Repairing Clay Buildings
and Cumbria's Clay Dabbins
|Traditional long-house at Durdar, Cumbria, which has since been thatched and, above, the long-house’s 1689 date stone|
Although this article focusses on the revival of interest in clay buildings in Cumbria, the repair and maintenance issues which are discussed here are relevant to all variants of earth construction in the UK.
Most clay dabbins, as they are known locally, are quite modest structures but some have survived remarkably well (right). Nearly all are rendered and it is often impossible to detect that a building is built of clay until the building starts to decay. When render is removed or falls off, or a clay wall collapses (sometimes without warning) the structure becomes visible. The horizontal clay layers, each of which is usually 50-150mm deep, are separated by thin layers of straw which are only millimetres thick.
Examples of this form of construction have been found in Wales and Scotland but they only appear to exist in England on the Solway Plain. They were once very common on both sides of the Solway but building clearance and improvement on the Scottish side has reduced this to three surviving examples.
In Cumbria several hundred examples have survived, dating from the 16th to the 19th centuries, and include cottages, farmhouses and farm buildings, many of which are cruck framed (illustrated below).
Numbers have been diminishing over the last century or longer, as the use of unburnt clay as a building material eventually ceased and the craftsmen capable of maintaining these buildings died out.
In temperate climates the main cause of decay in earth building is water penetration. It is important to protect the top and bottom of any earth wall and to ensure that any treatment applied to the interior or exterior of a clay wall is breathable. When water does occasionally find a way in, it can dry out naturally as long as the water vapour can reach the surface of the wall and evaporate.
If there are signs of water penetration these need to be investigated and treated. Cement render is a common cause of damp problems as it tends to prevent water vapour from reaching the surface. Unable to dry out, the wall begins to deteriorate and, if the problem is not addressed, it will eventually fail, often quite dramatically.
|Cottage at Castletown, Cumbria: the cement render fell away to reveal a waterlogged clay wall. After a week this had dried out completely. (Photo: Heritage Skills Initiative)|
If a clay wall is very damp and it is cement rendered, care should be taken when investigating the state of the clay underneath as, in the worst cases, saturated clay can literally pour out. Removing patches of render from the upper wall first and gradually working down to the plinth is the preferred course of action.
If the clay wall is excessively damp and soft then it should be left to dry out for several days. Once this section is dry then the removal of render can resume. Once the wall has dried out the strength of the clay wall will gradually return and it will then be possible to address any other problems that have been uncovered.
The most appropriate finish for a clay wall is the traditional solution of using either an earth or lime render or a limewash. No impervious coatings of any description should ever be used.
Protecting the top of the wall with deep eaves helps to keep the top and much of the face of the wall dry. If this is not possible then adequately sized, well maintained guttering and downpipes are paramount. Most clay buildings have a stone plinth on which the clay wall is set, and in dabbins this is generally about half a metre high; if this is free of vegetation and there is good drainage around the building this should prevent any damp attacking the base of the clay wall.
THE CLAY DABBIN PROJECT
A survey carried out for English Heritage by Oxford Archaeology North in 2006 identified just over 300 surviving clay dabbin structures in Cumbria. A significant number were in a poor condition and although it was possible to give advice on how repairs should be carried out, not a single suitable contractor could be found in the region.
As a result of the survey, English Heritage was aware of the situation and eventually agreed to fund a programme – the Clay Dabbins Project – to train contractors in clay building techniques and also to engage with clay dabbin owners to make them aware of the importance of their buildings and to reverse the widely held impression that these buildings were damp, dangerous and not worth retaining.
This view had been generated by many builders who, when faced with questions about how to repair a clay wall or get rid of its damp problems which they were unable to answer, suggested that nothing could be done and that the buildings should be demolished and replaced with breeze block. The damp problems, needless to say, were usually created by previous builders who had applied cement render to ‘solve’ a damp problem.
|Left: turning the mix (Photo: Heritage Skills Initiative) and right: treading in the clay, levelling the sides and laying the straw course|
The partners in the Clay Dabbins Project, which was initiated in 2009, were English Heritage, the North of England Civic Trust, Grampus Heritage and Training Ltd, and Carlisle City Council. The main aim was to give training to builders to enable them to tackle clay dabbin construction and repair. Further funding was obtained the following year from the Heritage Skills Initiative which enabled the project to expand to include professionals and the general public.
A programme was devised to train builders on a course that was centred on constructing a small bothy (a simple, one-room structure) using the clay dabbin tradition. This would encompass all of the essentials for dealing with clay dabbins of any size or scale. It would also see the construction of the first clay dabbin for more than a century.
The training was provided by Becky Little and Alison Davie from Fife, whose firm, Little and Davie Construction, has had a great deal of involvement in the repair and restoration of earth buildings, including one on the Solway Plain.
|Cruck barn at Drumburgh, Cumbria|
|When making clay blocks the mixture must be well compressed into the mould|
The traditional communal process of clay dabbin construction was to mix clay or mud and straw with the addition of water to bind the two. The straw is pressed into the wet clay by treading it (cattle were sometimes used to do this). Once thoroughly mixed, this material can be carried to the previously prepared plinth and it is then laid in thin layers, each separated by a thin layer of straw.
The straw helps the drying process and allows the next layer to be started immediately, without having to wait for any drying out. This is in contrast to other regions where the lifts (layers) are up to three feet deep and need up to a week before the wall is dry enough for a builder to stand on top and lay the next lift.
