External rendering - Undercoats

The undercoat performs a variety of functions besides keeping out the rain; it should level uneven surfaces, it should equalise the suction for the top coat and it should prevent the joints from grinning through. Single-coat renders, like plaster, can often be identified by the grinning of the joints; this is nparticularly noticeable after rain. However, recent developments in rendering materials have enabled single-coat techniques to be effectively used.

The thickness of the first coat should not exceed 15mm and should be a mix appropriate to the background. When the undercoat has begun to stiffen it should be combed, as shown in the photograph on the previous page. This is partly to provide a key for the final coat and partly to provide stress relief points, which prevent undue shrinkage occurring as the render hardens and sets. Cracking in a base coat can affect subsequent coats if they are applied too soon: a base coat should therefore be left for several days before applying additional render coats.