undulating medieval plaster with carved stone details under
a coat of limewash
promotion of modern gypsum-based plasters has led to the almost
complete demise of lime plastering, and of many of the traditional
skills associated with the craft. This has been exacerbated by
the plastering trade being divided into flat and decorative work,
with new 'fibrous plasterwork' being made in workshops. Many youngsters
entering the trade are now just taught the basic skills to enable
them to stick up plasterboard and skim plaster onto it. We are
told that it is all down to 'supply and demand'; if this is the
case, those of us involved in work on old buildings need to be
is a real need for skilled plasterers who can plaster with lime,
and also turn their hands to repairing and reinstating dado and
cornice mouldings in situ. The current training system works against
anyone gaining this set of skills.
article like this cannot resolve this skills shortage, nor attempt
to even describe the range of skills that a traditional plasterer
should have. There are, however, some general principles which
anyone involved with lime plastering should be aware of. Sadly
there are too many cases of lime plasters failing because the
people who have specified the work or the people carrying out
the work don't have adequate knowledge or experience.
using lime in old buildings have a vague understanding of the
benefits of a 'breathing' mortar or plaster, but if they perceive
that lime is too difficult to use they may decide not to bother
with it. We need to make it clear that the revival of the use
of lime is not some 'airy fairy' idea dreamt up by a bunch of
idealists. On the contrary, it is driven by the realisation that buildings are
suffering because they have been coated with inappropriate materials,
and the people living in them may be less healthy as a result.
There is compelling evidence that modern gypsum plasters encourage
condensation and consequent mould growth if used on walls that
are supposed to 'breathe'. We are beginning to see a revival in
the use of lime plasters and we need to encourage a revival of
the skills required to use them.
the lack of proper training for anyone wanting to learn traditional
plastering skills, it has to be said that lime plastering is not
rocket science. With a basic understanding and a willingness
to learn, most plasterers can pick up the skills required to produce
a reasonable job in a few days. This is not the same as the skill
required to repair plaster in a fine quality country house, or
the experience required to match a range of historic finishes,
but these things come with time, and we can hope that as more
plasterers learn to use lime they will be inspired to develop
their skills and understanding further.
two characteristics that differentiate lime plasters from modern
plastering materials. The first is that they set slowly by absorbing
carbon dioxide from the air, in the presence of moisture. The
second is that they will shrink as they dry.
limes, which set more quickly than white/fat limes, are occasionally
used for plastering in damp conditions, they are less flexible
and breathable than the latter, and their use internally should
generally be avoided. All the evidence on old buildings and in
written documentation indicates that for centuries, if not millennia,
plasterers have chosen to use white/fat limes for internal plastering.
is some debate about whether we really need to use traditionally
slaked lime putty, or if bags of dry hydrated white lime from
the builders' merchant are just as good. Although chemically they
are the same (both are calcium hydroxide), a traditionally slaked
lime, which has been matured for three months, will have broken
down into much smaller particles and started to form crystal
chains. This gives it better adhesive qualities, helping it to grip the wall more tenaciously. The difference seems to be in the maturing process; so
if recently hydrated lime from a fresh bag is left to soak in
water for three months, it should be as good as a traditionally
slaked and matured lime. However, in practice most of us find it is
easier to buy the matured stuff from a specialist supplier.
in most buildings from the second half of the 17th century onwards
was applied in three coats, which enabled a flat finish to be
achieved. In agricultural buildings two coats of plaster are common,
or even one single coat may be found where an undulating surface
was acceptable. Similarly, pre-18th century buildings often have
undulating plaster finishes, and this usually indicates that fewer
than three coats were used.
on any plastering project it is worth assessing the number of
coats used originally and/or the quality of finish required. On
the basis that three-coat work is the most common in historic
buildings, it is best to understand how to apply this and then
reduce to two or one coats where appropriate.
coat is known as the 'scratch coat', because the surface is scratched
with lines to give a key for the next coat. The mix used is usually
one part lime putty to two and a half parts of coarse, sharp,
well-graded sand. If the grading of the sand includes
more or less of a particular grain size the amount of lime may
need to be varied slightly. An experienced plasterer will be able
to tell instinctively whether another half part of lime or sand needs
to be added. Another way to tell is to take a sample of the dried
sand and measure the volume of water required to fill all the
voids between the grains; the amount used is equal to the amount
of lime required.
coat on the underside of a spiral staircase
Hair can be
added to the mix to give it tensile strength. Although this isn't
absolutely necessary when plastering onto stone or brick, its
benefit in the long term is that if the building moves or any
patches of plaster detach from the substrate, the hair will help
bridge over any gaps. Old plaster can sound hollow in places when
tapped, but is usually still sound if it contains hair. When plastering
onto laths, the addition of hair becomes a necessity, because
plaster does not adhere well to timber once it has dried; it relies
on interlocking fingers formed as the wet plaster squeezes though
the laths and slumps over, so the tensile strength imparted by
the fibres is vital.
It is important,
before applying new lime plaster, to ensure that it isn't going
to be sucked dry by the background it is applied to, as this will
cause it to shrink and potentially fail. The suction can be reduced
and controlled by wetting the substrate before applying the plaster.
