In order to care for traditionally constructed buildings – to exclude the weather and maintain comfortable living conditions, and to retain their unique character – it is important to appreciate the way in which they work. You will be familiar with the technology of a Gore-Tex or Harris Tweed jacket versus a plastic Macintosh. One manages moisture levels by allowing ‘breathing’ and evaporation; the other excludes all water (provided there are no holes) but causes unpleasant sweating. In the same way, the technology of traditional building construction is very different from that of most modern buildings. Traditionally constructed buildings tend to rely on an ability to handle and manage moisture within the building fabric. Modern construction aims to totally exclude moisture.
Traditional masonry structures were built as mass construction, generally without the application of impermeable finishes, and without the incorporation of membranes and damp proof courses which modern construction employs. Most masonry materials and the bedding mortars are porous and permeable to some degree. This improves their ability to take in and release water and water vapour. In addition, many buildings were originally finished with a coating of lime harl. Lime mortar used as an external coating material will exclude a certain amount of water but remains essentially porous and permeable. Access water taking into the wall finish will evaporate again, conditions are low. In a continuous cycle of wetting and drying, rainwater is retained mainly in the outer part of the wall; the lime finishing layer minimising water movement or retention, and associated deterioration, in the masonry itself. In severe conditions, a mass of lime in the core of the wall can mop up penetrating water and minimise the risk of penetration to the interior.
Another difference occurs in the way traditional and modern buildings react to movement. All buildings flex in response to thermal and seasonal changes, or even to ground movement. Traditional mass-wall construction handles this movement through a degree of overall flexibility, brought about by the use of numerous small units (stones and bricks) set in a relatively soft mortar. Modern rigid construction requires incorporation of specially designed ‘movement joints’ at intervals in the construction. The characteristics of breathability and flexibility are also important, and the availability of compatible, breathable materials is critical to the well-being of traditional houses.
Using Traditional Materials & Techniques
The most effective method of repair and maintenance will almost invariably involve the use of materials and techniques employed in the original construction of the house. These might include clay or earth mortars or traditional lime mortars, all of which are more permeable and more flexible than cement mortars and contain fewer potentially damaging salts. They also look better – it is simply not possible to achieve an authentic appearance without the use of authentic materials. The imposition of hard grey cement in a rigid pattern of jointing, emphasising each individual stone, or the application of a brittle cementitious render finished with modern paint system, is alien to the expected softness and warmth in the appearance of a traditional masonry or harled wall. As well as altering the appearance of traditional buildings, cement-based mortars will also alter the performance, almost invariably for the worse.
Damage to traditional masonry buildings as a result of inappropriate repairs is now a widespread problem. The search for permanent maintenance free treatments during the second half of the 20th century has left a legacy of accelerated decay in stone’s surfaces and has caused significant problems of dampness and timber decay. In selecting materials for the repair or replacement of mortars, renders and plasters, we should be considering the range of materials traditionally used. This will include earth and clay mortars, with or without various additives; lime mortars of various types; mixes containing combinations of earth and lime; and perhaps, some of the latter, harder setting, mortars.
Although lime seems to have been known as a building material for over 7000 years, its use in domestic buildings in Scotland is much more recent. Early mortars will most likely to have been earth-based materials, perhaps reinforced with vegetable fibres such as straw, or in the form of turf. Although lime mortars were used in many of the grander buildings from medieval times, they may not have been in common domestic use much before the 17th century. Once the use of lime for agricultural improvements was appreciated, it also quickly became accepted as a common building material. This took the form of construction mortar and mortar for external coatings and internal finishings, and for use as paint; and a parallel tradition developed alongside that of earth-based mortars. Lime-based mortars have been widely used from the late 17th century, both in urban development and in the standings and cottages of agricultural improvements.
During the 18th sets and 19th centuries use of lime mortars of various types became the norm, alongside a gradually declining local use of earthen mortars. In the early 20th century an increasing use of cementitious materials occurred and by the mid-20th century, their use was almost universal. A realisation of the damage that can be caused to traditional buildings by the use of excessively hard impenetrable mortars has resulted in the recent reintroduction of lime mortars for repair and conservation.
