Historical Note
Mud and fieldstone are the earliest walling materials within buildings which remain in use today. Both have continued in use throughout recorded time into the beginning of this century. A limited amount of rubble stonework is still built today and there are few people living who can give first-hand accounts of building with mud.
Materials
Mud
Mud is used in three forms: monolithic; in courses bonded with straw; and as unbaked bricks. The material is strong and perfectly stable as long as it is kept reasonably dry and for this reason it was always lime washed. The walls require some moisture to keep their binding and can fail if too dry. Insertion of damp proof courses are not recommended for mud walls for this reason.
Numerous vernacular buildings across Northern Ireland have mud walls. However, they are increasingly falling into disrepair or being repaired using replacement materials such as stone, brick or block. Sometimes openings may also have been part patched or built up in stone or brick and mud walls can be hard to recognise before work starts.
Mud was also used as a binder in rubble stone construction, when it was often mixed with lime.
Construction in these materials is necessarily massive and openings are small, giving buildings of these types very high qualities of thermal insulation.
Stone
‘In Northern Ireland stone has been used for generations because of its strength, longevity and aesthetics. It can be worked, dressed and carved to express the intent and inspiration of architect, sculptor and mason. Crucial to the understanding of a stone building is knowledge of how it is constructed, how the components are crafted together and how this intricate combination of the parts contributes to the final aesthetic and artistic qualities.
Exemplary stone repair requires specialist knowledge of materials and methods. This project provides some practical information for Northern Ireland to the custodians of our heritage and those that work to restore and conserve them. However, it has been widely recognised that there is a lack of information for stone conservation and any information that does exist is not available in one place.…..
In Northern Ireland the full range of rock types (igneous, sedimentary and metamorphic) have been used as building stone since Neolithic times. As transport systems developed in the 19th and 20th centuries, stone was moved further around the country as well as being shipped in from further afield for construction of the more prestigious buildings such as town halls, churches and banks.’
(Source – The Stone Database)
The Stone Database provides a tool for identifying which stone type has been used in a building, wall or structure.
The site also provides a template for recording stone decay and carrying out a stone risk assessment.
In Northern Ireland, the origin of stone used varies considerably, however stone is broadly categorised into two types according to how the walls are constructed – uncoursed or coursed. The finish of the stone can be described as ‘Rubble’ (stones once cut from the quarry or removed from the ground/field are relatively untouched) or ‘Ashlar’ (dressed, worked with tools/some elements are carved).
Rubble stone
Rubble stonework is found in all regions of Northern Ireland. Rubble stone can be split along the bedding planes giving shapes that can be knit tightly together. The quality of rubble stonework varies widely. The best has the stones selected in size to form a consistent pattern throughout, with special stones set to bind the corners and other weak points.
However carefully executed though, such stonework was rarely built to be seen. To make buildings weathertight and waterproof, the mortar which was squashed out of the joints as the work proceeded was plastered our over the face and then given a wash of lime or burnt earth. The best stones would be used to face the work, with the poorer material thrown in with generous supplies of mortar to level up the core. Lintels were either single large stones or a series of timbers. Attempts at arching are very rare in this type of construction because of the way in which the walls are made up. The enlargement of openings or breaking out new openings can be a very hazardous and destructive operation. Problems can sometimes result from the settlement of the core material away from the face work over time.
Ashlar
‘Belfast's built heritage has a full range of Scottish Carboniferous sandstones and English Jurassic limestones used from the mid-19th Century.’
(Source – The Stone Database)
Brick
When coal became widely available brick making was concentrated into industrial centres; previously bricks were made on site from locally dug clay. Because of the difficulties and expense of manufacture a place was found for every brick whatever its faults, the worst unburnt or deformed examples being built into internal partitions. During the 18th century it became accepted practice to bond masonry walls by making use of horizontal timbers built in as the work progressed.
Mortar
Stone and brick used for habitable buildings requires mortar. The simplest material was earth but the greater number of buildings that survive in use were built with a mortar using burnt limestone. These mixes of lime with earth or sand were used primarily to form a bedding for the unevenly shaped stones or bricks providing stability and keeping the weather out. Some superior work made use of hydraulic limes which, because of their silica and alumina content formed a chemical set through their entire depth. These are the forerunners of the modern proprietary cements. Most construction work made use of lime slaked from local limestone which in this region is quite pure or ‘fat’. These limes harden where they are in contact with the air (carbonate) they have little have chemical setting action. Historical mortars however seem to have had some chemical setting properties due to impurities such as ash in their production. . During the 19th century more complex mixes were developed using crushed furnace ash (so called black mortar) and similar bonding materials having a greater structural strength than pure lime.
A revolution in mortar mixes followed the development of Portland Cement about 1830. The material was designed to have a complete chemical setting quality and a structural strength up to five times that on any other known material. It was not long before it was discovered that Portland Cement mixed with a filler like sand or gravel and possibly reinforced with steel rods could be the principal building material, taking the place of both brick and stone, in the form of ferro concrete.
