Lime Harling
Craig Frew
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| Craigievar Castle, Aberdeenshire: hand-cast ‘hot lime’ harling and pigmented limewash (Photo: LTM Group) |
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.
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 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.
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| Lime harling which is at least 150 years old on an old church in Orkney | New, lightly pressed back lime harling and limewash finish at Brodie Castle, Forres |
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 cementbased 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.
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| Hand-cast harling – note the flat trowel used to throw the harl. (Photo: LimeRich Stonemasonry & Plastering) |
Applying harling by mechanical spray: in some circumstances this method may be appropriately used for
the pricking up or straightening coat, prior to the application of a hand-cast finishing coat. (Photo: LimeRich Stonemasonry & Plastering) |
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
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| Inchnadamph Church, Assynt: mechanical spray applied lime harling and plain limewash |
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 trowel’ to take up its initial stiffening. Prior to hardening, the mortar is compressed back with a trowel to ensure a tight bond with the adjacent stone. Suction control as previously described is critical in achieving a good bond between stone and mortar. Finally, the dubbing out coat can be scraped back with the edge of a trowel to open up the surface to aid the curing process, and also to provide a good key for the pricking-up coat. The dubbing out should bring the wall face to a suitable level for application of a thin harling coat. In certain circumstances, where the wall face is relatively sound and flat, a single application of harling will be sufficient, omitting the pricking-up and straightening coat.
Pricking-up coats These are applied in a similar manner to the harling, either by the traditional method of hand casting the mortar using a specially shaped trowel, or with mechanical spray equipment (where appropriate). Pricking-up coats generally comprise a thin, binder-rich and gritty coating which acts to even out the background suction levels. This is particularly important where high capillarity lime mortars exist alongside impervious stones. It also provides a good mechanical ‘key’ for the subsequent harling, hence the requirement for this mix to contain a sharp, gritty sand. No attempt is made to ‘straighten’ the wall faces with harling at this stage (this should have been done as part of the dubbing out process) as variable thicknesses can lead to an inconsistent appearance in the finished work and can impact on its performance.
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| Wardlaw Mausoleum, Kirkhill: typical surface weathering of ten-year-old lime harling and limewash |
Straightening coat This may be required where the background masonry is particularly uneven and/or where a flatter finish is required. It is important to bear in mind that the final finish coat will be a relatively thin coating of consistent thickness, so will not mask any unevenness in the background. Typically this coat is cast or sprayed on, for the best adhesion, scraping the mortar from the high points (a plasterer’s straight edge can be used) and filling the low points as work progresses – it is important not to overwork or ‘move’ the mortar around too much on the wall as this can weaken its bond with the background.
Finish coat Finally, the harl is applied to a consistent thickness and texture. Careful planning is required for large elevations without natural breaks such as internal or external returns or string courses. Visible day joints can be avoided by ‘fairing’ out the harling (gradually reducing its thickness) in a wave like shape (not a straight line) and blended in the following day. It is also essential that the scaffolding is designed specifically for the purposes of harling with removable inner boards to avoid the risk of ‘lift lines’ in the finish. Fairing into vertical faces of returns and reveals is a typical, traditional detail, and harling is normally faired out at the base of the wall to discourage moisture retention.
The number of coats and relative thicknesses of harling will vary from building to building depending on the type and condition of the substrate, and the type of finish and appearance desired. Similarly the timing between coats for curing and drying will vary depending on the materials selection, number of coats and thickness of coats. Individual coats of harl should generally be no thicker than about 10mm – often traditional Scottish lime harling is less than this in overall thickness. Once the lime harling is complete, it should be allowed additional drying time, after curing. Any patchiness in the moisture levels across the harling may lead to an inconsistent appearance when limewashing. This will be more pronounced when using stronger pigmented limewash.
THE STATE OF THE ART
Reinstatement of previously stripped harling finishes to traditional and historic buildings is now becoming more prevalent. One of the challenges over the last two decades has been re-learning the almost lost traditions in terms of both materials and methods. Much of the early ‘lime-revival’ harling work carried out in the early 1990s was founded on the practices employed in the application of cement harling/ pebbledash or plasterwork, where float applied base and straightening coats were commonly used. Today, much of the harling work carried out utilises excessively-strong modern hydraulic limes to over-compensate for some of these earlier failures, as the mechanisms involved in the application of lime mortars, along with the properties of the range of binders now available are still to be fully disseminated and applied across the industry.
Mechanical spray application of lime harling is also becoming more commonplace. There are advantages of spray application in that a lower water content is required for application, minimising the risk of drying shrinkage and creating a more compacted mortar finish. The action of spraying under pressure also helps to ensure a good contact between the substrate and the lime coating. However, if re-instating a harl finish to a traditional or historic property, it can be difficult, if not impossible, for a sprayed-on coating to match the texture and appearance of a traditional hand-cast finish. In such circumstances, a hand-cast finish should be specified for the finish coat, even if the underlying coats are to be mechanically sprayed.
Many of the materials and methods used for lime harling over the past two decades bear little resemblance to traditional lime harl coatings. We still have much to re-learn from traditional practice. Harling which has survived for over a century on many of our traditional and historic buildings can tell us much about what we should be doing today, and it is through organisations such as the Building Limes Forum that we can share experience and understanding of traditional building limes.
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Further Information
The Building Limes Forum exists to encourage expertise and understanding in the appropriate use of building limes and education in the standards of production, preparation, application and after-care. It is a charitable organisation with no commercial ties, and has about 350 members in the UK and overseas, the majority being actively concerned with either the repair of historic buildings or the use of lime in new build.
Courses
Several organisations provide short practical courses in the use of lime, including the Scottish Lime Centre Trust. See our short courses and events pages for more information.
Recommended Reading
Technical Advice Note 15: External Lime Coatings in Scotland, Historic Scotland
English Heritage, Practical Building Conservation: Mortars, Plasters and Renders, Ashgate, Farnham, 2011
