Troubleshooting Guide - Lime Plaster Cracking

Cracking in Lime Plaster can be caused by a number of reasons:

1. Over trowelling, especially a problem with the top coat as over vigorous working can lead to excessive fat being brought to the surface of the render/plaster which can then shrink and crack due to the mix being altered by the overworking resulting in a lack of aggregate in the surface of the plaster/render.
2. Sulphate rich (usually black) mortar beds causing complex chemical reactions in which the products have a greater volume and thus can cause expansion on the joints. Usually, this can be identified as a very uniform set of cracks conforming to the mortar beds themselves.
3. Lack of moisture control – Drying and carbonation is two different processes, just because a plaster/render is dry it does not follow that it has carbonated. Drying is just the process of moisture leaving the plaster/render, whereas carbonation is the absorption of Co2 from the atmosphere and this is how the mortar gains its strength. In carbonation of lime can only take place in the presence of water. It can years for a new plaster/render to reach its full strength. If plaster/render dries before sufficient carbonation has taken place then cracking can occur. Therefore the suction of water from each successive coat of the plaster/render must be controlled by wetting down the background beforehand, then regular misting with water and protection from the wind and the sun are crucial in tempering this process.
4. Plaster/render too wet – Pre-mixed fat lime mortars will rarely require the addition of any extra water. Lime is thixotropic, this is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. If a mortar is inadequately mixed or ‘knocked-up’ the addition of water can have the effect of making the mortar more fluid, however, this causes the mix to be overbulked. Lime reduces in volume when it dries and carbonates, as a result over bulking with water will exacerbate this effect, this applies also to hydraulic lime mixes.
5. Lack of protection from wind/sun – This will cause the same effects as point 3) (above).
6.  Inappropriate aggregate choice – The aggregate in the mortar improves strength and reduces the effects of shrinkage. It is recommended that the aggregate in the mortar is no smaller than 2/3 of the depth of the plaster/render. So for a 6mm graded aggregate, it can be used to plaster up to 9mm. If the aggregate is too small relative to the thickness of the plaster then excessive cracking will occur. We recommend that beach sands are not used as even after washing they can still contain salts that can be detrimental to the mortar. We produce a sand that we call ‘Beach’, it only had the appearance of a beach sand and is not sourced from the marine environment. 
7. Not properly compressed - In the first few days after plaster/render is applied as it carbonates a degree of shrinkage will occur and this is quite normal. The process of floating plasters and renders compresses them and helps to consolidate them; this must be done prior to carbonation whilst the mortar is still plastic. 
8. Too much plaster/render used, more than 25mm of mortar in joint – If the joints that have been raked out are deeper than approx 25mm, then the pointing must be built up in layers of 25mm leaving sufficient time between each application for shrinkage and carbonation to begin.
9. Not adequately mixed – For hydraulic limes leave to mix for at least 20 mins after all water has been added to aggregate and lime to ensure that all of the lime is fully hydrated and that the sand is evenly distributed throughout the mortar, longer mixing times will result in less shrinkage and cracking of the resultant mortar.  Fat lime mortars require ‘knocking-up’ before use to eliminate the need for the addition of water.
10.  Movement within the background- e.g. as caused by subsidence or ceilings that are subject to high stresses such as below high traffic areas within a building.  Insufficient numbers of fixings used on insulation boards can also contribute to this we recommend at least 8 per sqm.   
11. Incompatible materials abutting- Can be caused by differences in thermal expansion by different types of lime abutting each other. For example, a fat lime external render is patched with a strong hydraulic lime; this problem will be especially prevalent on south facing aspects. This can be identified usually by cracking along the join between the two material types.

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.

Cracking in the plaster/render should be investigated in such a manner as to determine the causes of the cracking. Cracking is perfectly normal in the basecoat and also the float during application, as long as the plaster/render has not delaminated from the background or previous coat of plaster then there is no cause for undue concern. If the cracking is however in the finished plaster/render then the causes must be investigated.

Externally if the render seems unsound, then affected areas can be removed and then must be re-applied following manufactures guidelines. If the cracking is confined to the top coat, and is extensive, the entirety of the render can be filled and painted using the Beeck Quartz Filler (breathable mineral paint system), this is a hard-wearing alternative to limewash. Please contact Ty-Mawr for details about this application.

Internally fine cracks can be filled with either casein filler or Agalia fine surface filler which can then be sanded back and decorated with an appropriate breathable finish.