Buildings and Their Repair
new extension contructed with cob walls, rendered and limewashed.
As a thermally efficient building material which involves minimal
energy-use in its production, earth is attracting renewed interest
for the construction of new buildings and extensions.
Most people associate buildings with earth walls with Africa, Arabia and South America. Yet, despite our damp climate, there are thousands
of earth buildings in the United Kingdom, some of which are over four
hundred years old.
region (see map) tends to have its own form of construction dependent
on the nature of the materials available locally. The principal forms
of earth building include:
The thick cob walled houses in the West Country are probably the best
known of all earth walled building types. Their thick walls are made
by piling a mixture of subsoil and straw, about 600mm thick, on the
wall and paring the rough edges flush with the wall. The next layer
is put on when the previous work has dried enough to bear the weight.
This method of building extends east as far as Basingstoke where the
subsoil used is mainly chalk. Similar forms of construction can be
found in the South West and in north west Wales.
Oxford and Aylesbury thin walls were constructed using the cob technique,
where it is known as 'whitchert', meaning 'white mud', and in the
Midlands the local earth is used to build in the same way.
the north, on the Solway Plain, and on both sides of the border, walls
are built with the continuous cob method in which the subsoil is placed
in thin layers alternating with layers of straw. Because the layers
are so thin, by the time one layer has been put right around the building
the previous layer has dried sufficiently for the building work to
or 'Adobe': The universal mudbrick, adobe is found only in East
Anglia in the area east of the A1 and south of the A47, where it was
introduced, from abroad, around the end of the 18th century. For about
100 years it was the principal walling material for every sort of
building on the chalky boulder clays of Norfolk and Suffolk. Although
adobe was reported in Perthshire before it was introduced in England,
none has been found. Construction is similar to brickwork with regular
bonded courses, but the dried blocks of adobe or clay-lump are much
larger and are usually laid in a mortar of fresh earth or clay.
|Areas of the British Isles in which earth buildings are commonly found
roofs of earth buildings are usually hipped, not gabled, because of
the difficulty of providing the necessary restraint for a gable wall.
Rafters are carried on wall plates positioned over the centre of the
wall and the spaces between the rafter feet are filled with subsoil.
are usually constructed of brickwork above the roof, although the
flues below are often of clay-lump. Where there are no chimney pots,
water is liable to get inside the chimney and erode the top of the
clay-lump. Many brick flues were lined with subsoil and some had their
brickwork laid in earth mortar.
in walls are bridged with wooden lintels and often have blocks of
wood built into the reveals for fixing doors or windows. In clay-lump
buildings the windows and doors were built in. Because monolithic
walls shrink as they dry, the openings were formed as the wall was
built and the doors and windows were fitted afterwards.
clay-lump houses have one or more elevations faced with brickwork
which was either built with the house or put on as an improvement
later. The brickwork was fixed to the clay-lump with bands of hoop-iron
which were nailed to the clay-lump or were built into the mortar joints.
In time these ties rust and fail. They can be replaced using remedial
ties designed for cavity walls.
and Daub: The most common form of non-load-bearing earth construction
is wattle and daub, which has been widely used to infill the panels
of timber-framed buildings. Like all the other forms of earth-based
building materials the 'daub' is made of a subsoil which must contain
a small proportion of clay mixed with animal or vegetable fibre. In
this case the earth mixture is supported by an interwoven lattice
of sticks and laths (the 'wattle').
variation of this technique can be found in Lincolnshire where there
are a number of houses with mud and stud walls. In this case the mixture
of straw and subsoil is placed around and between earth-fast posts.
These houses are small and generally well recorded, well repaired
and fiercely conserved.
In addition to the techniques described above, there are numerous
variations and hybrids, such as shuttered clay (a form of cob in which
the plastic subsoil is placed between boards) and pisť de terre, in
which shuttered earth is rammed in thin layers to form an extremely
dense, hard material. In Scotland, walls can be found which are made
using grass turves and peat turves.
strength of earth walls is proportional to their moisture content.
At a moisture content of 13 per cent of its dry weight the strength
of the material falls to a point where it can no longer resist the
pressure exerted by an average wall (c. 0.1N/mm2). At this level the
wall may collapse. Damp is usually caused by poor alterations, inadequate
maintenance, or a lack of ventilation.
Damp:Most earth buildings have shallow rubble foundations with
footing walls of brickwork or rubble masonry, varying from about 150
- 1200mm in height. Rising-damp seldom crosses from the footing wall
into an earth wall. However, where a damp course is necessary, it
should be inserted in the footing wall to avoid any damage to the
earth walls have no footing walls, rising damp is more likely to be
a problem, but in these cases, damp-proof courses which provide a
barrier should not be used, as they prevent the drying of the part
of the wall below by isolating it from the wick effect of the wall
above. This can lead to the collapse of the wall as the moisture level
increases. Alternative methods of reducing damp should be introduced
on the advice of a specialist, such as improvements to land drainage
and the rapid removal of surface water from around the building when
Coatings and Renders: Earth walls are traditionally finished externally
with lime or earth renders and internally with similar but finer earth
or lime plasters. Renders with a cement content of more than 10 per
cent should not be used as they are not vapour permeable, and inevitably
trap moisture within the structure. They also provide a cold surface
on which condensation will form within the structure and, furthermore,
cement expands when warmed (it has the same coefficient of thermal
expansion as steel) while earth tends to shrink. As a result, differential
thermal movement causes cement renders to crack, allowing water to
enter. This moisture, combined with condensation on the back of the
render, percolates to the base of the wall where it accumulates, causing
the earth wall to deteriorate.
due to the use of cement renders have been dramatic, involving the
collapse of large sections of walls. Painting the exterior of the
building can have the same effect, as many modern paints trap moisture
in the wall. This sort of damage may not be covered by normal household
insurance policies. Therefore, where repairs are required, a lime
or earth render should always be used which matches the original as
closely as possible, and when dry, they should be painted with limewash
which is not only the traditional finish but also vapour permeable.
