Lime Harling

Harling is a Scottish name for what is often called wet dash or roughcast. This is probably the most traditional finish that is still common today. Like all external finishes, its purpose is to act as a protective outer layer that gives good weathering characteristics. This is mainly due to the combination of large and small aggregate. This combination gives a large surface area, which can shed moisture easily.

Introduction

The lime coating known as harling or wet dash is the most commonly applied external finish to be found on vernacular architecture in Ireland. The harling material is a combination of aggregates and lime, mixed into a slurry consistency and applied in a fluid state. Historically the harling was applied directly onto the masonry walls which had previously been evened out by pointing the wall flush and filling small holes with stone pinnings and mortar. Towards the end of the nineteenth century it became commonplace to apply one or two towelled undercoats to flatten the background before casting on the lime harling.

An extension of this practice became known as dry dashing or pebble dashing, where dry shingle is cast into a wet adhesive coat. However, in traditional harling the finish coat is applied directly to the masonry background.

Preparation of the Masonry Background

Successful application of lime harling depends greatly on the amount of preparation that is carried out on the masonry background. External lime coatings are generally applied in relatively thin coats, therefore any masonry defects in the form of hollows or missing pointing must be corrected before the application of the harling coat. The correction of these defects should be carried out during masonry preparation and not rectified through coats of “dubbing out”. Areas of varying thickness are prone to shrinkage, carbonation and curing problems. Careful background preparation plays a vital part in the weather resistance capability of the wall.

Materials to be used in background preparation, should, wherever possible, be matched to the existing fabric. In doing so, the repair will be compatible and produce a similar performance pattern. Where previous remedial work has taken place with unsuitable materials, (i.e. dense cement mortars), these should be removed and repaired with matching mortars or stone.

The background must be free from dirt, grease and vegetation. These elements should be removed several weeks before repairs are underway. The removal of biological growths should be thoroughly carried out, as any remaining lichen, algae, etc, will grow back and attack the bonding between lime finishes and the background.

Suction Control and Bonding

Before the application of any new lime coatings it is vitally important to check the degree of suction within the background. Poor or excessive suction can result in a weak bonding with the substrate caused by rapid de-maturing of the newly applied render. This will result in a weak and powdery interface which could lead to later failure and separation.

In situations where suction needs to be controlled, wetting down will be required. On dense blocks or near impervious masonry, simply dampening the surface with a mist spray may be all that is required.

On very porous surfaces such as old brickwork considerable wetting will be required. Wet the wall with a hose, working from the top of the structure, downwards, this may need to be carried out the previous day and several times on the day before rendering commences depending on the substrate.

The objective of the suction control is to achieve a thoroughly damp surface, but not wet, i.e., the surface must not have running or standing water remaining on the masonry or brick, this could form a barrier between the coating and substrate.

A certain amount of suction is required for lime mortars to adhere and stiffen so the exact amount of wetting down is decided by the plasterer’s experience.

On dense or near impervious background, it may be necessary to apply a sand/splatter dash coat to the background to act as a mechanical key.

Salt Contamination‍

Where new lime coatings are to be applied to masonry which is salt contaminated, the masonry should be allowed to dry fully before applying new renders. This will allow salt to be detected on the masonry and mortar joint surfaces, if excessive salt is identified clay or lime mortar poulticing may be required. Specialist advice should be sought.

Salt contaminates should never be washed from the surface, as this will result in the crystallized salt returning to a soluble state and retreating back into the pores of the masonry or brick. Salts should be brushed from the surface and cleaned away from the structure.