Earth-based materials were normally extracted locally, tempered by working and sometimes reinforced with vegetable fibres. Earth or loam might be used with or without the addition of sand or gravel. Clay would normally be combined with a coarse sand or gravel filler. Pointing up with lime mortar where this was available might protect joints in earth-bound masonry walls, or the external wall surface may be coated with clay or lime-based render, protected by a coating of limewash. Internally, wall surfaces might be plastered with clay, with or without a limewash finish or perhaps coated in a lime plaster. Other special mortars, such as clay/cow dung mixes, could be utilized for chimney lining or for external weatherproof finishes. Where lime was available, there is evidence for mortars and plasters containing clay/lime mixes. Clay-based building materials are excellent for managing moisture and humidity levels in buildings and, provided they are maintained and not compromised by the introduction of impermeable materials such as a cement-like plastic paint, it will continue to function effectively.
Lime Production In Scotland
It is reasonable to assume that local sources of lime where utilised where available, although there is evidence that these were being supplemented by the output of larger scale lime works by the late 18th century. Within the complex geology of Scotland, limestone deposits were variable, giving rise, historically, to locally varied mortars, as well as providing a rich source of potential types and strengths in lime mortars throughout the country. The majority of Scottish limestones produced feebly hydraulic lime mortars; others were not hydraulic or strongly hydraulic. Even within clearly defined geological areas, considerable variation occurred in the type and strength of limes produced.
Lime production involves bonding or seating limestone to convert calcium carbonate it to calcium oxide with the driving off of carbon dioxide. This process is normally carried out in a lime kiln, and the remains of many old kilns survive throughout Scotland. After burning, the calcium oxide (or quicklime) is converted to lime calcium hydroxide by combining with water in a process known as ‘slaking’. Lime mortars comprise a mixture of lime with a sand filler and, depending on the chemical composition and method of production, harden either simply by reabsorbing carbon dioxide or in a more complex process that also involves chemical hardening.
As explained above, the use of matching or compatible materials in repair and maintenance work is important. Earth and clay-based materials for repair or maintenance are generally obtained locally, although some commercial supplies are starting to become available. Although unfortunately, lime is not produced commercially in Scotland at present, this could change in the foreseeable future. In the meantime, supplies of English and continental limes are available from Scottish outlets. Lime comes in a wide range of different properties and strengths, enabling the characteristics of surviving historic materials to be matched and providing the new material suitable for a variety of applications and exposure conditions. Current programs of research are investigating the influence of lime burning and production methods on performance, with the objective of reintroducing some of the complex materials found in surviving historic mortars.
Lime mortars, whether for use in bedding or pointing masonry or for external or internal coatings, are made by combining lime and a filler such as coarse sand. Fine sands may be used in finishing plaster and mortar repointing fine ashlar joints. The softer setting mortars (non-hydraulic mortars) can be made up in advance of use and are available, ready-made, from suppliers of traditional mortars. Harder setting mortars and hydraulic mortars must be made up shortly before use and are normally made on-site by combining dry bag hydraulic lime with sand and water. The selection of mortar type appropriate to the job and the location is important.
Repair And Maintenance Strategies
Before undertaking a programme of repair works, careful investigation is required to establish the original materials and to understand the dynamics of the building. Investigation and understanding of the architectural development of a historic building have long been accepted as the norm, whilst current practice also puts emphasis on the social and cultural importance. Alongside these concerns, an understanding of the technology of the building is essential if repairs are to be effective. Effective repair of traditional buildings relies on his holistic approach, taking account of the overall behaviour pattern of the building and its local environment.
Traditional permeable mortar materials require an overall environment of good building repair to maintain their decorative and protective qualities. Where other buildings elements are not adequately maintained, the poor performance of traditional mortar materials, whether in the form of coatings or jointing mortar, will be compromised. Once the dynamics of building performance, including patterns of decay, have been understood, a strategy for conservation or repair can be developed. In the past, the decay of lime harling, for example, has too often been regarded as an underlying failure of the material. This has led to its replacement by cementitious coating without investigation and resolution of the casual factors of failure, such as inadequate maintenance of rainwater goods or inappropriate building details.
Repairs might be required to jointing mortar, due to either gradual deterioration over many years, or, for example, to lack of maintenance of rainwater goods. Reinstatement of lime mortar jointing might also be required following inappropriate cementitious pointing, where the hard mortar has encouraged exhilarated decay in the stones themselves.
