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Design Review: Volume 1, Issue 3 (September 1948)

External Rendering Or Plastering

External Rendering Or Plastering

External cement-rendering of a masonry wall is commonly adopted for the exclusion of moisture from the building. It has developed from the old lime plaster which deteriorated rapidly when exposed to rain or frost. Lime plasters were therefore protected by a coat of paint or a simple wash such as lime-tallow. Much of this painted plasterwork remains in the work of Nash in London, but still requires regular painting for its maintenance.

A later development was the use of hydraulic limes, and during the latter part of the last century the use of Portland cement in renderings was introduced. This brought with it the cracking and crazing of rendering so well known to-day.

In New Zealand, we seem to have followed the English tradition of using a 1:3 cement:sand mix, to which about 10 per cent of lime is added to improve workability. Cracking and moisture penetration of these finishes either on solid masonry or “Wood-frame and Stucco” buildings is common and well known.

As a result of numerous inquiries about rendering failures, the Building Research Station in England carried out an extensive programme of work. Several Continental countries were visited to study their methods, as cracked and crazed renderings are uncommon there.

Though mixes vary widely on the Continent, cement is seldom used as the sole binding material. It is usual to include a high proportion of lime.

It was found at the Building Research Station near London that a 1:2:9 cement:lime:sand mix was very satisfactory, if it was properly applied. However, under the more severe conditions of Scotland, these weak absorbent renderings failed. It was found finally that a 1:1:6 mix was generally satisfactory, when a good quality fat lime was used. In all the mixes tried, the binder:sand ratio was about 1:3. It was the cement:lime ratio that was altered.

The general method of application of renderings involves “laying on” and then finishing with a wood float. On the Continent, the material is thrown on at a fairly wet consistency, and finished in a variety of ways, generally in the nature of a scraping technique, but never by trowelling. The cement is allowed to take its initial set generally after four to six hours, depending on the weather. It is then scraped with a steel straight-edge, hack-saw blade or other tool to give the required texture.

The reasons for the success of this technique will be discussed in the next issue.