School buildings in developing countries
Practical Action
framework structure within the walls. The elements of the ring beam, bracing or structural
framework would need to be made of a tough and flexible material such as timber or
reinforced concrete with adequate steel reinforcement. Any locally applicable seismic design
and building codes must be strictly adhered to.
Where there could be a termite problem a termite barrier would need to be inserted near the
bottom of the wall. One possible option is shown below. This is of concrete and would need
to be well-made to ensure that there are no cracks in it which termites can penetrate and
enlarge.
5 cm
5 cm
10 cm
drip 15 cm
Ground Level
Floor level
Figure 5: Termite barrier
An external plaster or render would help to protect the building fabric from weather damage,
improve appearance and fill in any holes which could harbour insects and dirt. As with the
mortar it is important that the plaster is no stronger or harder than the material to which it is
bonded, particularly for relatively weak and friable materials such as earth or soft stone. A
plaster made only with Ordinary Portland Cement as the binder would be too hard for most
materials, and there is the risk that water penetrates behind it causing the plaster to fall off
and sometimes taking pieces of the material to which it is bonded with it. Use of a lime and
cement mixture, lime only, or lime-pozzolana for the binder would almost always be
preferable to just using cement. Typical mix proportions of plaster mixes are one part lime to
two of sand, one part lime to two of pozzolana (such as some types of volcanic ash or finely
ground burnt clay) to nine of sand, or one part cement to two of lime to nine of sand. Plaster
needs to be well-bonded in to the surface to which it is fixed, so a good key needs to be
provided. This can be done by removing some of the mortar from the joints to create grooves
(raking), making grooves in the surface or fixing a wire frame tightly to the wall. Plaster is
applied in two, but preferably three thin layers. The layers or coats would preferably each be
less than one centimetre thick, but certainly so for the final or finishing coat which needs to
be the thinnest.
The first coat is applied, scratched to create a key for the second, then allowed to harden for
several days before the second is applied. Keeping a cement-based coat moist would help it
to harden, while with a lime-based coat allowing the drying to take place only very slowly
would help to reduce cracking. The procedure is repeated for the second coat if a third coat
is to be added. The finishing coat is made with a richer mixture (containing more binder),
and a fine sand so that it can produce a surface of smooth appearance.
Walls can also be plastered or painted internally. A white gypsum or lime-based surface
makes a room seem lighter, can help to improve acoustics and creates a smooth surface
where insects and dirt have difficulty accumulating. Gypsum is a particularly useful
plastering material, but is not recommended for external use, except in dry climates, as it is
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