‍Techniques of Application‍

New applications of lime harling are generally applied in 2 coats with an approximate thickness 8mm to 6mm respectively, single coats can be applied, but it must be remembered that these thin coats will not have the same durability. The overall thickness will of course be effected by the size and shape of the aggregates used. To replicate a traditional harled finish, the selection of tools used and method of application is essential. The best tools for harling are the purpose made harling or scudding trowels (many plasterers use a small coal shovel with great effect), these tools allow an even spread of material over the whole blade of the trowel, which helps with an even coat of material against the wall. The technique of application requires that a small amount of the lime harling is scooped up with the trowel, and with a flick of the wrist dashed against the wall. This method requires a degree of practice to achieve an even material thickness. Forehand and backhand casting techniques will allow difficult and awkward areas to be reached and ensure total coverage. Variation of texture and pattern are typical of traditional harled finishes and are an indication of different casting techniques and style of tradesman’s personal preference.

Machine applied sprayed finishes have been introduced into lime finishing, and although their performance is technically sound, the finished work bears little resemblance to a traditional finished harl.

‍Harling up to Details

‍Where details such as stone quoin of window dressing are encountered, which stand proud, the harling can be tucked neatly behind the finished edge, this will protect against water penetration, where stone details have no distinct edge the harling can be feathered out, this can be achieved by using progressively thinner material.

Harling Finishes

It is generally considered that there are 2 distinct styles of finish to lime harling, the first style is the “as cast” style, where the material is simply left as cast onto the masonry without any further adjustments, the second style is a flatter finish in which the cast harling is pressed back against the wall with the aid of a timber float to even out the surface. Both styles are traditional and choice of finish should follow existing examples.

Health and Safety

Harling by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to eye and skin protection.

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Summary

• Harling is a textured finish that was traditionally thrown by hand using a large flat trowel.

• Harling comes in many types and textures from very small aggregate to aggregate up to 20mm.

• A new application of lime harling onto a sound, even background should generally be in two coats of approximately 8mm and 6mm thickness respectively. A thin single coat application is also possible but this may be less durable in exposed locations.

• Where a thicker harl is required, the material should be built up in several thin coats. As in modern construction, it is good practice for each subsequent coat to be thinner, weaker and more permeable than the preceding coat.

• Before work commences, the background needs to be cleaned down of any loose material and vegetation. Any moss or lichen must be thoroughly cleaned off and the walls should be sprayed with a moss-controlling agent as per the manufacturer’s instructions.

• Having cleaned the background, some localised patch pointing may need to be undertaken.

• Before work commences, mask off and cover exposed windows and stone etc.

• When mixing the dry materials use gauging buckets to ensure consistency of mix otherwise variations could show up in the finished product.

• The harling mortar should be mixed to a rather fluid consistency which when thrown should spread easily but not slump on the wall. The best tool for applying harling is a purpose made ‘harling trowel’.

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• Before throwing on the dash, damp down the background but do not kill the suction by over wetting.

• Modern methods of mechanical application, although they may be technically sound in some situations, are unlikely to produce an acceptable result in traditional harling work. If spray application is used, it should always be finished with a hand cast top coat.

• The first coat of harl may need to be lightly pressed back with the back of the harling trowel, or with a stiff bristle brush, as it starts to set, to remove any high spots. Care should be taken not to over-work or smooth the surface, but just to push it back. Each layer of lime material should be allowed to cure, normally under protective coverings, and then re-dampened before applying another layer.

• If two or more coats are applied, they should be applied while the previous coat is still fresh/green. This is important for better adhesion between coats.

• On masonry features such as quoins etc. the harling may be feathered out (the material can be cast progressively thinner as it approaches the detail so as to die away to nothing) to expose these features. Where harling stops against dressed stone masonry care must be taken not to form raised edges. These edges are vulnerable to water penetration which may lead in the future to detachment of coating. Details such as raised margins and string courses offer protection, allowing the harl to be tucked behind. Details without a positive edge, such as crow-steps, and external angles, require extra thought and skill in application.

• If patch repairs are to be undertaken, then the surrounding material needs to be analysed to ensure a correct match. With this information it is possible to repair damaged areas of harling quiet successfully. These will have to be built up in wet thin even coats so as not to stand out.

• Continue to dampen down the finished work for up to ten days.

• The final finish should be painted with a breathable paint finish such as limewash or a silicate masonry paint, to provide an extra barrier to the weather and to complement the overall effect.

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