Mud plasters and renders
Practical Action
Erosion resistance
The main cause of erosion is heavy rain, and high winds driving the rain hard onto walls at an
angle will increase erosion further still. Heavy rain, even for a short time, is much more
damaging than prolonged light rain. A knowledge of local weather patterns and an analysis of
meteorological data can give an indication of erosion risk and hence appropriate plastering
materials and methods. It is advisable to study local traditional buildings and practices, as
their evolution will have been influenced by the local climate.
Impact resistance
The durability of mud plasters depends on their ability to withstand the impact of humans and
animals by bumping, scratching, or scraping. Impact resistance is closely linked with the
quality of the plaster, which is determined by its density, methods of application, number of
coats used, and maintenance practices. The texture of the plaster is also important.
Good bonding
The bonding of earth plasters to walls is very
important. When plastering a stone or earth wall
the composition of the mix as well as its
application are both crucial in producing a good
bond (the join between the two materials). The
plaster and the wall itself should ideally be
compatible so that shear forces are transmitted
between them and not terminated at the bond.
Good bonding reduces the incidence of cracking
caused by changes in ambient temperature and
humidity. The plaster must be applied in coats
of recommended thickness to prevent excessive
strain at the bond.
Testing the performance
A simple soil test which will show whether the
soil is suitable is to plaster an area of wall and to
observe the development of cracks on drying. A
number of different compositions can be tried to
find the one which produces the least cracks
and satisfies the need for hardness and water
resistance in that particular situation.
Simulation tests in the laboratory, such as the
spray erosion test, can only be indicative
because factors such as changes in scale,
influence of true climatic conditions, building
usage and maintenance practices are not easily
replicated. One of the most realistic simulation
methods is to expose small test-walls to natural
weather conditions; this has been done in
Australia, the United States, Senegal, and
France, for example. This test is a good
indicator of the durability of different plasters
and allows a realistic comparison between
plasters with different compositions and
methods of application. The main drawback
with this test is the length of time needed to
obtain meaningful results, and building projects
cannot always afford to wait that long.
Figure 2: There are many ways to
prepare wall's surface to help the
render to form a good bond.
2