Page 157 - The Building Conservation Directory 2013

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4.1
Services & Treatment : Protection & Remedial Treatment
Lime harling which is at least 150 years old on an old church in Orkney
country houses throughout the 18th and 19th
centuries. Limewash finishes were typically,
but not exclusively, applied over the harling
surface, as a protective and decorative finish,
often using natural earth pigments.
Like many traditional building materials,
lime harling and other forms of lime-based
renders and coatings are vapour permeable
to some degree and can help to manage the
effects of moisture on mass masonry walls.
The additional layer generally reduces the
depth of rainwater penetration through the
masonry, while the inherent permeability
of the lime promotes evaporation from the
underlying wall fabric and from the harling
itself under drying conditions. The damage
which can be caused to traditional solid-wall
buildings by the use of inappropriate cement-
based and other impermeable coatings is
now well recognised. If correctly specified
and used, lime harling can provide an
attractive and durable, weather-protective
coating to most traditional buildings.
General principles
As a basic principle, lime harling and
other external coatings should generally
be weaker than the background material
over which they are applied, and each
successive coat should be weaker and
thinner than the preceding one. The
exception is in areas of severe exposure
where it may be advantageous to maintain
consistency throughout the thickness.
In most instances, however, weaker lime
mortars are best, as they generally provide
greater flexibility and vapour permeability.
An appropriate method of work allied to an
appropriate specification for materials and
mix design is critical in achieving the best
outcome for the building or structure.
When harling onto an existing
masonry wall, several factors require
careful consideration before deciding on
a suitable approach and specification. It is
important to establish the function of the
coating; is it to improve the appearance of a
building, to protect vulnerable underlying
masonry, or to help dry out a damp building
or wall? The specification of both the
materials and methodology for undertaking
the work can be designed to achieve a
particular outcome, and where appropriate
address several, or all of the foregoing.
The type and condition of the substrate
will also have a significant bearing on the
specification. Most importantly, any coating
must achieve an adequate bond with its
background. Where loose, friable substrate
materials exist, remedial repairs and/or
consolidation of this will be essential before
any lime coating can be applied. In some
instances, a few applications of diluted
limewash can be helpful on a weak substrate.
Adequate bonding may be achieved
solely through a well-controlled suction
bond, a mechanical key, or a combination
of both. Inappropriate suction control is
a significant cause of failures in mortar
coatings – over-wetting as well as insufficient
damping down will adversely affect the
bond between the existing wall surface and
the new coating. Where the wall contains
both dense impervious stone and highly
porous mortar, there is a need to thoroughly
dampen the mortar joints, while ensuring
that the stone surfaces are dry to the touch,
prior to applying any lime coating.
A ‘pricking-up’ or ‘bonding’ coat may
be helpful on difficult backgrounds – this is
typically a thin, binder-rich, gritty coating
which provides a good bond with the substrate
and can help to even out the background
suction across variable substrates.
A non-hydraulic lime mortar (aggregate
and lime putty) may work well when applied
to a relatively dry wall, but would be at risk
of failure on a wet wall as saturated lime
mortars will not carbonate, and therefore
not harden. A hydraulic lime mortar
would also be at risk if saturated, in frosty
conditions. Despite its initial hydraulic
set, it relies on carbonation to achieve full
strength and frost resistance. To avoid
failure, any damp, uncarbonated lime
must therefore be protected from frosts
through the winter. At the risk of stating
the obvious, frost requires moisture, so
a dry mortar in freezing conditions will
not become damaged by frost action.
When contemplating winter working,
bear in mind that colder temperatures
will slow down the curing process (as with
cement mortars), so greater care must
be exercised in planning the works to
ensure adequate protection. Scaffolding
must be enclosed and protected and
background heating may be required.
Where walls are found to be damp, as is
often the case where a cement-based coating
has just been removed, specialist advice on
materials selection should be sought. The
time of working, length of contract and
level of protection possible (for working
and curing) are all important factors which
must be considered at specification stage.
Application process
There is no ‘standard’ methodology for harling
an existing masonry wall, as there are so many
variables. However, the works can generally be
divided into the following applications:
1. Substrate preparation
2. Dubbing (daubing/flushing) out
3. Pricking-up (tack/scud) coat
4. Straightening coat
5. Finish coat
6. Limewash
Substrate preparation This will
generally involve repairing any defects and
filling any voids or cracks in the masonry. All
masonry must be clean and free from dust or
debris; soft friable stone should be brushed
back to a relatively sound surface and in some
instances these may need to be consolidated,
either with diluted limewash applications, or
the masonry repaired by some other method
to ensure a satisfactory background.
Dubbing out This is the process of
filling previously prepared open joints,
building out the mortar until flush with the
face of the masonry. As with most filling of
rubble stone masonry joints, pinning stones
are incorporated where necessary to avoid
large volumes of mortar accumulating. The
mortar must be well compacted to fill any
voids and can be finished fairly rough ‘off the
New, lightly pressed back lime harling and limewash finish at Brodie Castle, Forres