What is the Best Render Mix for Walls?

Rendering of Walls Gives Them Added Protection

Walls are often made from brick or stone to give them the required stability and strength. The surfaces of these materials are often quite rough and can be quite inconvenient for the residents or users of any living space. It is, therefore, customary to render walls with a render made from cement and sand and occasionally lime.

This cement rendering is a thin mixture made of all these materials that are applied as a surface coat on walls. Quite often this rendering is used on both the insides and outsides of walls. This surface coating also helps the wall to resist dampness and make it weatherproof. The rendering can be smooth or coarse, and it is customary to have smooth surfaces on inside wall surfaces and coarse rendering on outside surfaces. What then is the best render mix for walls? This decision on the best mix can depend on the weather conditions in an area, especially where outside wall surfaces are being treated. Higher cement ratios lead to rendering being more vulnerable to cracks.

Quite often architects and engineers may specify definite mixes for the rendering, but as a rule, a proportion of one part of cement to four parts of sand is conventional for walls that are exposed to the elements. For interior surfaces, the sand quantity is increased to six parts, and some lime may be added. The lime allows for a smoother surface that allows for paint to be applied directly. Many plasterers prefer to use two coats of plaster on wall surfaces. In such cases, the first layer is made of a render mix design that may have a higher proportion of cement. This coat will be of almost three fourth of the thickness of the final rendering. This initial layer is left rough and even scored to leave a proper gripping surface for the final rendering. The top coat will we very thin and may contain a larger proportion of lime. It is possible to make this very smooth.

In many cases, external walls are also given a two-layer treatment. Outside wall plaster thicknesses are normally more than that which is used for inside walls. A 3/4 inch thickness of wall plaster is a minimum for outside wall surfaces, and 1/2 inch for inside walls. Wall preparation of walls is essential for proper rendering and is as important as deciding on the right mix for it. Walls must be clean and free of any fungus or other material. The joints between layers of brick or stone must be raked out to provide a key for the render. The render must be mixed to a consistency that allows the material to be pliable but at the same time enable it to have the desired stiffness. Additives or plasticisers can be used to ensure this, but this can increase costs. 

render mix

Care has to be taken to ensure that any quantity of render that is mixed is used or applied to the wall before the cement achieves its initial set. If this is not done, it may become difficult to finish the wall surface to the desired level of smoothness.