Masonry repair
Today there are very few local commercial quarries producing building stone and none of these are suitable for fine ashlar work. Clay bricks are made outside Dungannon but their appearance is quite different from earlier bricks made here so that matching material often has to be imported or salvaged. Pure lime either hydrated or in putty form is available in Northern Ireland and is recommended for repair work to all historic masonry walls.
Faults and repair
Ground settlement:
Symptom: Deformation and cracking, generally orientated vertically; sometimes outward leaning walls.
Repair: Most settlement takes place early in the life of a building. If this is the case it may be sufficient to replace any fractured lintels or other structural members and then grout the cracks. Old settlement patterns are unlikely to be the cause of further movement or structural collapse. If the settlement is new or progressive the course must be located – the local water table may have been altered by new building work nearby, or a drain may have been fractured. After dealing with the cause it may be necessary to provide new foundations by underpinning the old foundations with additional structural support.
Damp:
Symptom: Discoloration: paper not staying on the wall; appearance of powdery or crystalline deposits on the wall face; frost damage to brick, stone or mortar.
Repair: The position of the damp will be a good guide to its origin – failures of the roof; rainwater drainage and external plasters are dealt with in other sections. Ground damp can be counteracted in two ways:
- Piping water away before it reaches the structure. This will involve a trench around the building, piped to falls, together with manholes and catch pits for maintenance, and the excavation back-filled with stones – sometimes called a ‘french drain’.
- Making the wall itself resistant to damp. If the ground level is high and cannot be reduced then the wall must be ‘tanked’. This means that the wall is excavated and protected by a waterproof membrane, which itself is protected from puncture by a second skin of masonry and then back-filled.
If ground levels relate correctly to the walls, ie, are below the level of the floor and damp is showing at the lower part, then it may be necessary to provide a horizontal barrier within the wall. However the necessity of this should be carefully assessed before embarking on potentially costly remedial works. Ensure that the cause of the problem is isolated – don’t confuse condensation (which can commonly occur in this area in modern moisture rich and draught proofed houses) with rising damp. Repairs in recent times may have exacerbated the problem by applying a dense coating outside and inside and to the floor. It may be possible to replace modern cement plaster with lime plaster and modern paint for example with a more breathable alternative or to reduce the problem by opening up blocked vents. Bear in mind that most historic buildings were designed to avoid moisture problems. What has changed and can it be reversed?
If a Damp proof course is felt to be necessary, the most common methods are:
- To cut through the thickness of the wall and insert an impervious sheet, usually copper. This method is only suitable for walls with continuous horizontal mortar bed joints, e.g., brick or ashlar stonework.
- To inject the wall with a chemical which prevents water movement. This method requires holes to be drilled at regular intervals and is visually unacceptable in most historic buildings – listed building or scheduled monument consent should always be obtained before undertaking works which involve these risks to the passage of moisture out of the walls and this level of alteration. In solid rubble masonry walls the core often contains voids and decayed mortar which will result in an uneven distribution of the chemical. To avoid this, the wall are often prepared by washing out the loose material at the base of the wall and then grouting to fill the voids. The grout should be of similar strength to the original mortar, and be lime-based. The chemical is then expected to form an even and continuous barrier. This method is uncertain in its coverage and operatives normally require the removal of a meter of plaster on the interior and its replacement with hard cement render to avoid bridging. This hard covering will also disguise any failure of coverage leading to long-term potential damage, and so this method is rarely advised in historic buildings..
- To prevent capillary action in the wall by electro-osmosis. This type of installation is specialist work. The wall is prepared by stripping the plaster to a height of about 1 m. A continuous metal strip is then fastened to the masonry and the surface made good. The long term effectiveness of such systems have been questioned but they have the advantage of causing less disruption to the historic fabric of the wall.
Damp higher up the wall can sometimes be related to flues no longer in use. Flues on external walls reduce the effective thickness of the walls and where there is no protection at the head of the stack considerable amounts of water can drive in. This may not dry out because there is no air movement. This problem can often be cured by fitting ventilating caps at the top of the stack which prevent rain entering but allow air to circulate. Lower down in the flue fit a vent so that air can move through the length of the flue.
In walls where weather exposure is severe and a very dense stone or brick has been used, the wind will sometimes force rainwater right through the full thickness of the wall. One option is to render the outer face using a lime based render which will encourage the moisture back out of the wall and release it. Cement based renders should not be used, as hairline cracks are formed over time which allow water to penetrate, but not to escape, causing a built up of moisture over time. Some exposed gables are slated to throw water off. For appearance reasons, specialist advice is necessary and statutory consent is required.
