The NV Technique
The AVN version of the Nubian Vault
The Nubian Vault (NV) technique is an age-old method of timberless vault construction, originating in upper Egypt. It uses only earth bricks and earth mortar. Nubian vaults built over 3,000 years ago at the Ramesseum mortuary temple, Luxor, are still standing.
Ramesseum, Luxor, 1,250 BC
AVN masons in Boromo, Burkina Faso, 2009
During the last ten years, AVN has successfully introduced a simplified, standardised version of this ancient technique in Burkina Faso, Mali, Senegal, and Zambia. This standardised NV technique is:
- ecologically sustainable - no corrugated iron roofing sheets, nor timber beams, rafters, or supports;
- carbon neutral - none of the construction materials are manufactured, or transported long distances, nor do any trees need to be cut down;
- economically viable - only locally available raw materials (earth, rocks, and water) are used, favouring local economic circuits and self-sufficiency;
- comfortable - due to the excellent thermal and acoustic insulation properties of earth construction;
- durable - NV buildings have a far longer lifetime than those with corrugated iron and timber roofs, and maintenance is simple;
- modular - applicable to a wide range of buildings (houses, schools, healthcentres...), of different styles (flat terrace roofs, two-storey buildings, courtyard buildings...), which are easily extendable;
- vernacular - incorporating tradtional practices and aesthetics of earth architecture.
AVN's simplified and standardised version of the technique has been adapted to the climatic conditions and traditional know-how of the Sahel region, making it easy to learn on-the-job:
- roofs are exclusively vaulted (no domes), and can incorporate a traditional flat roof-terrace
- vaults are a standard width (3,25 metres), made from mud bricks of a standard size
- a guide cable is used to define the curvature of the vault
- walls are a standard thickness, and openings for doors and windows a standard size
- plastic sheeting is incorporated in the roof to reduce water penetration and erosion.
The major cost element in using the VN method is labour, often provided by family members and neighbours on an exchange / barter / self-build basis, thus keeping cash in the local economy; the raw materials (earth, rocks, water) are locally available and ecologically sound; construction with mud bricks and mortar is traditional in the Sahel region - the innovation of vault construction can easily be incorporated into existing practice.
Building a Nubian Vault house: an overview
The main critical stages in the construction of a NV house are summarised and illustrated below: learning to build a vault, however, is a skill that can only be acquired through experience on a NV building site.
Timing is crucial: construction can only occur during the dry season (September to May in the Sahel, May to September in Zambia), and must be completed before the seasonal rains start. Fortunately, the dry season is also the time when there is little agricultural work to do in the village fields, and men are available to help as unskilled labor, or to train as apprentices.
Choosing a site
It is essential to choose a site that is not in danger of seasonal flooding; dry gullies or low-lying sites near rivers should be avoided, as should sites which might lie close to rainwater run-offs.
A house destroyed by seasonal floding in Burkina Faso
Given the modularity of the NV concept, it's also a good idea, especially on plots of limited size, to plan ahead for future extension of the house, or the addition of an extra storey - in the latter case, the foundations need to be strong enough to take the extra weight.
Obtaining building materials
Advance scheduling of the sourcing of building materials (earth, water, rocks) and the making of the mud earth bricks is essential: ideally, when the team of NV masons arrives to start work, the rocks for the foundations, the water and earth for the mortar, and the bricks for the walls, should all be ready and on-site. Any delays here will not only cause problems between mason and client, but also run the risk of being unable to complete the house before the start of the next rainy season.
Depending on the properties of the ground and the site of the building, foundations can vary from 40 to 80 cm deep; they need to be 70 cm wide for load-bearing walls, and around 50 cm wide for the gable walls. They are filled with rocks, bound with an ordinary earth mortar.
In sites where there may be strong flows of surface water during the rainy season, the foundations should be raised 10 – 15 cm above ground level.
Load-bearing walls which carry the vault are 60 cm thick, and are made up of rows of super-imposed mud bricks. Each course is made up of a row of bricks laid lengthwise alongside a row laid width wise, using an earth mortar. The orientation of the bricks alternates with each course.