External lime coatings can survive for several hundred years given a favourable environment and appropriate building maintenance, but more frequent renewal is often necessary. Removal of cementitious coatings and the reinstatement of lime finishes is a common requirement. It will also be found that cement renders have trapped moisture in the wall fabric, leading to dampness, timber decay and, of course, to stone decay associated with the presence of sulphates or other salts behind the cement render.
The Life Expectancy Of Traditional Building Materials
Internal lime plaster finishes normally survive for a long period, except where decay or movement of timber substrates cause mechanical failure. Most internal plasters before the early 20th century are lime based, although many from the 18th century onwards all also have small gypsum content. The behaviour and properties of modern, wholly gypsum-based plasters are significantly different from those of lime-based plasters and these never newer materials should never be used to patch traditional plasterwork.
Techniques for use of traditional mortars, renders and plasters require skills which have been learned by practice. Major repair or reinstatement of and clay-based mortars should be only tackled by workers familiar with the materials and their properties. Similarly, the repair and reinstatement of lime-based materials require an informed approach.
Unfortunately, modern lime materials and skills do not, at present, always achieve the long activity of many traditional applications – but we are working on the problem. The successful use of
new lime mortars depends both on the materials themselves and on appropriate conditions and techniques of use. Skilled craftsmen and good site practice are of fundamental importance, as is the availability of high quality, durable lime mortars. The former can be addressed by training and the latter by research and development work, such as that currently practised at Charlestown in Fife.
The life expectancy of traditional permeable mortar materials can be directly influenced by any of the following: availability and selection of an appropriate good quality mortar; techniques and quality workmanship (including effective preparation of materials and backgrounds, removal of vegetation, careful and knowledgeable application and appropriate curing after application, usually involving some form of protection; building details; local environmental conditions; and the maintenance regime.
Traditional masonry buildings require regular routine maintenance as well as long-term maintenance. For many building owners a simple routine of checking, and do it yourself or local tradesmen care on the level of housekeeping maintenance, would minimise the need for large-scale repairs. At this level special skills are not essential – a commonsense approach and a basic understanding of the building and its materials are all that is required. For those owners who do not feel sufficiently confident to tackle basic housekeeping maintenance, help is at hand. This comes in the form of short courses or one-day workshops run by the National Trust for Scotland, the Society for the Protection of Ancient Buildings, and the Building Limes Forum, the Scottish Lime Trust, and others. Published guidance is also available from Historic Scotland in the form of the technical advice notes.
A good maintenance strategy will focus attention both on mortar materials themselves and on other aspects of a building’s fabric. Along with the routine tasks of keeping rhones clear of debris, checking and fixing all loose slates, and checking ground drainage, the touching up of any minor degradation of finishes should be normal practice. The lifespan of traditional external finishes will be significantly enhanced by prompt attention to minor defects. Monitoring of rainwater disposal systems – a basic requirement for any type of building – is critical to the performance of traditional mortar finishes.
Regular Building Inspections
Maintenance requirements for buildings containing earth and clay involve regular inspection and prompt making good of minor defects by patching in with matching materials. No alien materials should be introduced and the use of cement-based mortar or modern paint systems must be totally avoided. Earth-based materials rely on an appropriate level of moisture – if they become totally dried out they will crumble and disintegrate. On the other hand, too much water will also cause failure.
Where buildings have a limewash finish, routine housekeeping maintenance on a domestic scale can be undertaken very simply by brush application of a limewash to minor defects if and when necessary. Limewash can be kept in a resealing container for this purpose. Similarly, minor degradation of harling can be patched in by hand, using a matching ready mixed lime mortar kept for this purpose (Premixed basic lime mortars be stored for long periods of time.)
Traditional domestic buildings almost invariably contain permeable mortars, either lime-based or, less frequently, earth based. These may be in the form of construction mortars, external coatings or internal plaster finishes. Effective performance of these mortars is critical to the overall well-being of a building and, conversely, effective maintenance of a building is required to ensure the performance of mortars. A holistic approach is required to the care of traditional buildings. Major repair or reinstatement of traditional mortar materials requires selection of appropriate materials and informed and skilled workers; but with a little understanding, common sense and enthusiasm, owners or local tradesmen can tackle routine housekeeping maintenance which, performed regularly, will significantly reduce the need for major repairs.