Adobe is a natural building material made from clay, sand, water, and sometimes with a fibrous or organic material (straw, dung) added, which is shaped into bricks using forms, and dried in the sun. Adobe structures are extremely durable and account for the oldest surviving buildings on the planet.
The word adobe can be traced from the Middle Egyptian (c. 2000 BC) word dj-b-t (sun dried) “mud brick”. This evolved over the following 1400 years to become the Coptic word tobe (mud) “brick”, and subsequently into the Arabic at-tub “the tub”; “Tub” meaning (mud) brick. ‘At-tub’ was in turn assimilated into Old Spanish as adobe (a-dobe), still literally meaning “mud brick”. Buildings made of sun-dried earth are the single most common form of building type in the world and can be found throughout Africa, the Middle East, South America, Central America, Southwestern North America, China, India, and in Spain.
Adobe had been in use by indigenous peoples throughout the world for several thousand years. Its popularity and wide use can be at- tributed to the ready availability of the raw material, the simplicity of design, manufacture, and construction, making it easy and therefore cheap to build with.
In hot climates, compared to wooden, fired brick, cement and concrete buildings, adobe buildings offer significant advan- tages due to their greater thermal mass in keeping and maintaining the interior of a building cool. In cool climates, the thermal mass of adobe is ideal for use as a means of collecting passive solar energy and thereby warming the interior of the building.
An adobe brick is a composite material made from soil mixed with water and sometimes an organic material such as straw or dung. The soil composition typically contains clay, silt, fine sand, and course (sharp) sand. The ideal mixture is roughly 20% coarse sand, 40% fine sand, 20% silt, and 20% clay. This formula however will vary depending on the type of clay and sand available, and test samples must be made to determine the most suitable percentages for the mix, and what materials, if any, need to be added.
A simple and effective way to test the composition of the available soil is to take a handful of the soil, place it into a clear container (jam jar or similar), and add water, creating an almost completely saturated liquid. After the jar is sealed, the container is shaken vigorously for at least one minute. It is then allowed to sit on a flat surface for at least 12 hours to allow the various types of soil to stratify in the jar. The sediment that collects on the bottom of the jay will be the coarser sand, followed by finer sand, then silt, and lastly the clay on top. The strata should be quite clear and the approximate percentage of each in the jar obvious. If there is a shortage or excess of any of the composites when making the adobe mix, then an adjustment needs to be made by adding necessary material to approximate the above percentages.
The addition of straw can be useful in binding the brick together if there is low clay content in the soil, and it can also help in allowing the brick to dry evenly. Dung can also be added, and it offers the same advantages as straw but with the added benefit of its ability to repel insects.
Adobe bricks are made in an open frame known as a form. A commonly used size form creates bricks 250 mm wide x 360 mm long x 100 mm high as used in Central America, Mexico, New Mexico, Arizona, as it gives good thermal properties for the building, and is considered a convenient and efficient sized brick for workers to lay. The clay, sand, water mixture is made in a mixing pond, making sure that all parts are thoroughly mixed together, and any clay lumps are broken down. Once the mix is about the consistency of porridge, it is taken from the pond and pushed firmly into the form, filling it completely, and levelled to the top of the form; the form is then removed quickly revealing the formed brick.
After drying for several hours or days depending on available sun and temperature, the bricks are then turned on edge to finish drying. In very hot climates, drying the bricks in a shady area is advisable, as it slows down the curing process, and therefore reduces the potential for cracking. The brick’s thickness is determined partially due to its thermal ability, and also the structural stability of a thicker brick over the standard brick dimension.
An adobe brick can be made in nearly any shape or size, depending on the form that the mixture is pressed into, and provided that the drying time is even. Larger bricks or bricks that will be exposed to weather can be ‘stabilised’ to increase their strength and durability. The adobes are stabilised naturally by adding 5%-7% of lime, dung, or straw or artificially by adding 5%-7% cement or bitumen into the mixing process.
