Rendering inappropriate surfaces

Painted or coated substrates offer low suction and often poor key for render finishes. Additionally, the bond strength of the coating to the substrate is usually too low to carry the weight of a render coat.

Construction tolerances or dimensional accuracy of blockwork and concrete is often at odds with the accuracy needed for good rendering. Steps in the surface or excessively thick areas of render can give rise to potential cracking or shading problems.

Existing cracking or the potential for differential movement arising from dissimilar substrate materials can transfer stress into a new render, resulting in cracking.

Renders applied to substrates that are too weak or to those with weak surfaces are likely to fail along the plane of the interface as the surface of the substrate itself fails.

A detailed assessment of the condition and flatness of the substrate surface must be made By the specifier. The assessment should consider whether the surface will allow for direct application of render or if additional mechanically-fixed render carriers are required. Sometimes the surface will require dubbing out (making level) before rendering can proceed.