The clay used was whatever was available locally and it can vary enormously in composition (from having considerable proportions of aggregate in the clay to almost all silt). A serviceable mixture today could contain (by weight) 30 per cent stone/gravel (5-40mm), 30 per cent coarse and fine sand, 15 per cent silt and 25 per cent clay.
The other main ingredient is straw. Both wheat and barley straw were used and approximately 25kg is used per cubic metre of subsoil. Water must be added to the mix to allow the clay to break down and coat the aggregate and straw. This is done by treading the mixture and breaking up all of the subsoil until there is no dry material left. The amount of water is critical because too much will make the mixture very sloppy and impossible to apply. Once mixed, it must be turned and trodden again (above left).
The process may have to be repeated several times to get the right consistency, which can be an arduous task. Mechanical means (a JCB or a tractor with a bucket to turn the mix) can speed up production.
Part of the training involved making clay blocks (right) which would be used in the repair of clay dabbins as well as their construction. The same dabbin mixture is used to make the blocks using a timber or metal mould made up to an appropriate and convenient size for the task. The blocks need to be laid out to dry under cover and turned after three weeks. The longer they are left to dry the better.
The process of building the clay wall is straightforward but needs skill and judgement in laying the material, treading it down firmly but without squeezing it off the wall, and protecting the edges and corners to ensure that they remain vertical and solid. Beating the sides can maintain a vertical wall and where there are any bulges these can usually be trimmed off at the end of the day.
The process continues until the required eaves height is reached and the gables can then be constructed. In this trial the upper half of the building and the gables were completed in clay block with the purlins and ridge built into the clay wall (below). The walls were plastered with an earth render and a thatched roof was added consisting of turf laid as an undercoat onto pole rafters and heather thatch laid on top.
Deterioration can be caused by damp, water penetration, cement renders and defective rainwater goods as mentioned above. Other common defects include cracks which extend through the clay wall, and hollows and cavities such as rat runs. Sections of clay wall sometimes need to be reconstructed, for example when inappropriate repair materials have been used.
|Finishing off the gables of the clay dabbin|
In many of these instances the preferred method of repair is to use air dried clay blocks laid with a clay dabbin mix as a mortar. In some cases clay tile or thinly bedded sandstone can be used to tie new work to old. As a modern alternative lightweight, twisted stainless steel reinforcing rods (Helibar for example) can be fixed into the original clay wall and laid across the new clay blocks (below right).
Small cracks which do not continue all the way through the wall can be treated by stitching across the crack using small adobe blocks. A small trench, which is large enough to take the block and a quantity of the clay dabbin mix as mortar, is cut horizontally across the crack. The trench is well wetted before the clay block is pushed into the mortar so that the entire cavity is filled by the block and the mortar and the excess is squeezed out of the front.
Depending on the length of the crack there should be a stitch about every 350mm, starting at the base and doing one stitch at a time. The gaps between the stitches can be opened up slightly, damped down and filled with mortar and small pieces of adobe block (bottom right).
Major cracks that carry all the way through the wall will probably need to be investigated by a surveyor or other professional, as the cause of the cracking may need to be addressed. Any remedial work will need to be carried out by a competent professional, particularly if propping is necessary. Sometimes when the gaps are wide the cracks have been built up in brick or stone, often using cement mortar. If it is thought prudent to remove the brickwork and replace it with clay block, then this needs to be very carefully controlled.
Surface hollows are very common in clay dabbins and if they do not create large overhangs then they could be left alone and simply rendered. These surface depressions add to the character of the building. Hollows that are 100mm deep can be treated by cutting into the clay wall, providing a flat base on which to lay clay blocks bedded in a clay dabbin mix used as mortar.
|Helibar ties laid along the courses of clay blocks (Photo: Little and Davie Construction)|
|Stitching minor cracking (Photo: Little and Davie Construction)|
As before, the cavity should be thoroughly wetted before starting. On larger areas it is often necessary to key the blocks firmly to the original wall. Various armatures have been used in the past. Oak pegs or clay tiles can be fixed into the wall, although tiles can be awkward to fix and can detach from the slots that have been cut for them. Lightweight Helibar rods can also be effective in this situation.
Deeper hollows can be treated in the same way but with greater caution as this kind of repair is essentially underpinning and there may be a need to support the wall while someone is working on it underneath. If in any doubt about the structural integrity of the wall, specialist advice should be obtained.
Finally a word on rat runs. These are continuous cavities that are sometimes found inside a clay wall. They are fairly common in buildings near or on farms and have probably been excavated by rodents. Attempts may have been made to seal or block them up with anything that came to hand, including broken glass and barbed wire.
If the runs are extensive then seek advice as to the stability of the wall. If all or part of the cavity can be cleaned out, bearing in mind the hazardous material you might find, then it can be filled with a grout of lime and coarse sand. Again dampen as much of the cavity as possible beforehand without saturating the clay wall. If the holes are large, mix in coarser aggregate and if necessary push it in with a rod. This may not completely fill the run but it will have reduced the amount of unsupported clay and improved the wall’s stability.
If the run is close to the surface then it may be possible to expose the cavity and treat it as a hollow (see above).
The Clay Dabbins Project made both professionals and the public aware of this important part of Cumbria’s built heritage, one that has been diminishing rapidly over the past century. Since the project began in 2009, the public’s appreciation of clay dabbins has dramatically increased, and the skills and knowledge of builders, contractors and professionals have improved to the point where the repair of existing clay buildings is no longer a lottery. There is even the possibility, with so much focus on sustainability, that Cumbria might see a revival in the clay dabbin tradition with the construction of new homes in the near future.
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