‘Harl’, also known as ‘harling’ (or lime harling), is a technique for weatherproofing the exterior of masonry buildings, traditional to Scotland and Northern Ireland. It is commonly found on Scottish castles as well as more common building types, and was favoured due to its practicality and suitability for the harsh, wet climate.
The term ‘harling’ derives from the action of hurling wet mortar at a solid wall. This is known as a ‘cast-on’ finish, as opposed to a floated base coat (traditional to England), or rough-cast work such as pebbledash. In Ireland, it is commonly known as ‘wet dash’.
Harling consists of a slaked lime and coarse aggregate mortar which is thrown onto a stone wall, using a slurry of small pebbles or fine stone chips. By embedding a pigment in this material, the need for repainting can be avoided. A specially-shaped trowel is used to throw and then press the material into the surface. Cast-on coatings tend to provide better resistance to weather as the mortar is better compacted and more uniform throughout its thickness compared to trowelled-on coatings.
As the harl is mostly lime render it cures chemically rather than by drying, to provide a weather-protective and decorative coating. Once the harl is set it can be lime washed in various colours.
 Related articles on Designing Buildings Wiki
‘Structures and structural failure’ at IHBC’s Nottingham School, with Ed Morton (ex Canterbury, York and Westminster to St Paul’s) and John Ruddy.
Ageing gracefully - restorations which retain historical decay.