Last edited 31 Oct 2021

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Institute of Historic Building Conservation Institute / association Website

Vernacular earthen architecture

Earthen material, used in vernacular architecture worldwide for centuries, could have enormous potential for building with local resources in an age of climate change.

Mud bricks.jpg
Mud bricks and a mud-brick mould.

Earth is a gift bestowed to mankind by nature to be used as a building material, and it has been, since the neolithic era. In 2004, 563 world heritage sites listed by UNESCO were built entirely or partially with earthen material. In 2012, 150 were added, and more are to be included in the future.

In the 20th century half of the Earth’s population lived in earthen buildings. In the 21st century there is a growing interest in the material’s ecological properties and its use as a contemporary building material. In the UK, it is traditionally known as cob, or wattle and daub. It is even better known for its widespread use in hot and dry climates.

Earthen material’s global and timeless use provides a strong incentive to investigate its history, indigenous styles, performance, structural techniques and potential. In Cyprus, archaeological excavations have unearthed mudbrick structures in settlements (Kalavasos and Khirokitia) from the Cypriot Aceramic neolithic era, dating from 7000 BC. In the UK, earthen material was used in part for the construction of Hadrian’s wall.

In hot and dry climates, we have even more impressive structures. In ancient Mesopotamia (modern-day Iraq), high-quality clay made up the mudbricks that were used in the construction of Ziggurat (6th century BC). In Egypt, the fertile soil near the river Nile was used to build one of the finest examples of mudbrick construction, the vaults in the Ramesseum (1250 BC) in Thebes (Luxor). In Yemen, the old city of Shibam (16th century AD) has the world’s first high-rise buildings, built of mudbricks. Earthen material was used through the centuries to build simple single-storey dwellings, fortifications and mud skyscrapers.

Indigenous styles and ingenious creations reflect natural and social parameters. These provide us with examples that come from continuous refinement. In the mountainous regions of Mali, nomads have built their earthen huts on hill slopes, with thatch umbrellas and raised floors, allowing water to flow underneath the structures during storms. The densely populated earthen built villages in Draa valley, Morocco, have flat rooves which become a living space, a path or part of the street.

The indigenous integrity and authenticity have allowed endless creativity (shapes, forms, colour) in the plasticity of the forms. Decoration becomes a method of conservation and preservation. In Nigeria, some dwellings have decorative, protective coating: a thick layer of lime or juice from local fruits is used as a waterproof finish on the walls. The coatings are renewed annually, in a ritual that follows the rainy season. In Mali, dwellings have decorative geometrically shaped openings to provide ventilation. In Saudi Arabia, the tall, earthen dwellings have layers of slates to protect the monolithic structure from shrinking in rainstorms.

Earthen material’s performance, attributes and qualities are remarkable. It is economical, immediately available, inexpensive and easy to use. It is ecologically sound: a natural material, reusable, durable, degradable. It has excellent thermal qualities (cool in summer, warm in winter), and it is both soundproof and fireproof. Its seismic attributes are notable: it is flexible and behaves well under compression. Seismic performance varies according to the structural technique used. Being an organic material, it has low fracture resistance and its greater enemy is water in its several forms, particularly rainwater or rising humidity.

Its shear-force resistance needs to be improved by enhancing its structural qualities and achieving equilibrium. The vernacular builder, who was usually the user, was aware of the material’s potential. To avoid the material shrinking, he or she followed the principles of the buttress-type loadbearing wall, which creates central thrust against bending. The walls grow thinner as the building rises, or the walls are buttressed, as in the construction of ziggurats. The walls are tied up with bonding timbers to achieve horizontal reinforcement.

Enhancing its structural integrity and achieving cohesion, the material requires high-quality additives, which create a form of natural glue. The additives include sand, gravel, animal hair, straw, seaweed, bitumen, dung, urine and animal blood. These ingredients act as binders strengthening the mixture. They help the earth dry faster and allow the structure to shrink uniformly. The properties of the additives were well known to builders since the neolithic era. Archaeologists have found traces of straw, seaweed and crushed local sandstone in Khirokitia (Cyprus). The addition of salty water (freshwater with a small amount of salt added) to the clay mixture prevents vegetation growth and provides a waterproof layer, since it becomes crystallised after the first rain. This method has been widely used for the construction of flat roofs, as in Cyprus.

Protective measures are required to improve the material’s water resistance. This can be achieved ‘with a good hat and stout boots’, as the Welsh saying goes. Earthen structures are usually built over a stone-built plinth or underpinning course, and are protected by the overhanging eaves of the roof. Regular maintenance (re-plastering) of the walls is vital.

The main component of earthen structures is clay/ soil. The chemical composition of the soil defines its quality and its suitability for construction. For example, soils in coniferous forest regions tend to be acidic, hence unsuitable for building, whereas the grassland soils found in prairies are widely used. The desertic soils of broken rock and sand, which retain many of the characteristics of rock, become hard and encrusted when exposed to air, and so are good construction material. This type of soil was used in the construction of multi-storey buildings in Yemen. The colour of the clay is determined by the amount of iron oxide: the more iron oxide, the redder and deeper the colour.

There are three main techniques in earthen architecture (with variations): structure (cob), monolithic (rammed earth) and brickwork (adobe). Cob is the use of the earthen material as a structural component. The uses of clay include its use as a filling material in timber-frame structures, and as the infill between two walls. Contemporary cob architecture is currently built in the UK, US and France.

Rammed earth is used to build a monolithic structure. The material is reinforced with additives (sand, gravel and lime) and compressed mechanically into an externally supported frame to create a solid wall, similar to concrete. The methods and techniques remained similar through the centuries. Contemporary techniques (with additives such as cement, asphalt and bitumen) can enhance the material’s performance (thermal mass with low embodied energy), its structural qualities, and its appearance. Contemporary rammed earth is popular in the UK (at the Eden project, for example), Australia and Canada.

Adobe structures are built of sun-dried mudbricks. The mudbricks are made of clay with organic additives and are formed into moulds. The construction procedure, proportions of ingredients and dimensions vary according to local traditions and peculiarities. Adobe structures are extremely durable in hot and dry climates, but they are susceptible to earthquake damage. The Getty Institute has carried out experiments to improve adobe’s seismic performance. The growing interest in reusing adobe in contemporary architecture is widespread in the southern United States.

In recent years many organisations, such as CRATerre, the Getty Institute, Rammed-earth, Earth Building UK and EBUKI, have been experimenting in building new earthen structures, not only to promote earthen architecture but also to improve the construction material’s performance.

As UNESCO stated in its manifesto in 2012: ‘Earthen architecture is one of the most original and powerful expressions of our human ability to create a built environment using locally available resources.’ It is an architecture that ‘reflects cultural diversity, the ingenuity and identity of communities and involves sustainable building techniques that often convey true artistic expression’.

The growing interest in its revival will contribute to social, ecological and cultural development, influencing and inspiring future interventions in our rural landscape that are needed not only because of the changes in the climate but also as a response to the Earth’s rapidly increasing population.

Bibliography


This article originally appeared in Context 143, published by the Institute of Historic Building Conservation (IHBC) in March 2016. It was written by Maria Yioutani-Iacovides, an accredited conservation architect with special interests in vernacular and ecclesiastical architecture.

--Institute of Historic Building Conservation

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