External finishes need to be both durable, attractive and have
some degree of resistance to moisture penetration.
This may not mean actual waterproofing.
A relatively absorbent rendering, which can 'breathe', is often the most
satisfactory.
A strong, dense mix is far more likely to crack, water will then penetrate
the cracks, be unable to escape because of the denseness of the mix, and
will travel inwards. This could cause dampness inside a building or continual
dampness behind the rendering, eventually causing it to part company with
the background or backing.
The materials used in external cement work are Portland Cement, Sand-which
should be both sharp and clean - And sometimes hydrated lime, coloured
Portland cement or various pigments for colouring.
Other aggregates used in obtaining textured finishes are shingle or crushed
stone for rough cast and pea shingle or spar for pebble dashing.
Also available are several materials comprising PVA plus fine natural aggregates.
If rendering is done on a hot and sunny day, measures will have to be taken
to ensure that it does not dry out too quickly, and/or, bake.
Rendering must not be done over a frozen surface. |
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