Wednesday, June 4, 2008

Adobe - from Wikipedia

Adobe - Wikipedia, the free encyclopedia

Composition of adobe

An adobe brick is made of clay mixed with water and an organic material such as straw or dung. The soil composition typically contains clay and sand. Straw is useful in binding the brick together and allowing the brick to dry evenly. Dung offers the same advantage and is also added to repel insects. The mixture is roughly half sand (50%), one-third clay (35%), and one-sixth straw (15%).

Adobe bricks

Bricks are made in an open frame, 25 cm (10 inches) by 36 cm (14 inches) being a reasonable size, but any convenient size is acceptable. The mixture is molded by the frame, and then the frame is removed quickly. After drying a few hours, the bricks are turned on edge to finish drying. Slow drying in shade reduces cracking.

The same mixture to make bricks, without the straw, is used for mortar and often for plaster on interior and exterior walls. Some ancient cultures used lime-based cement for the plaster to protect against rain damage.

The brick's thickness is preferred partially due to its thermal capabilities, and partially due to the stability of a thicker brick versus a more standard size brick. Depending on the form that the mixture is pressed into, adobe can encompass nearly any shape or size, provided drying time is even and the mixture includes reinforcement for larger bricks. Reinforcement can include manure, straw, cement, rebar or wooden posts. Experience has shown that straw, cement, or manure added to a standard adobe mixture can all produce a strong brick. A general testing is done on the soil content first. To do so, a sample of the soil is mixed into a clear container with some 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 until the soil sediment has either collected on the bottom or remained a blended liquid. If the sediment collects on the bottom, that indicates there is a high clay content and is good for adobe. If the mixture remains a liquid, then there is little clay in the soil and using it would yield weak bricks.

The largest structure ever made from adobe (bricks) was the Bam Citadel, which suffered serious damage (up to 80%) by an earthquake on December 26, 2003. Other large adobe structures are the Huaca del Sol in Peru, with 100 million signed bricks, the ciudellas of Chan Chan and Tambo Colorado, both in Peru.

Thermal properties

An adobe wall can serve as a significant heat reservoir. A south-facing (in the Northern Hemisphere) adobe wall may be left uninsulated to moderate heating and cooling. Ideally, it should be thick enough to remain cool on the inside during the heat of the day but thin enough to transfer heat through the wall during the evening. The exterior of such a wall can be covered with glass to increase heat collection. In a passive solar home, this is called a Trombe wall. Adobe has a relatively dense thermal mass, and is most useful in tropical climates. In temperate climates it is less effective to heat a structure this way due to heat leaching by the ground and walls.

Adobe wall construction

When building an adobe structure, the ground should be compressed because the weight of adobe bricks is significantly greater than a frame house and may cause cracking in the wall. The footing is dug and compressed once again. Footing depth depends on the region and its ground frost level. The footing and stem wall are commonly 24" and 14", much larger than a frame house because of the weight of the walls. Adobe bricks are laid by course. Each course is laid the whole length of the wall, overlapping at the corners on a layer of adobe mortar. Adobe walls usually never rise above 2 stories because they're load bearing and have low structural strength. When placing window and door openings, a lintel is placed on top of the opening to support the bricks above. Within the last courses of brick, bond beams are laid across the top of the bricks to provide a horizontal bearing plate for the roof to distribute the weight more evenly along the wall. To protect the interior and exterior adobe wall, finishes can be applied, such as mud plaster, whitewash or stucco. These finishes protect the adobe wall from water damage, but need to be reapplied periodically, or the walls can be finished with other nontraditional plasters providing longer protection.

Adobe roof

The traditional adobe roof has been generally constructed using a mixture of soil/clay, water, sand, and other available organic materials. The mixture was then formed and pressed into wood forms producing rows of dried, earth bricks that would then be laid across a support structure of wood and plastered into place with more adobe. For a deeper understanding of adobe, one might examine a cob building. Cob, a close cousin to adobe, contains proportioned amounts of soil, clay, water, manure, and straw. This is blended, but not formed like adobe. Cob is spread and piled around a frame and allowed to air dry for several months before habitation. Adobe, then, can be described as dried bricks of cob, stacked and mortared together with more adobe mixture to create a thick wall and/or roof.

Roof materials

Depending on the materials available, a roof can be assembled using lengths of wood or metal to create a frame work to begin layering adobe bricks. Depending on the thickness of the adobe bricks, the frame work has been performed using a steel framing and a layering of a metal fencing or wiring over the framework to allow an even load as masses of adobe are spread across the metal fencing like cob and allowed to air dry accordingly. This method was demonstrated with an adobe blend heavily impregnated with cement to allow even drying and prevent major cracking.

Traditional adobe roof

More traditional adobe roofs were often flatter than the familiar steeped roof as the native climate yielded more sun and heat than mass amounts of snow or rain that would find use in precipitous roofs. Moisture, however, is often foe to a composite of mud and organic matter, so the introduction of cement is often more common to help ward off any undue water damage. It is at this turn that sense is required before the construction of any adobe is begun, be sure that the location for such a structure is similar to the climate it naturally comes from, that is, a hot, arid climate. Cool and moist climates would do well with moisture precautions planned out.

Raising a traditional adobe roof

To raise a flattened adobe roof, beams of wood or metal should be assembled and span the extent of the building. The ends of the beams should then be fixed to the tops of the walls using the builder's preferred choice of attachments. Taking into account the material the beams and walls are made from, choosing the attachments may prove difficult. In combination to the bricks and adobe mortar that are laid across the beams creates an even load-bearing pressure that can last for many years depending on attrition. Once the beams are laid across the building, it is then time to begin the placing of adobe bricks to create the roof. An adobe roof is often laid with bricks slightly larger in width to ensure a larger expanse is covered when placing the bricks onto the beams. This wider shape also provides the future homeowner with thermal protection enough to stabilize an even temperature through out the year. Following each individual brick should be a layer of adobe mortar, recommended to be at least an inch thick to make certain there is ample strength between the brick's edges and also to provide a relative moisture barrier during the seasons where the arid climate does produce rain.


Adobe roofs can be inherently fire-proof, an attribute well received when the fireplace is kept lit during the cold nights, depending on the materials used. This feature leads the homeowner and builders to begin thinking about the installation of a chimney, a feat regarded as a necessity in any adobe building. The construction of the chimney can also greatly influence the construction of the roof supports, creating an extra need for care in choosing the right materials. An adobe chimney can be made from simple adobe bricks and stacked in similar fashion as the surrounding walls. Basically outline the location and perimeter of the hearth, minding the safety elements common to a fireplace, and begin to stack and mortar the walls with pre-made adobe bricks, cut to size.

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