Technical papers

Published with permission of Scott & Company – Chartered Surveyors and Historic Building Consultants 

Introduction

A building may be being re-plastered for various reasons. The original plaster may have failed, or possibly a modern plaster has been applied to an older building which has proved to be defective. This fact sheet is prepared to give some background information as to why the re-plastering may be necessary and some of the problems that may arise because of earlier errors and failures.

Why re-plaster?

In many instances the plaster that is on a traditional building has come to the end of its natural life. Plaster is only a sacrificial decorative coat. Originally, very thin skim plasters or multiple layers of limewash were applied to walls to give a surface and decorative treatment. Sometimes, these earlier systems can be seen under the plaster when it is removed. In older buildings express care is needed to ensure that there are no vestiges of mediaeval wall paintings below subsequent plaster systems. This needs to be carefully checked.

Traditional plaster was built up using lime mortars. Some early plasters were very thin and were barely more than lime washes. Some plasters were almost a neat lime mix and some show large lumps of unslaked lime within an aggregate mix.

Over the last 130 years cement has come into use and has become more predominant as a material for plastering in the last 50 years. It is a quicker and less labour intensive material to use, and hence has many attractions and supporters. It is not a traditional material and has now proved to be problematic.

The breathable wall

Traditional mortars and plasters using lime enable the wall to breathe. Lime mortars permit evaporation of moisture from within the core of the wall thus enabling an equilibrium of damp to be maintained. This applies both to lime pointing and lime plaster. If the wall is not allowed to breathe, moisture will build up within the core and change its character and performance capabilities. With this moisture accumulation comes various degrees of salts. These are natural salts attracted up from the ground, nitrates and, in buildings relatively near to the sea, the structure will get affected by sea salts.

Problems with cement

Many buildings have had cementicious based plasters applied internally and cement mortar used externally. Cement is considerably less vapour permeable than lime and it is a rigid material. Lime mortar will allow free cyclical movement of the building, which is inevitable; if any minor fissures or cracks arise in the lime mortar there will be a natural migration of free lime through the mortar to these fissures which will self heal this cracking. This is enabled by the vapour permeability and the wetting and drying process along with the evaporation and free lime within the lime mortar.

With the cementicious mortar, which is rigid, cracks will arise when the building moves. Every building moves cyclically due to thermal expansion, variations in moisture, seasonal changes and temperature differences. In a controlled manner, this is not something to be of concern in traditional construction.

With a cement mortar, however, cracks arise around the stones and through the mortar pointing itself externally. Moisture then enters through the cracks and accumulates within the core of the wall, changing the character and moisture content. As evaporation is not possible through the exterior cementicious pointing to the same degree as lime, inevitably problems will arise within the interior. If a hard cementicious plaster has been used on the internal walling this will encapsulate the moisture, and salt levels within the core of the wall will increase. In some situations, the earthy, cobby, lime based core mortar of the wall turns to liquid mud because of this encapsulation. Clearly, this is most unsatisfactory.

Re-plastering

We have learnt from our errors. It is essential, before deciding how to adopt any re-plastering scheme, to assess the extent of cementicious pointing and internal plastering that the building has been subjected to. It is also necessary to assess the moisture within the core of the wall. If the building has been pointed externally and plastered internally with a cementicious plaster, it is strongly recommended that the wall be allowed to dry out for as long as possible. Ideally this should be permitted if either or both cementicious applications have been made. It is because of this that we leave the walls without plaster for a reasonable period of time to try and gain some equilibrium in the moisture content within the core of the walls. At this time transmission of salts to the wall surface may be apparent. This may show itself in furry, powdery growth. This should be carefully brushed off and removed.

In the meantime an equilibrium will be achieved and a decision will have to be made over the time of plastering. It is not possible to ascertain how much salt is within the core of the wall, as clearly this would be destructive. A further application of a moist plaster to the walling may reactivate salts and bring them to the surface. The new plaster surface will, in itself, act as a vapour permeable material which, because of its good drying and evaporation qualities, will act in a similar way to blotting paper. This may encourage further drying out of the core of the wall and again, with it, transmission of salts.

Ideally a decorative lime wash should be applied to the plaster. This again is vapour permeable and will act as blotting paper and will again attract salts and moisture to the surface. This will continue until a further equilibrium has been achieved.

It is important to note that:

It is not possible, without substantial investigation and expense, along with possible fabric destruction, to ascertain exactly how much salt is within the core of the wall and how this will react. It should also be noted that having a vapour permeable breathable surface to the interior and a traditional lime pointing to the exterior, that the moisture content of the wall will be affected by good and bad periods of weather. In excessively wet periods when there is continual rain and little change of evaporation, the moisture content within the wall will increase and this may show itself as further salting and damp to the interior. The aim of this exercise and therefore the consolation that must be taken from such appearances of moisture, is that the plaster and pointing system is vapour permeable and will therefore allow water to evaporate out, thus maintaining a satisfactory condition and equilibrium to the walls.

Summary

Regretfully many buildings have poor inheritances due to the inappropriate use of modern cementicious or vapour impermeable surfaces. This can also apply to maintenance free and polymer based paint systems. Many of these historic buildings have existed quite happily over hundreds of years with traditional lime mortars enabling evaporation and breathing to continue unabated.

In some instances, historic or older lime plasters become addled because of continual salting and damp, and after a while will need to be replaced. It is only correct and natural to replace them with a lime based plaster to maintain the character and equilibrium of the walls. In certain instances buildings have to have the inappropriate cementicous plaster taken off and therefore it is inevitable that because of long periods of inappropriate plasters and pointing, that the wall will need to relax and become balanced. The use of inappropriate materials earlier in the building’s life may result in substantial salt migration to the plaster which may cause major problems to the decorations and may cause problems to the plaster. This needs to be carefully watched and assessed. In many instances only very limited, if any, problems arise.

In conclusion, one must expect some damp and salting. The degree of damp and salting cannot be ascertained at an early stage. Consolation, however, must be taken from the fact that one is reinstating a breathable vapour permeable system within the building, thus maintaining the historical integrity and wellbeing of the course structure. Without any question of doubt, it is totally inappropriate for moisture to be encapsulated within the core of the wall following the application of cementicious internal plasters and external pointing.

David N Scott
B.Sc Dip Bldg Cons FRICS
Chartered Building Surveyor