MUD PLASTERS AND
RENDERS
The traditional use of mud plasters and renders to coat and protect walls dates back a very
long time and is found in almost all regions of the world. Finishing a house with mud plaster
when the house itself has been built with earth is a natural, complementary technique, but
mud plasters can also be used for buildings of stone and fired brick provided they incorporate
an earth-based mortar for the joints.
Earth-based plasters often use earth in combination with other natural materials such as wheat
straw or cow dung, or with mineral additives such as bitumen, to improve the basic qualities of
the earth by acting as stabilisers, hardeners, and waterproofers. Even without additives,
however, mud plasters and renders can give excellent results provided that they are made and
applied with skill and care, and maintained regularly. Today, with low-cost mass housing a
priority and with the increasing interest in the preservation of architectural heritage, the need
for plastering materials which are efficient and economical has awakened a new interest in
earth. Earth-based plasters are completely compatible with traditional materials and building
techniques and the almost universal availability of suitable earth for building gives them a
distinct advantage over some modern synthetic plasters.
Figure 1: Walls built using traditional methods, such as cob, are very suitable for mud renderings.
Fundamental properties
The need for a plaster and the type of plaster that should be used depends particularly on the
method of construction and quality of construction. The provision of adequate footings,
basements, eaves, and overhangs to a roof can in certain circumstances eliminate the need for
a plaster coating altogether. As plastering can amount to 15 to 20 per cent of the total cost of
a house, its benefits need to be considered relative to alternative options.
In general, except in the case of highly exposed walls in areas of heavy rain, a plaster should
protect against wind, rain, knocks and abrasion, and should improve the thermal insulation
and appearance of a wall. At the same time it has to be easy to apply without requiring
expensive and elaborate tools, and must be affordable. All types of mud plasters, but
especially those on external surfaces, need to offer erosion resistance, impermeability to
moisture, and impact resistance, and be well bonded to the wall.
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