Nubian Vault

The Nubian vault is a way of roofing a room with a barrel vault of sun-dried earth bricks, built without centring, formwork or any other temporary support.⁵² The mason leans each course of bricks against the course before it, starting from an end wall. Each new ring holds itself in place while its mortar sets, so a vault can be raised using nothing but earth, water and a few hand tools.⁴² The technique takes its name from Nubia, the Nile valley region of southern Egypt and northern Sudan. It was revived in the twentieth century by the Egyptian architect Hassan Fathy.⁵ Since 2000 it has been promoted across the West African Sahel as a timber-free, metal-free roof for houses, schools and public buildings.⁵¹

In French, the language of most Sahelian programmes, the technique is called voûte nubienne. That is also the name of the association that has spread it, the Association la Voûte Nubienne (AVN).²¹ Archaeologists describe the underlying method as pitched-brick vaulting, because the bricks are pitched (inclined) rather than laid in radial courses over a frame.⁴

History

Ancient origins

Vaults that need no centring are among the oldest masonry roofs. In Mesopotamia, pitched-brick vaults are known from excavations of the third and second millennia BCE. Their bricks are set on edge and leant at an angle, so that the vault can be completed without formwork.⁴ Assyrian builders used sun-dried brick vaults of this type for gates, underground tombs and drains.⁴

In Egypt, brick vaults appear from the early third millennium BCE and were in wide use by the end of the eighth century BCE.⁴ The granaries behind the Ramesseum, the mortuary temple of Ramesses II at Thebes (19th dynasty), are thought to be the earliest barrel vaults in Egypt. They span about 3.7 m (12 ft).⁴ Their lower third was built in horizontal courses. Above that, the rings were tilted back so that each held together as it was laid, needing no centring of any kind. The method gave the vaults an elliptical cross-section.⁴ The same principle, of brick rings leaning against a back wall, was used in Iran for the ṭāq-e żarbī barrel vault, built where timber was scarce.⁸

Nubia and Hassan Fathy

In Nubia the craft of building earth-brick vaults and domes without formwork survived into the twentieth century.⁵ The Egyptian architect Hassan Fathy (1900–1989) rediscovered it in the Nubian village of Abu al-Riche. In the 1940s he revived it as the basis of an architecture for rural Egypt built from local earth.⁵⁶ His best-known project was New Gourna, on the west bank of the Nile at Luxor. It was designed and built between 1946 and 1952 to rehouse the people of Old Gourna, whose houses stood above the ancient Theban tombs.⁶ Fathy described the experiment in Architecture for the Poor: An Experiment in Rural Egypt, published in English in 1973.⁷

New Gourna won wide architectural acclaim, but many of the families it was built for soon returned to their old village.⁵ Over the following decades the village fell into serious disrepair. Major buildings collapsed for lack of maintenance, many original mud-brick houses were demolished, and reinforced concrete buildings encroached on the site.⁶ New Gourna lies inside the World Heritage property of Ancient Thebes with its Necropolis, inscribed in 1979.⁶ UNESCO began a safeguarding project with Egypt's Ministry of Culture and the Governorate of Luxor in 2009.⁶ A later project, "Safeguarding Hassan Fathy's Architectural Legacy in New Gourna", worked on the village's khan, mosque, theatre and Fathy's own house in two phases, from 2019 to 2021 and from 2023 to 2024.⁷

The Sahel programme

The modern spread of the technique began in Burkina Faso. In spring 1998 the Burkinabè builder Séri Youlou and the French builder Thomas Granier built two prototype vaulted buildings at Boromo, in the centre-west of the country.² The Association la Voûte Nubienne was formally set up in 2000 as the base for a programme called "Earth Roofs in the Sahel". The association simplified and standardised the method so that it could be taught to local masons.²⁵ By about 2006 more than 200 vaults had been built and 35 builders trained, with 40 more apprentices in training. That October the association received an Ashoka Changemakers innovation award.²

Growth since then has been rapid. By 2012 more than 1,600 vaulted buildings had been completed in Burkina Faso, Mali and Senegal, and more than 260 masons trained. A programme had also begun in Zambia in 2009 under AVN's Belgian branch.⁵ The association now reports about 8,500 buildings, including homes, schools and health centres. It reports 4,900 masons trained and certified and about 108,000 direct and indirect beneficiaries in seven countries.¹ It estimates that using local earth instead of imported materials has saved about 215,000 tonnes of CO₂.¹

Materials

Everything structural in a Nubian vault building is made of earth:³²

• Bricks. Bricks for the vault are moulded on site from good-quality local earth mixed with straw and dried in the sun. Typical vault bricks measure about 24 × 12 × 4.5 cm (9½ × 4¾ × 1¾ in), deliberately thin and light so that they stick to the course behind. Walls use larger bricks.
• Mortar. Earth mortar, made from the same soil, is used for laying.
• Foundations. Trenches 20 to 45 cm (8 to 18 in) deep are filled with rock and earth mortar. They are raised 10 to 15 cm (4 to 6 in) above ground where surface water runs.
• Walls. Load-bearing side walls are about 60 cm (24 in) thick. Gable walls are about 40 cm (16 in) thick and lean slightly inward, about 1.5 cm per metre of height.

