Externally, plaster
correctly applied provides excellent protection under even the most exposed conditions.
Inside farm buildings it gives hardwearing easily cleaned and hygienic wall surfaces.
Cement, lime and sand
together with water and sometimes an admixture, are the typical materials used for
plastering. The use of a clean sharp plastering sand together with the correct choice of
mix proportions has a fundamental influence on the finished quality. Recommendations for
the normal range of mixes are provided in Table II.
Local practice may indicate slight adjustments to the proportions given. Caution should be
exercised in the use of richer mixes due to the risk of cracking.
Normally, scud followed by undercoat and finish is adequate. For increased
protection and on exposed sites, apply a second undercoat. Increasing the thickness of
individual plaster coats does not provide extra protection and is not recommended.
Premixed dashing mixes incorporating lime are available in some areas.
They reflect local tradition and are usually gauged 1: 6-7 with cement.
In order to produce a high quality durable and uniform finish to plaster
applied over existing blockwork the following precautions are recommended:
All movement joints in blockwork should be carried through the
plaster to avoid potential cracking.
As joints will always show up eventually, it is important that they are
concealed or tied in with some feature of the building.
The surface of the blocks should be clean and free from loose mortar.
All mortar joints should be raked to a depth of 10mm.
The first coat is called a Scud Coat. This is best applied by throwing the
mixture onto the surface of the wall from a hand scoop. The wall surface should be covered
with a single layer of scud coat 3 to 5 mm thick and no steps should be taken to smooth
the surface. This provides a good key and uniform suction for later work.
As a rule, to avoid failure due to shrinkage and loss of bond, successive plaster coats
should be no stronger then the previous coat (or background); this is achieved either by
reducing the strength of the mix or by reducing the thickness of the later coat. The
undercoat should not exceed 12mm in thickness. Always comb fresh undercoats to provide a
sufficient key for the following coat.
Roughcast finishes - known also as wet dash - are the most popular of the
textured finishes and provide excellent weather protection. The mix is thrown onto the
wall with close attention to achieving a wide even spread and obtaining uniform texture.
Alternatively decorative finishes can be produced by the use of trowels and other small
tools on the surface of the fresh plaster.
Use Irish Cement, which has been stored in dry conditions.
Use only clean sharp sand found to perform well locally.
Avoid excessively strong mixes, which frequently cause cracking.
Do not use fresh blocks.
Prevent plaster from rapidly drying out, particularly in sunny weather.
Always allow a minimum of 2/3 days between the application of successive
coats of plaster.
Never apply strong plaster over weak undercoats.
Advisory Engineers are always
available to discuss problems relating to the use of concrete on the farm.
