Applied exterior finishes

Mosaic tesserae were a common external finishing treatment to many buildings during the 1960s and 1970s. Comprising small marble or ceramic tiles bonded to a background material such as concrete or sometimes brick, the finish has been found to be durable, but problems of adhesion failure are common, leading to the loss of individual tiles or more serious delamination of large areas of backing material.

To correct a common error, the term 'mosaic' strictly relates to an artistic design that has been commissioned and not to a uniform finish of small tiles or 'tesserae', often applied to entire elevations or to exposed structural components.

There are no British Standards by which to select and compare tesserae suitable for external applications although guidance as to the application can be found in BS 8000-11.1:1989. The subject is also covered in more detail in BS 5385 Part 2, 1991 (formerly CP 212, Part 2). Neither document was available for designs executed during the 1960s but they do now give us a useful standard by which to comment on the earlier levels of work.

Tesserae come in a variety of styles and sizes - for external applications commonly around 20 x 20mm in glazed or unglazed ceramics, marble or glass. Some rough finished stone materials were also used and these required special care in fixing.

Individual tiles were bedded face down on a paper backing (or alternatively nylon strips or filaments) to facilitate handling and fixing. Paper-faced tiles were pre-grouted before fixing to prevent adhesive from squeezing up between joints. The paper facing was soaked off before final setting to enable minor adjustments to be made.

Defective tesserae applied to a concrete parapet. Both the background render as well as the tile finish has failed, possibly as a result of lack of key and water ingress via the parapet capping.