A vaulted house uses about three times as many earth bricks as a house of the same size with a corrugated iron roof.²

Construction

AVN's method breaks the work into a standard sequence.³²

1. Bricks. Mud bricks are made on site from local earth and left to dry.
2. Foundations and side walls. These are built first, with door and window openings. The arched lintels over openings are formed over oil drums used as temporary supports, the only formwork on site.
3. Gable walls. These are raised at each end of the room, leaning slightly inward.
4. Guide line. A steel cable is stretched between the gables along the base of the future vault. A sliding ring and cord on the cable keep the radius of each course constant.
5. Vault. Starting against a gable wall, the mason lays rings of bricks inclined at about 60° from the horizontal, so each new ring rests on the one before it. The section starts as a semicircle. Above about 70° the radius is lengthened slightly with each course, so the crown becomes elliptical. Masons alternate between the crown, where the mortar must be left to grip, and the easier sides.
6. Buttressing. The side walls are raised by eight to ten courses of large bricks above the springing, and the spaces between vault and wall are filled with earth. This gives the haunches weight and resists outward thrust.
7. Waterproofing and finish. The roof is coated with smooth mud mortar, then covered with plastic sheeting, then given an enriched earth render at least 5 to 6 cm (2 to 2½ in) thick. A roof terrace or an upper floor can be added on top.

The same principle of leaning courses can be used to build domes.⁵⁴

Dimensions and forms

Because the vault is built from small, thin bricks, its span is limited. The maximum inside width of a Sahelian Nubian vault is about 3.2 m (10½ ft).² Larger buildings are made by placing vaults side by side or at right angles, or by stacking a second storey on a vaulted ground floor.²³ Examples include:²

• Two-vault house. About 50 m² (540 ft²) of floor space, built with about 4,300 large bricks and 150 barrels of water.
• Two-storey house at Boromo. 110 m² (1,180 ft²) of usable floor area.
• Mosque at Kagalé. The first Nubian vault mosque in Burkina Faso, built in the winter of 2005–2006, with a 70 m² (750 ft²) ground floor under two large vaults.

Performance

Thermal comfort. Thick earth walls and vaults give a building high thermal mass, whereas thin uninsulated metal sheets offer almost none.⁹² In informal steel-roofed dwellings studied in South Africa, combining thermal mass with shading was the most effective way to reduce heat stress. A Nubian vault is a high-mass roof of exactly this kind.⁹

Durability and maintenance. The weak point of any earth roof is water. Nubian vaults rely on their waterproofing layer and render. The render needs renewing roughly every two years, after the rainy season.² Good drainage and foundations raised above runoff protect the base of the walls.²

Weight and thrust. A vault pushes outward as well as down. Thick side walls and the extra courses and earth fill over the haunches resist this thrust.²

Skills. Building a stable vault without formwork needs trained masons. AVN trains apprentices on site under master builders, using familiar tools.²

Cost and economics

In rural Burkina Faso, a Nubian vault house can cost up to 30% less than an earth-walled house of the same size with a corrugated iron roof.² That advantage disappears in towns, where bricks or earth have to be brought in.² Examples from the mid-2000s give a sense of scale:²

• A two-vault house of about 50 m² cost about US$800, including labour, materials and finishes but not land.
• The two-storey Boromo house cost about US$2,000: $650 for labour, $600 for materials and $750 for finishes.
• The Kagalé mosque cost about US$1,050.

Labour takes a larger share of the cost than it does with sheet-roofed houses. Much of the money therefore stays in the local economy as wages for masons and apprentices. Apprentices were paid about 25,000 CFA francs a month, depending on experience.²

Why it is used

• No timber. The vault needs no timber at all, neither for beams nor for formwork.²⁵
• No imported metal. The vault replaces a corrugated iron roof, and in rural areas the vaulted house is the cheaper of the two.²⁹
• Local materials and lower carbon. Using raw local earth cuts the carbon footprint of building.¹
• Jobs and skills. Building the vaults creates paid work and a recognised trade for trained masons.²¹

Limitations

Nubian vaults depend on an earth render that must be renewed after the rains.² Spans of a little over 3 m limit room sizes.² In towns the cost advantage over sheet roofing largely disappears.² New Gourna shows that buildings of this type depend on continuing maintenance and on the support of the people who live in them. Without both, earth buildings decay quickly.⁶⁷

Notable examples

• Ramesseum granaries, Thebes, Egypt (19th dynasty). The earliest known Egyptian barrel vaults, built without centring.⁴
• New Gourna, Luxor, Egypt (1946–1952). Hassan Fathy's earth-built village, now the subject of UNESCO-backed conservation.⁶⁷
• Prototype buildings at Boromo, Burkina Faso (1998). The first buildings of the modern Sahel programme.²
• Kagalé mosque, Burkina Faso (2005–2006). The first Nubian vault mosque in the country.²