Page 156 - The Building Conservation Directory 2013

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4.1
Services & Treatment : Protection & Remedial TreatmentLime Harling
Craig Frew
Lime harling is a thrown, or cast-on,
finish consisting of a slaked lime and
coarse aggregate mortar, and it usually
has a rough-textured surface. It is the most
common type of traditional surface finish
found in Scotland on masonry buildings of
solid wall construction. South of the border it
is known as roughcast and is widely found in
vernacular architecture. In Ireland it is known
as wet dash. Like other forms of lime render,
harling provides both a weather protective and
decorative coating, commonly covering what
was perceived to be poor quality rubble stone
or brickwork.
This article contains general advice on
materials and methods for undertaking lime
harling and associated works.
“The principal purpose for which any
of these processes (‘outside plastering’
including harling) is adopted on the
outside wall of a cottage is, to keep them
dry; and a second purpose is, to render
them ornamental, either by imitating
stone, or producing a surface more
curious or agreeable to the eye, than the
rude materials concealed behind it.”
(JC Loudon, Encyclopaedia of Cottage,
Farm and Villa Architecture, 1833)
Perceptions and fashions change over time,
and Loudon’s appreciation for the finish was
not shared by later Victorians. In the late
19th century external harling and render
was commonly stripped from the exterior
of older buildings in favour of exposing the
underlying stone or brick, and this practice is
still all too common in some areas today. The
vast majority of pre-Victorian era buildings
constructed of rubble stone, certainly in
Scotland, were generally plastered, harled
and/or limewashed externally. There are some
exceptions, such as rear and side elevations to
many Georgian properties which were often
flush pointed and lined out like ashlar.
The term ‘harling’ is generally understood
to derive from the action of hurling wet
mortar at a wall – hence the terms roughcast
and wet dash used elsewhere in the UK. In
Scottish vernacular architecture, harling
was almost exclusively applied as a cast
finish and not onto a floated base coat, as
is common in England (see Recommended
Reading) and on modern cement roughcast
work such as pebbledash. Cast-on textured
finishes will generally give better adhesion
than trowel-applied coatings, particularly
on rough or uneven surfaces, and those
with low suction properties. Cast-on
coatings also provide better weather
resistance, as the mortar is generally better
compacted and more uniform throughout
its thickness than trowelled-on coatings.
Like most other traditional materials
and techniques, knowledge and experience
were passed down through generations
and adapted to suit the local environment,
materials and labour availability. Typically
though in Scotland, pre-Victorian lime
harling would consist of a hand-cast, thin
coating, usually no more than around 10mm
in overall thickness, applied as a single coat
or maybe two coats at most. As with the
vast majority of traditional building limes,
quicklime would be mixed with coarse,
varied aggregates, slaked with water and used
while still warm as a ‘hot lime’ mortar. It was
typically the same mix as that used for the
masonry bedding mortar, but a wetter mix.
Craigievar Castle, Aberdeenshire: hand-cast ‘hot lime’ harling and pigmented limewash (Photo: LTM Group)
It was the size and shape of the larger
aggregate particles, allied to the degree of
preparation and straightening carried out
to the wall, in advance of application of the
finish coat, which dictated the characteristics
of the harling finish.
The most commonly used traditional
finish for harling was ‘as cast’, as this was
the quickest and most efficient option. There
are, however, some historic examples where
the cast finishes were gently pushed back
to remove the high-spots, or floated back to
give a flatter finish. Flat finishes were often
lined out to provide a more formal finish –
the best examples imitating a high quality
ashlar stone facade. Such finishes became
more common on town buildings and