When laying adobes, the mortar used between the bricks is the same mixture and consistency as used to make the bricks (without straw, if straw was used in the bricks). The same mix can also be used for mud plastering both interior and exterior walls, whether it is a 2-coat or 3-coat finish. For added rain protection and resilience to the exterior, dung or lime can be added when mixed. Adobe has a relatively dense thermal mass.
In cool climates, an adobe wall can serve as a significant heat sink. A northfacing adobe wall (south facing in the northern hemisphere) can moderate heating and cooling providing that the wall is thick enough to remain cool on the inside during the heat of the day, but not too thick to prevent heat transferring through the wall during the evening. In warm or tropical climates, shading the adobe wall from the sun on the north side of the building by using roof overhangs, pergolas, etc, will cause the reverse of the cool climate to occur, thus allowing the interior rooms to remain relatively cool throughout both the day and night.
In either warm or cool climates, if designed correctly, the interior of an adobe walled building will only vary in temperature by about 4-5 degrees Centigrade (approx 17C – 22C) between summer and winter thereby making it either relatively warm or relatively cool depending on the outside temperature.
An adobe wall will need a solid foundation below it, as the adobe bricks in an unstabilised form are not suitable for laying below or on the ground. Ideally the foundations and foundation walls are built out of stone if locally available; if stone is not available, then stabilised adobe bricks, standard size fired clay bricks or as a last resort, cement blocks can be used. The width of the foundation walls are about 1-1/2 times the width of the adobe brick and should be brought up to about 200mm above ground level.
In wet areas a moisture barrier should be provided between the top of the foundation wall and the first course of adobe bricks. Adobe bricks are laid one course at a time around the perimeter of the building and the adjoining walls. Each course of bricks is laid the whole length of the wall on a layer of adobe mortar approximately 12mm thick. No more than six or seven courses of bricks should be built in a day, as the weight of the bricks may cause the wall to collapse. For two or more stories, a double thick adobe brick wall must be used from the ground floor to the first floor.
At window and door openings, a wooden lintel, slightly narrower than the wall width is placed over of the opening extending 300mm on either side to support the wall above (old railway sleepers are ideal for this!). At the bearing height of either roof or floor beams, a ‘ring’ or ‘bond’ beam is required above all the walls both external and internal to provide a horizontal structural plate for the roof to ‘tie’ the building together and to distribute the roof or floor load evenly along the wall. The ‘ring’ or bond beam can be of wood or structurally reinforced cement, and should be a minimum of two-thirds of the wall width and approximately 200 mm high.
The wood ring beam is attached to wall by using 750 threaded steel placed ap- proximately 1 metre on centres, and at each corner and held in place with a nut and washer. The reinforced concrete ring ‘beam’ is tied into the adobe wall using reinforced steel bars, also placed approximately 1 metre on centre, and at each corner. In both cases the steel goes down 6 courses into the wall, with the lower end bent at 90 degrees to go under the brick.
Metal tie-straps are set into the concrete ring beam at the location of the roof and/ or floor beams in order to fasten them securely. The finished adobe wall is typically plastered for protection with either two or three coats of lime or mud plaster, as the adobe wall needs to ‘breathe’. The exterior plaster can be protected from rain and other weather conditions with 3 or 4 coats of lime wash, which has the consistency of paint and can be applied with a brush. Natural pigment can be added to the lime wash to give the wall colour if desired. The interior walls can also be plastered with 2 or 3 coats of ‘Cretestone’, which is a gypsum plaster, and my particular favourite, and can be subsequently sealed with a mix of beeswax and mineral oil.
Under no circumstances must any cement products be used for mortar joints or to plaster walls, as cement does not bond well with the adobe, and it absorbs and retains moisture preventing the adobes from drying out. Adobe is a very forgiving and enjoyable material to build with; it allows for any shape and size of wall or building, up to 3 floors in height. The installation of plumbing and electrical lines is simple, as is designing and building built-in wardrobes, bookshelves, niches, etc.
When completed, the exterior of an adobe house is soft and easy on the eye, and can easily harmonise with its surroundings, while the interiors have a very serene and calming elegance, making an adobe building a very positive, healthy, and enjoyable environment to live or work in.