Ferro Concrete or Reinforced Concrete
The most frequent cause of failure is corrosion of the reinforcement. Poor compaction when originally placed can also cause later decay. Contained chemical impurities, possibly introduced from the sand or aggregate, or added intentionally at the time of construction to increase plasticity in the mix/make possible work in frosty weather, are another cause.
Mud
Stone
Local weather conditions caused repeated wetting and drying, and atmospheric pollution are severe enemies of all stones. The incorrect bedding of sedimentary stonework can result in the total breakdown of individual stones. Iron, built in as ties or cramps, rusts and swells eventually bursting the stonework open. Where the stone itself is extremely durable for example basalt and granite the problems are found at the joints. All these decay patterns are aggravated by frost.
Basic structural damage to stonework often results from impact or explosions. Ill considered structural alterations can provide the initial weakness, for example the creation of large ground floor shop fronts and clear sales areas cutting lateral stability to a minimum or perhaps the creation of rooms in a roof space causing the essential structural tie members to be severed.
Brickwork
Brickwork behaves in a similar way to stone. The bricks themselves can be broken down by chemicals present in the clay at the firing or from adjacent building materials. Poor firing is another problem. When bricks were expensive every brick possible was found a job in the structure often in cross walls and flues where they were not originally seen.
Pointing
The general principle of pointing is that the joint should be weaker than the brick or stone with which it is used. In this way it is the joint which decays and not the masonry face. Consequently it is necessary after a period of time to renew mortar joints. The frequency of renewal will depend very much on the quality of the work and the weather conditions. There are many examples of work upwards of two centuries old which are perfectly sound. It must also be remembered that older buildings do not have specially designed joints to accommodate thermal movements. It is in fact the flexibility of the relatively weak mortar beds which provides the necessary movement, and this must not be inhibited by unsuitable repairs or the use of incorrect materials such as inflexible cements.
Repointing is a job which, if it is to have any life and be at all effective, must be carefully done. The joint must be cut back square to an even depth. It must not be feathered off at the ends into old work. Just before repointing flush out to clear all loose material; this will ensure that the new mortar has a good grip and does not suffer from too much suction. Work must start at the top of the wall, and the new mortar must be well rammed in. The finished joint must not project beyond the face of the adjoining masonry. The strength of the new mortar must always be matched to the original work and it should never be stronger than the stone or brick from which the walls are built. Any mortar used in historic work should be of the range – 1:3 lime to sand, hydraulic lime mixes are recommended for exposed conditions). Gauged cement lime sand mixes have been accepted by HED to repair historic buildings in Northern Ireland, but traditional lime/sand mixes are much more appropriate for the long term conservation of these buildings and there has been an increase in availability of the material and the skills to use it in recent years. Sand cement mixes have proven to be detrimental to historic buildings and are not acceptable.
Tips for the preparation of Contract Specifications
- Identify the areas to be worked on, and for this photographs can sometimes be as valuable as drawings. Describe the types of work necessary in each area, i.e., repointing; individual stones or bricks to be cut out and replaced; parts of the walling to be taken down and rebuilt; location and size of ties, pins, blockbonding, ring beams, underpinning, etc.
- List what materials from cutting out or demolition should be set aside and numbered if necessary for reuse and any cleaning or other preparation which must be carried out.
- Describe replacement materials whether they are to be new or re-claimed and in this context it may be necessary to say from where they can be obtained.
- Limes, cements, sands, aggregates and mortar mixes should be included with descriptions of colouring, sizing, etc. Indicate how these should be stored and protected before use.
- Describe damp-proofing courses, and any pinning or other reinforcing techniques to be employed.
- Describe the required quality and appearance of the finished product, any bond pattern or finish to the joints that is particularly required and name any sample areas that must be approved before the general work proceeds.
Sources of Relevant Information.
- The Georgian Group Guides No2, Brickwork, 2018
- TAN 06 - Earth Structures and Construction in Scotland, Historic Scotland, 1996
- TAN 31 - Stone Masonry Materials and Skills, Historic Scotland, 2007
- Alan Gailey, Rural Houses of the North of Ireland, John Donald Publishers, Edinburgh, 1984.
- Stone, Practical Building Conservation series, English Heritage, 2012
- Earth, Brick and Terracotta, Practical Building Conservation series, English Heritage, 2012
- Bricks, Heritage Advice Series, Government of Ireland 2009
- Historic Environment Scotland Inform Guide series.
- BS 7913:2013, Guide to the conservation of historic buildings, BSI Standards Ltd.
Contractors
The Department cannot recommend specific contractors for restoration work. However, this is a skilled field and owners and their agents should satisfy themselves that those they employ have the required skills. Previous examples of their work should be inspected, and recommendations sought.
UK and Ireland contractors who claim to have restoration experience are now listed online; The Directory of Traditional Building Skills, compiled and published by Ulster Architectural Heritage in association with HED can be accessed via the UAH website.
The Irish Georgian Society publishes a list of specialist contractors based mainly in the Republic of Ireland.