Mud plasters and renders
Practical Action
Clay content
The composition of traditional mud plasters varies from place to place and is an important
factor in determining durability. The clay content is particularly significant, because if it is
too low the plaster will lack strength and cohesion, and if it is too high there will be a risk of
cracking due to shrinkage, which will weaken the bond to the wall. A suitable clay content is
usually around 10 to 15 per cent, but values outside this range could also be suitable
depending on the type of
clay. Soils with unstable or
swelling clays must be used
with great care. The sand-
to-silt ratio is also very
important in determining
the quality of a plaster.
Traditionally, clay plasters
were often applied in one
coat both internally and
externally. If applied in two
coats, the first can contain
more clay, even if cracks
develop, while the second,
containing more sand, is
applied in a thinner layer.
The second coat will help to
close the micro-cracks in
the first, provided the
surface has been lightly
dampened before
plastering. Finally, lime
distemper or whitewash can
be applied to give some
additional weatherproofing.
This will need to be re-
applied periodically.
Figure 3: Rammed earth walls can be built in many ways; all can
be protected with mud render or plaster.
Clay renders are commonly improved by adding natural fibres such as cereal straw, animal
hair, pine needles, bark, and wood shavings. Long straw or hair is chopped into short lengths
(2 to 5cm) for easier mixing: the function of the fibres is to resist cracks and facilitate the
drying process. They also make the plaster less dense and improve its insulation properties.
The amount of fibres required will vary depending on soil characteristics and can be from 35
to 70kg per cubic metre for straw; 50kg per cubic metre is a typical figure. In India, paddy
straw (blusa) is added at a rate of 6 per cent by weight, or 60 to 65kg per cubic metre. The
straw is soaked for several days in water to facilitate a rotting process, and the complete
mixing process can take 10 to 15 days.
Another traditional practice is the addition of cow dung, which improves the cohesion and
plasticity of soils of low clay content. Sometimes the dung is applied to mud plaster which is
partially dry to help stop the development of cracks. A traditional waterproofing in India,
known as Gohber leaping, consists of one part cow dung and five parts earth by weight, made
into a fine paste with water and applied to fill up surface cracks. Another practice is the
addition of horse urine, which acts as a hardener and improves impermeability and impact
resistance.
Improving the composition
It is possible to improve the quality of mud plasters by:
• controlling the quantity of the sand fraction in the soil; to no less than three parts sand to
one of clay, for example. This helps reduce cracks without compromising cohesion. A
3