Recesses and openings are built into the thickness of the walls, for doors, windows, cupboards, alcoves, and shelving. These are headed with arched lintels (built over oil drums as a temporary support); their use can be modified according to the needs of the owner. These are built before construction of the main vaults starts.
Gable walls are built in courses of bricks laid lengthwise, and are around 40 cm thick. They are raised so as to lean slightly inwards (by around 1.5 cm for each metre of height).
Construction of the vault
Flat bricks (24cm X 12 cm X 4 cm) for the vaults are made in advance from good quality earth, such as that traditionally used for building granaries.
No formwork or shuttering is used to support the vault during construction. The mason, using a mortar made from the same earth as the bricks, starts the vault by laying the first courses against one of the gable walls.
A cable made of 6 steel wires coiled together is stretched between the two gable walls, at the height of the base of the vault. This defines the central axis of the vault, and a cord fixed to a sliding ring on the cable acts as a guide to ensure that the mason maintains a constant radius for the vault. Towards the top of the vault, the mason adds a couple of fingers width to each joint to develop an ogival (catenary) shape.
The builder alternates work on the top of the vault, which requires some waiting time between each course for the mortar to dry, and the sides, which are easier to construct.
This 1-min video shows an NV mason laying a course of vault bricks, using the guide cable.
Buttresses are formed on the completed vaults by raising the side walls by 8 to 10 courses of large bricks, and by filling the gap so created with plain earth. The height of the buttresses depends on the client’s needs - many prefer to raise them high enough to make a flat roof terrace over the vault (and the flatter the roof, the more resistant it is to erosion during the rainy season).
Finally, the roof is water-proofed, using plastic sheeting (locally manufactured FasoPlast, costing around 400 CFA francs / square metre), over a smooth coat of mud mortar. The sheeting is then covered with a rendering of enriched mud mortar (using traditional adjuvants), which both protects the plastic from damage by the sun, and provides additional protection against heavy rains. It also acts as a warning signal to the proprietor - if the (black) plastic sheeting is revealed after heavy rains - as in the RH photo above - then it’s time to repair the rendering!
Until recently, VN builders tended to deal only with the main structure of a building - the foundations, walls, openings, and vaults - leaving the finishing work to other local trades. But given the commercial incentive to become entrepreneurs able to advise their clients on, and to estimate costs for, all aspects of the work, more and more VN builders are taking on the finishing tasks (rendering, tiling, electrical work, plumbing...). Such tasks can be carried out during the rainy season, when major construction work is not feasible.
Depending on the resources and the tastes of the owner, various options are possible:
- renderings and floors of cement mortar might replace those made with mud
- coloured or plain lime washes are inexpensive to apply to the interior walls, and help reduce insect infestation
- installation of an electrical supply during construction is straightforward
- interior showers, with cement floors and tiling, can be installed
- interior partition walls (20cm thick) can be fitted.
Clients wishing to reduce regular maintenance to a minimum can, as an alternative to enriched earth, use a final rendering coat of thin fibro-cement mortar over an earth base. It is advisable to wait one year after construction before doing this, to allow for any settlement to occur. The fibro-cement rendering can then be further waterproofed with a coating of hot tar. However, such precautions may often be unnecessary, and clients are advised to wait at least a year to judge the state of the roof after one rainy season before making this extra expenditure.
Several technical options are available to eliminate the need for re-rendering of external walls:
- on earth brick walls, to use a thin cement rendering (very common in the Sahel regions), or a mix of tar/sand/earth/lime, which gives a very attractive finishing coat
- to use laterite bricks or concrete blocks for the outer layer of load-bearing walls, with cement mortar joints.
Such modifications can lead to major cost increases for purchase of imported materials such as cement; regular maintenance and re-rendering with enriched earth mortars is in fact sufficient to guarantee the durability of the buildings (as demonstrated by the traditional architecture of Gourounsi in Burkina Faso).
> NV Building Guidelines A summary of the NV guidelines and instructions for builders and clients, together with a sample quantities estimate (PDF, 224 KB)
> Comparison of principal building techniques in the Sahel A 1-page table comparing the pros and cons of the five main construction techniques (PDF, 4.1 MB)