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Design Review: Volume 1, Issue 4 (December 1948)

External Rendering Of Plastering

External Rendering Of Plastering

In the previous issue appeared some notes on the type of material used and the method of its application to produce a rendered surface that is less liable to cracking and crazing than is the finish generally applied in this country. This article deals with the reasons for the success of the procedure.

This type of failure has its origin in moisture movement or the change of volume which cement products undergo as their moisture content changes. Cement renderings must be applied in the wet state, and as they dry out shrinkage stresses are set up. These are partly tensile stresses within the material itself and tend to form cracks, and partly shear stresses between the rendering and its backing which is frequently rigid. These latter stresses therefore tend to cause failure by loss of adhesion.

When a rendering has cracked, water running down the wall is quickly drawn into the cracks by capillary action. Thus the material behind the rendering may become very wet and the trapped water cannot escape readily if the plaster is strong and impervious. Wet walls due to this cause are common and are very difficult to deal with effectively.

Again, when water gets behind a rendering as described above, it frequently leaches soluble salts from the brick-work or whatever lies behind and, when drying eventually does take place, these salts are brought out as a disfiguring efflorescence, or worse still, sulphates are brought out into contact with the cement mortar, producing sulpho-aluminate expansion and consequent serious failure.

Another, trouble is caused by differential moisture movement. When a rendering dries on the surface and thus shrinks slightly while the back remains wet for a prolonged period, stresses are set up which tend to vanse the surface to become concave. The result may be failure by loss of adhesion.

If, however, a weak and open-textured rendering such as the 1:1:6 mix has been used, water is absorbed into the wall evenly over the whole surface and can evaporate comparatively quickly. It has been found more waterproof than the stronger and denser types.

These renderings do not crack as badly as the strong ones. Stresses built up as a result of shrinkage or movement in the building itself are quickly relieved by the formation of minute hair cracks all over the surface. These cracks do not allow entry of water any more readily than does the more or less porous surface itself. The partial relief of stresses in weak renderings by “creep” with the prevention of at least some cracking is described below.

Crazing is another form of failure and consists of a disfiguring network of fine surface cracks. These may not page 15 be very deep, but some frequently develop into shrinkage cracks. Crazing arises from the action of the carbon dioxide of the atmosphere forming a skin which shrinks as it is formed.

When a cement rendering is finished by steel trowelling it is particularly liable to craze due to the thin layer of cement that is brought to the surface. This can be obviated by the use of the weak mixes mentioned in the previous article, and by scraping the surface to remove the rich cement layer.

The phenomenon of “creep” is important, and involves the plastic flow of materials under sustained stress. Two points about creep in concrete are important:—


Creep is greater in lean mixes than in rich ones.


Creep is less for concrete stored wet than for similar specimens stored in air.

The first point indicates that shrinkage stresses can be relieved by creep more readily in weak than in strong mixes.

The second point explains why rendering in wet weather tends to be less successful than in dry weather. Experiments have shown that rendered panels kept damp for ten days cracked badly compared with panels dried naturally in the sun.

The weak rendering has the advantage of weathering uniformly over the surface, instead of showing the dirty streaks and runs so common on the smooth impermeable surfaces of strong renderings. Buildings thus retain a good appearance longer.

A common sight is the development of irregularities in colour soon after the plasterer has finished. It is characteristic of work finished with a wood float that the surface darkens at different rates from one area to another. This is due to differences in texture, some parts of the surface being more porous and absorbent than others. Buildings finished in this way soon look shabby.

Rough textured surfaces tend to cause water to drip down the wall rather than to run down in an unbroken film. The amount that does run down is reduced by absorption into the rendering as already described. Thus with these surfaces there is less tendency for the plaster to fail more seriously near the bottom than at the top of a wall—a fairly common experience with strong, dense renderings. The roughcast wall, a weaker mix merely thrown on, is usually the most satisfactory.