If the wall is very dry and porous it may need to be sprayed with
a hose pipe a couple of times on the day before, and then once
again on the day of application, but if it is less porous and
the environment is relatively humid, spraying with a hand-held
spray on the day of application may suffice. There needs to be
enough moisture in the wall for it still to be damp to the touch
after an hour, but no longer glistening with droplets of water.
coat should be no thicker than 15mm (5/8 inch). Any deep recesses
or holes should be 'dubbed out' beforehand, using a stiffer (drier)
mix, and allowed to dry to avoid deep pockets in the scratch
coat. If plastering onto laths it is important to apply the plaster
diagonally to the line of the laths, joining up each time with
the previous area laid, to achieve a consistent key between the
wet, the surface should be scratched with a three pronged lath
scratcher or a single pointed lath (which is slower but gives
a better job). The scratching should be in straight lines, diagonally
to the laths or the line of the wall, in both directions, to create
a diamond or lattice pattern. The quality of the scratching affects
the keying of the next coat, so it should be done carefully to
achieve an even pattern, and, on laths, particular care should
be taken not to cut through to the lathing.
coat should then be left to dry and shrink before attempting to
apply the next coat. In most circumstances it will need two weeks
to dry out, but can take up to four weeks in some cases. Shrinkage
cracks are likely to appear as it dries, but this is not a problem.
The important thing is to avoid it drying too rapidly, which can
cause it to fail. Exposed areas of plaster (adjacent to open windows,
for example) may need to be covered with hessian or polythene,
and the use of dehumidifiers should be avoided.
Once the pad
of a thumb can no longer indent the scratch coat it is ready to
take the next coat. At this stage the surface should still be
slightly damp to the touch and will just need brushing down to
remove any loose grains and then lightly dampened with clean water,
using a hand-held spray. If it has been left too long and has
dried out completely, more water will be required.
coat with devil-float scratch marks
coat is known as the 'floating' or 'straightening' coat, and is
used to bring the surface to a level plane. The mix is usually
slightly less rich than that of the base coat, typically one part
of lime putty to three parts of coarse, sharp, well-graded sand,
and normally without any hair. Again, it should not exceed 15mm
(5/8 inch) in thickness. A level surface is achieved using long
'floating rules' or 'straight edges', passed over the wet surface
to remove undulations.
In the best quality
work, wooden blocks (known as dots) are temporarily applied and
plumbed and levelled; lines of plaster (known as screeds) then
join between the dots, and are levelled using a floating rule;
finally the spaces between are filled using trowels and levelled
with a floating rule with its ends bearing on the screeds. This
method was used in finer quality Georgian and Victorian buildings.
Once it has
begun to stiffen up, the floating coat needs to be consolidated
by 'rubbing up' the surface using a wooden float to counteract
shrinkage. This is likely to be required once or twice on the
day of application, and may be necessary on the following day
as well. The timing depends on the speed of drying. Sprinkling
the surface with water, using a brush, assists the circular rubbing
action if it has dried out too much.
Once the surface
has been compacted, a 'devil float' (wooden float with nails or
screws driven through the corners to project about 2mm) is rubbed
over the surface to form a key for the finish coat. It should
then be left for about a week or so before it is ready for the
coat is known as the 'setting' or 'finishing' coat. It is usually
thinner than the other two coats and uses a finer sharp sand.
The mix can vary depending on the hardness and the type of finish
required; the richest mix being three parts of lime to one of
fine sharp sand, and the leanest mix being one part of lime to
three parts of sand. More sand will give a harder finish and is
more suitable for open textured float finishes; more lime will
give a softer surface but allows it to be polished smoother. For
standard work, a mix of one to one is suitable.
coat finished with sponge floats and lined out to imitate
repair (top left) to fine 18th century plaster, gauged with
gypsum to achieve a completely flush finish between old and
of this coat can vary between 2mm (1/16 inch) and 5mm (3/16 inch).
In order to achieve an even finish on the surface, it needs to
dry out consistently, so is applied in two or three very thin
layers. The additional time and labour required for this is well
worthwhile because it controls the drying of the surface and allows
the plasterer to achieve the finish required.
the setting coat the floating coat needs to be lightly dampened
with water to control the suction. Each layer is skimmed on as
thinly as possible, working in alternate directions each time,
and is laid over the previous one as soon as it has had a chance
to 'steady-up' (usually in about half an hour). When the work
is firm enough it should then be scoured to compact and consolidate
float (that is to say, one with the grain running the length of
the float) can be used for the scouring process, but if a very
flat surface is required, a cross-grain float is better. The 'cross-grain'
prevents the edges wearing down and ensures that any projections
are shaved off as the float passes over the surface. Cross-grain
floats are only used to rub or scour over a surface and any specification
that refers to using them to apply lime plaster is clearly wrong.
on what is required, the surface can then be worked over using
either a trowel, to achieve a fine closed finish, or a combination
of wooden and sponge floats, to create an open textured finish.
Some water is likely to be required in this process, splashed
on with a brush.
and scouring process required to achieve a suitable finish can
present a problem when patching up to old plaster. The first important
point to note is that the exposed edge of the old plaster is likely
to suck more moisture out of the new plaster than the wall itself,
so more water will be required to control this suction at the
edges. Once the area has been patched, particular care will be
required when rubbing over the setting coat, to avoid forming
an indent at the junction with the old plaster.
In some cases,
where an absolutely blemish free surface is required, the setting
coat mix can be gauged with Plaster of Paris (a form of gypsum)
to minimise the need for scouring. However, the decision to add
another material needs to be taken carefully, and the visual compatibility
of the repair needs to be balanced with its technical compatibility.
This brings us back to the need for traditionally skilled plasterers
who understand the materials they are working with. Without these
skills we are lost.
This article is reproduced from The
Building Conservation Directory, 2006
RATCLIFFE spent two years as a labourer with masons and
plasterers working on historic buildings, and 12 years with
two leading architectural conservation practices, before
setting up his own practice in Oswestry in 2000. He was
awarded the SPAB Lethaby Scholarship in 1987 and regularly
gives lectures and demonstrations on lime-related subjects.EMAIL
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