MIXTURES FOR REPAIRS
the subsoil varies so much, there are no definitive recipes for render
mixes. However, all earth mixtures contain two essential ingredients
in water; an aggregate such as sand or chalk, and clay which coats
the aggregate particles and acts as a binder. Other ingredients might
include set retardants such as hydrated lime, and fibre reinforcements
such as straw and ox hair.
clay content of the render shrinks as it dries and cracks develop.
Unlike the cracking of cement renders, these cracks are essentially
an aesthetic problem only, because the whole of the render is porous
and water can therefore evaporate as readily as it is absorbed. The
earth mixture is therefore selected with the aim of decreasing the
size of the cracks and increasing their number; ideally there should
be countless invisible cracks. Reducing the amount of water in the
mixture reduces the cracking but also makes it more difficult to apply.
By adding any sharp sand, straw, chalk or hydrated lime, the proportion
of clay in the render is reduced and the cracks reduced in size. Chopped
straw, produced for chicken litter, is ideal because more of this
type of straw can be mixed in. Chalk (agricultural lime) should be
graded 6mm down (ie sieved to include only particles of 6mm diameter
and less). Adding hydrated lime also reduces the size of the cracks
by slowing the rate at which render dries. Deflocculating agents (which
affect the polarity of molecules) may be used to reduce the amount
of water necessary to make the render workable. These include urine,
isinglass, fresh cow droppings (from a milking parlour), and waterglass.
scheme of rendering must start with a trial patch which is best carried
out on a sheltered elevation in case it can be kept, even though the
mixture has to be improved. Quite severe cracking can often be re-worked
by brushing the face of the wall with a stiff broom before starting
dry wall will suck the moisture from a render mix in moments. Therefore,
many operatives wet the wall with a hose before rendering to reduce
the suction; while others prefer to make the mix very wet to enable
it to be reworked later. Clay renders perform best if they are forced
on to the wall. More pressure can be exerted if the rendering starts
at the bottom of the wall. If there is any doubt about the condition
of the surface of the wall being suitable, then lime renders, in particular,
should be put on to a stainless steel or other non-ferrous metal mesh
which is fixed with spring head sheeting nails.
damage to earth walls can be patched. However the size of holes which
can be patched is limited by the shrinkage which will occur when large
volumes of new material are applied in a wet form. Large holes therefore
have to be filled in layers; each one being left to dry before being
scored to provide a mechanical key before the next is applied. The
hole should be over-filled and finally the surface is cut back, or
'pared' to the line of the wall. These techniques are fully described
in several publications and it is probably better to contact the local
Earth Building Association before attempting them for the first time.
repairs are made by cutting out the damage and rebuilding using cob-blocks
or clay-lumps laid in mortar. Cob-blocks are made for sale in the
West Country and should be made of subsoil similar to the wall to
be repaired. The material which is cut from the wall can be crushed
and riddled, to remove large stones, and used for the mortar in the
repairs. When cob-blocks and clay-lumps are laid in wet mortar the
moisture in the mortar is absorbed very quickly into the blocks so
the work is not delayed while the mortar dries.
and daub often contains more archaeological evidence than the timber
frame, and it can be quickly and cheaply repaired. Yet it is often
discarded during works.
daub should be salvaged and reconstituted to a paste with added barley
straw cut about 100mm long. Tie hardwood coppiced sticks (20-40mm
diameter) to the frame in the same way that the originals were tied,
spaced so an open hand will go between. The County Conservation Officer
will know where to get coppice sticks. Working on both sides, press
the daub through the wattle to cover it to a 25mm thickness. As the
daub dries it will pull away from the frame. If another coat of daub
or plaster is to be put on, prick a key into the surface otherwise
press the daub with a float to close the gap.
burrow in earth walls especially if they are damp and near a source
of food. Grout with an earth slurry only when confident that the extra
moisture will not weaken a wall already weakened by the burrows. Burrows
are probably better cut out so new blocks can be put in.
on problems to do with earth buildings can be obtained by contacting
your local authority conservation officer or Earth Buildings Association
Earth Buildings Association, Environment Directorate, Devon County Council,
County Hall, Exeter EX2 4QW
East Anglian Earth Buildings Group, Paperhouse, West Harling, Norfolk
Midlands Earth Structures Society, The North Wing, Harrington Hall,
for Earthen Architecture, University of Plymouth, School of Architecture,
Notte Street, Plymouth, Devon PL1 2AR
- Hugo Houben and Hubert Guillard, Earth Construction: A Comprehensive Guide, Intermediate Technology, London, 1994
- Larry Keefe, The Cob Buildings of Devon 2: Repair and Maintenance,
Devon Historic Buildings Trust, 1993
- John Norton, Building with Earth: A Handbook, Intermediate Technology, London, 1986
- Gordon Pearson, Conservation of Clay and Chalk Buildings, Donhead, Shaftesbury, 1992
- Jane Schofield, Lime in Building, Black Dog Press, Crediton, 1995
- Bruce Walker and Christopher McGregor, Earth Structures and Construction in Scotland, Historic Scotland, Edinburgh, 1996
article is reproduced from The Building Conservation Directory, 1997
BOUWENS is Secretary of East Anglian Earth Buildings Group (EARTHA),
which encourages the proper repair and conservation of earth buildings
and promotes the use of earth in new buildings.
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