Plaster and Render Reinforcement
well haired plaster should have hairs evenly distributed through
the mix with spaces between the hairs of no more than a few millimetres.
The earliest structures
to which plasters were applied took the form of panels of woven hazel
or willow spars supported by timber. When first applied, some of the plaster
would protrude through the spars, creating interlocking 'nibs' in the
void behind. The nibs help to secure the plaster to the lattice, reinforcing
the key or bond between plaster and wood. For centuries hair and other
fibres have been added to lime and gypsum plasters to give greater strength
to these nibs and stop them breaking off.
For the reinforcement
of lime plasters and renders, hair should be strong, long and free from
grease or other impurities. Ox hair is the preferred choice, but horse,
goat, donkey, and a variety of other hair, including reindeer, are suitable.
Human hair, being relatively fine and of poor strength, should not be
to hair include chopped straw, reed, manilla hemp, jute, sisal, and even
sawdust. Modern synthetic fibres such as glass and polypropylene which
have been designed for use with Portland cement mortars have also been
used successfully in pure lime mortars, despite their smooth and almost
shiny appearance when viewed under a microscope. Natural animal hairs,
by comparison, have a much rougher texture, and are generally more appropriate
for historic buildings.
woven hazel or willow spars work well and are often found in surviving
wattle and daub, the practice of splitting oak and chestnut lath to produce
riven laths became popular early in the 15th century. Oak and chestnut
make particularly good riven lath as they both contain natural oils, thus
ensuring long life.
the 19th century sawn lath started to be used, although there is no doubt
that riven lath is stronger, and its textured surface and exposed grain
affords a far better key.
should be spaced about one centimetre (3/8") apart, which is the distance
between the top of your little finger nail and the underlying pad. (Spacing
can be gauged simply by resting your little finger on top of the last
lath to get a sufficiently accurate gap - you seldom see a true craftsman
with a modern rule, measuring as he goes!) If the laths are fixed any
closer, the first coat (or 'scratch coat') of plaster (often prepared
from mature slaked lime putty and well graded, sharp aggregate ranging
from up to 3mm through to fines) will not be able to pass through to form
good nibs. Larger gaps will allow heavier nibs to form which are liable
to break off, filling the void behind the laths.
the 1999 edition of The Building Conservation Directory, the view was
expressed that it was a mistake to think of a 1:3 lime/aggregate mortar
mix as 'standard', as the proportions depend on the choice of aggregate
and in particular its surface area and void. Suitable mixes can vary from
beyond 1:3 down to 2:1, but if the proportions are not relevant to the
aggregate the mix could well be totally unworkable. The same problem can
also occur with haired mortar specifications as the weight to volume ratio
of animal hair varies considerably from species to species. There is no
point in specifying a particular weight of hair for a given quantity of
lime mortar if the source and type of hair is not identified.
importance of ensuring that sufficient hair is evenly distributed throughout
the mix cannot be over-emphasised. In the past, apprentice plasterers
often started their training by spending several weeks at a bench, beating
bundles of hair with lengths of riven lath to break up any lumps and separate
the fibres. Allowing lumps of hair into a plaster mix is almost worse
than not putting any hair in at all, as the lumps have no binding power
and create weak spots in the plaster, causing it to fail.
haired lime plaster is to be applied to a masonry background, the plaster
relies largely upon suction for its bond, so it does not need as much
hair as a plaster which is to be applied to lath, which relies almost
entirely for its key on the nibs that protrude between the laths. Insufficient
hair reinforcement in a plaster mix on lath will result in weak nibs and
the risk of early failure.
should be added to plaster just before spreading as the alkalinity of
the lime attacks the protein in the hair. Millar, in his 19th century
masterpiece Plastering Plain and Decorative (see Recommended Reading),
writes of an experiment where haired lime plaster was stored in wet conditions
for nine months: at the end of this period he found that the hair had
'been consumed by the action of the lime'. Recent analysis of a failed
ceiling plaster from an important civic building showed that, while the
mortar mix proportions were adequate and the appropriate quantity of hair
was almost certainly used, it had been introduced into the mix and 'wet
stored' for several weeks prior to application. The weakened fibres were
unable to maintain the integrity of the nibs which could then not support
the weight of plaster, resulting in a failure that could have been catastrophic
had the hall been occupied at the time.
many thousands of years, lime plaster has been applied to masonry substrates
and various forms of wooden lath. An increasing number of specifications
now call for lime plaster to be applied to inappropriate backgrounds such
as plywood, and a variety of alternative lathing materials have been introduced,
including expanded metal and patent forms of lath product such as Riblath.
Even chicken wire has been used. As there will be little or no bond with
a parent background such as plywood, the integrity of the render is dependent
on the choice of lath and the quality of fixing. All these attempts to
bypass the proven traditional methods will fail unless the fixings are
capable of taking the load and the mesh, or lath, is fixed in such a way
that mortar can encompass the structure that is to support it. If expanded
metal is specified, a system of spacers, such as simple timber batten,
should be fixed behind the mesh to ensure that the plaster can surround
working on historic and, in particular, listed structures, repairs should
ideally be carried out using similar materials to the original. Not only
are they more appropriate to the historic character of the architecture,
but they usually work better than modern alternatives, especially when
used in conjunction with other traditional materials and construction
- William Millar, Plastering: Plain and Decorative, (1897), Donhead, Shaftesbury, 1998
- Ashurst, John and Nicola, Practical Building Conservation Volume 3, Mortars, Plasters and Renders. English Heritage, Gower Technical Press, Aldershot, 1988
For information on training, please refer to our course pages.
article is reproduced from The Building Conservation Directory, 2002
BENNETT MBE is Director of
The Lime Centre in Winchester
Lime Mortars and Renders
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