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.

  • 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’.

New Built Barn finished with lime harling.

  • 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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