Cement rendering

Cement Rendering is the application of portland cement and sand to exterior wall surfaces such as Blockwork or Fletton Bricks. Whether it is to be painted or not, it is better rough-trowelled using a wood float rather than a steel one, which will give a sandy rougher finish.

Cement render is strongly alkaline and will attack solvent-based paints until it has substantially dried out. As the rendering ages and weathers, cracks caused by shrinkage may appear. This allows moisture to gain entry and, if frosted, may result in loss of adhesion or spalling. If rendering has to be painted, check whether it is over blockwork or brickwork. If the backing is blockwork, as little as two or three months are needed for drying out before painting. But if it is applied over brickwork, then it should not be painted for at least four to six months. If left during the winter, as much as nine months to a year may be needed for the drying out process. Concrete should be dry to a depth of three-quarters of an inch to one inch before any paint is applied. Check moisture content with a conductivity type moisture meter by driving masonry pins into the concrete to the stated depth and connecting the meter to them.

Where the environment is mild or moderate, an emulsion type paint can be used before the substrate is completely dry, as this type of paint is permeable and allows drying to continue naturally. With severe or very severe environments, an emulsion type paint can be used initially, provided that the substrate is in good condition. After drying is complete, an acrylated rubber paint can be applied over it.