External Lime Putty Renders

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Non-hydraulic lime putty based external render produced from 6 month mature Chalk lime putty mixed with particular sands in proportions of 1:2 or 1:2.5. Lime putty renders are normally reserved for early buildings. Natural hydraulic lime is more often used for external renders.

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Description

Lime putty renders are normally reserved for early buildings. Natural hydraulic lime (RJ1.05) is more often used for external renders. Find technical advice in our advice page on:

Internal Plain Lime Plastering
Hair/fibre reinforcement in basecoat plasters
Use of Haired Base-coat & Fine Finish Plaster

Environmental Considerations

Lime putty render is an environmentally sustainable product.

Usage/Purpose

For all coats of external render. Do not use below a temperature of 5°C or above 25°C. Do not use if there is risk of frost.

Coverage

50m2 /tonne at 10mm thick and 62m2 /tonne at 8mm thick.

Shelf Life

Standard basecoat renders now contain fibres and sisal, as there is no issue associated with deterioration of reinforcement. Animal hair deteriorates rapidly in wet plaster and hair reinforced plasters must be used immediately, preferably adding the hair on site immediately before use. Lime putty renders stiffen during storage, but can be plasticised by thorough re-mixing normally without the addition of water. The render will begin to set if a pozzolan is added. Do not remix pozzolan gauged render.

Colour

Mortar colour is determined by the sand or aggregate mixed with the putty.

Observations

It is very important that tradesmen with experience of the successful use of lime putty renders are employed, as they do not perform as hydraulic lime or cement based materials.

Application

Fibre reinforcement is added to basecoats at a rate equivalent to 2.5 kg/tonne (or 3.5 kg/tonne when applied to lath). Basecoats are applied maximum 10mm thick and top-coats normally c.8mm. Top coats are normally finished with a wood or sponge float, but can be thrown or sprayed on as roughcast or harling. It is often appropriate to site-gauge with a pozzolanic additive (RJ7.01). Lime putty renders must be finished with external grade casein or tallow limewash (RJ3.02/RJ3.03). It is very important that tradesmen with experience of the successful use of lime putty renders are employed, as they do not perform as hydraulic lime or cement based materials. Although there is a weak hydraulic reaction if a pozzolan is added, the primary setting mechanism is the reaction with atmospheric CO2 in the presence of moisture. This ‘carbonation’ is slow, and control of the drying and curing process by mist-spraying, ‘tending’ and protecting is crucial to success and long-term performance. Protections are essential. Do not use below a temperature of 5°C or above 25°C. Do not use if there is risk of frost.