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Lime render is the first coat of lime "plaster or the like" [1] applied to the external surfaces of traditionally-built stone or brick buildings. It allows the building to 'breathe' – as lime is porous, it allows for the collection and evaporation of moisture .
For internal walls, two coats is the standard and follows the same method as for external rendering but with a weaker mix of five or six sand to one cement and one lime. However, instead of being finished with a sponge, the second coat is left rough and sometimes will be scored by nails inserted in the float.
Harling is a rough-cast wall finish consisting of lime and aggregate, known for its rough texture. [1] Many castles and other buildings in Scotland and Ulster have walls finished with harling. It is also used on contemporary buildings, where it protects against the wet Scottish and Ulster climates and eliminates the need for paint.
Lime comes from Old English lim ('sticky substance, birdlime, mortar, cement, gluten'), and is related to Latin limus ('slime, mud, mire'), and linere ('to smear'). [7] Mortar is a mixture with cement and comes from Old French mortier ('builder's mortar, plaster; bowl for mixing') in the late 13th century and Latin mortarium ('mortar'). [7]
Traditionally, a render would be manufactured on site by a plasterer mixing sand, cement and sometimes lime material together with water to produce his render. This would then be applied to the walls, usually in either two or three coats. [6] When painting, there is usually a primer, an undercoat and a topcoat.
It is usually not recommended to replace more than 20% of the lime content with cement when rendering the facade, and it is a matter of contention whether adding any concrete is ever appropriate in order to maintain the benefits of lime over concrete. Lime plaster is permeable and allows for the diffusion and evaporation of moisture.
In old buildings with walls that shift over time, cracks can be found which allow rain water into the structure. The lime mortar allows this moisture to escape through evaporation and keeps the wall dry. Re−pointing or rendering an old wall with cement mortar stops the evaporation and can cause problems associated with moisture behind the cement.
Cement render or cement plaster is the application of a mortar mix of sand and cement, (optionally lime) and water to brick, concrete, stone, or mud brick. It is often textured, colored, or painted after application. It is generally used on exterior walls but can be used to feature an interior wall. [1]