standard code of practice on the subject is in course of publication [BS 5262:1976]. Defects are still common, however, and inspection usually reveals that they result from some transgression of basic principles. These notes are therefore intended to re-emphasize a few main points with particular reference to new work where a rendering is applied for its appearance or to resist rain penetration.

The use of design details, such as wide eaves, to reduce the amount of rain falling on a wall, is recommended. Mixes for external rendering should be appropriate for the exposure conditions and are listed in a table. Proprietary mixes and aerated ('plasticized') mortars are also discussed.