Troubleshooting Guide - Lime Plaster Crumbling

Crumbling Lime Plaster can be caused by a number of reasons:

1) Crumbling plaster/render can usually be attributed to a lack of carbonation of the lime within the plaster/render meaning that the plaster/render will have little if any, strength. Lack of carbonation can be caused by exposure to freezing temperatures causing expansion of the lime as it is absorbing CO2 and prohibiting a coherent plaster/render being formed. It must be remembered that it can take years for a fat lime to reach its full strength and so should be applied as soon in the year as the weather will permit, usually from March onwards.

2) The use of an aggregate that is too fine or poorly graded can restrict the movement of CO2 into the plaster/render, vastly reducing the speed at which it carbonates thereby leaving it venerable to attack from frosts.

3) If the render is exposed to heavy rain soon after application lime can be washed out of the mortar leaving the affected areas devoid of a binder and as such crumbly to the touch.

4) Inappropriate lime selection for the exposure of the render. This can lead to undue weathering and ultimately failure. It is crucial that the different types of lime are understood so that an informed decision can be made for the job at hand

5) Poorly maintained or defective rainwater goods leading to water being shed down the face of the building causing the render to become saturated and ultimately fail during periods of extended cold weather.

Resolution

Ty-Mawr products use the highest quality British quicklime and are produced under the strictest of conditions in accordance with ISO9001:2000. It must be remembered that the carbonation of lime is a very simple chemical reaction and there is very little to go wrong with it, problems are usually caused by the inappropriate selection or use of materials.

We start to issue winter weather warnings with all of our lime based products from the middle of August as lime products continue to gain strength markedly up to and beyond 90 days. So you must think well ahead and consider what the temperature is likely to be over the coming months when undertaking work with lime. If the temperature falls below 5ºC for any length of time before the mortar has gained sufficient strength then there is likely to be problems in the future.

Rainwater goods should be checked annually towards the end of the summer for the upcoming winter. Protection of new work from extremes of weather is vitally important in ensuring the longevity of the plaster/render.

The reason for the failure should be identified before any remedial work takes place so that the problem is not repeated. The plaster/render should be raked out and reapplied in accordance with the manufacturer's instructions.  Cement and modern admixtures must be avoided at all times as these will have a deleterious effect on the mortar.

As well as providing training courses and producing the instructional ‘The Lime Handbook’, Ty-Mawr does provide an on-site supervision and advice service that can be requested.