by Nick Gromicko, CMI®
Adobe is a natural building material made from clay and sand mixed with water and an organic binder, such as sticks, straw or dung. Adobe structures are more common in inhospitable climates and where lumber is scarce. They're commonly built in low-income communities that lack the resources to construct more complex or conventional designs whose components are more costly. InterNACHI inspectors should be prepared to find adobe buildings in all types of communities -- even wealthy ones -- as green building advocates and experimental builders have become more attracted to adobe construction in recent years.
The History of Adobe
Due to the abundance of its constituents and ease by which it is produced and shaped, adobe construction is truly ancient and universal. Even the word “adobe” has existed for around 4,000 years, with little change in its pronunciation or meaning; it can be traced from the Middle Egyptian word for “mud brick” and was borrowed by Late Egyptian, Demotic, Coptic, Arabic, Old Spanish and, finally, English. Entire cities have been made from the material and many adobe buildings have seen continuous use for thousands of years. Even within the United States, many of the oldest buildings, indigenous and European, were made from adobe. Two such examples are the San Miguel Mission of Santa Fe, New Mexico, thought to be the oldest church in the country, and Pueblo towns and villages that have withstood the winds of the American West since 750 AD. Today, the use of adobe is still widespread across the American Southwest, North Africa, West Africa, western Asia, South America and southern Europe.
Perhaps not surprisingly, adobe is an ideal building material for the climates in which it is most commonly found, such as deserts and other regions typified by hot days and cool nights. The material’s great thermal mass cannot transfer heat without a relatively long input of sun exposure, keeping the interior cool during the daytime when the sun burns intensely. By the time the sun sets, the thick adobe walls will have absorbed the sun’s heat, which is then slowly released into the living space when the outside temperature is at its lowest. By the time the structure has exhausted its heat reserves, the sun will rise again, starting the cycle anew.
Adobe provides excellent soundproofing and fire-resistance, which is helpful when a fire must be kept lit during cold nights. It is also easy to produce; according to Sustainable Sources, a green building journal, adobe requires less than 1/150th of the energy required to manufacture a similar amount of Portland cement, and less than five times the energy required to produce ordinary brick.
Adobe Construction Elements
The following materials are commonly found in adobe construction:
- Bricks: While adobe can simply be piled up and shaped into a structure (as did American Indians before their contact with the Spaniards and their architectural influences), it is generally cast into uniformly sized bricks before they are assembled. The adobe mixture, by weight, is roughly half sand, one-third clay, and one-sixth straw or some other organic, fibrous material. Modern bricks are 14 inches long, 10 inches wide and 4 inches thick. Water is used to turn the clay and sand into a more fluid, malleable consistency, and the straw helps the bricks shrink more uniformly while they dry. Visible alkali salts or brackish water should be avoided for mixing adobe. A test brick is sometimes made to ensure the suitability of local soil. To form bricks, the wet adobe mix is poured into molds or pressure-molded using special machinery and then left to cure for weeks. Bricks are sometimes kiln-fired in low temperatures, but their use at the exterior is discouraged in climate zones with daily freeze-thaw cycles.
- Mortar: Traditionally, mud mortar was used to lay adobe bricks, as the two have the same rates of thermal expansion. Modern adobe bricks may be laid with cement (or some other strong mortar) if the adobe bricks are stabilized during production using certain admixtures, such as asphalt, in order to limit the adobe’s water adsorption. Because the components of cement renders it more resilient than the adobe, adding cement mortar will accelerate the deterioration of natural (or non-stabilized) adobe bricks.
- Foundations: The foundations of historic adobe buildings were made from a variety of natural materials found locally, including bricks and tile fragments from other structures, and field stones and rocks. Adobe structures were rarely constructed over basements or crawlspaces. Modern building codes prohibit the use of adobe as a foundation material due to its low structural strength.
- Walls: To compensate for their structural weakness and to support a heavy roof, adobe walls are massive. They may account for 15% to 20% of the weight of the whole house, while frame-house walls account for about 5% of the structure’s weight. The aspect ratio, or the ratio of the height of the wall to its thickness, should not be higher than 10 or the structure will be unstable. For this reason, adobe structures are almost exclusively only one or two stories tall and used for single-family housing.
- Floors: Floors may be flagstone, tile, fired brick or adobe brick.
- Roofs: Older adobes of the American Southwest were typically built with flat roofs with parapet walls. Logs and wooden poles were incorporated into the roof for support. Sawn planks and boards are used in newer adobe roofs and for repairs in older adobe. Adobes built over the last hundred years in New Mexico have sheet metal for roofing.
- Bond beams: On top of the highest layer of brick is the bond beam, which provides a horizontal bearing plate for the roof to distribute the weight more evenly along the wall. They help anchor the roof against wind loads and secure the structure against earthquakes and the gradual effects of gravity. The International Building Code (IBC) states:
The wood tie beam shall be a minimum of 6 inches in thickness, except as provided for walls thicker than 10 inches above.
- Lintels: Lintels are used to distribute loads over entrance ways and window openings, as well as for decoration. Concerning lintels in adobe, the IBC states:
Lintels shall be minimum in size, 6 inches by wall thickness. All ends shall have a wall bearing of at least 12 inches. All lintels, wood or concrete, in excess of 9 feet shall have specific approval of the building official.
- Footings and stem walls: The footing and stem wall of adobe houses should be 24 inches and 14 inches, respectively, both of which are somewhat larger than those for frame houses whose difference is due to the greater weight of adobe walls. In colder regions, such as the mountains of New Mexico, adobe footings are dug even deeper to avoid the expansion and contraction caused by freezing cycles.
Adobe walls that are not stabilized require exterior coatings to protect against moisture intrusion. Even in stabilized adobe, protective coatings can retard surface deterioration caused by sand, wind and insects.
While inspecting adobe homes, InterNACHI inspectors may encounter the following types of coatings:
- Mud plaster: This was typically used on historic adobe houses because it bonds easily with the brick and exhibits the same expansion under heat. The mud plaster must be smoothed manually, which can be a time-consuming process.
- Lime plaster: Consisting of lime, sand and water, lime plaster is stronger than mud plaster but it tends to crack easily. Walls are sometimes cut with hatchets to create grooves that encourage the lime plaster to adhere to the adobe. This style became popular during the early 20th century.
- Whitewash: Similar to mud plaster, whitewash includes gypsum and acts as a sealant when it’s brushed onto the adobe. It has fallen out of favor in recent years due to its impermanence and high maintenance, as it must be re-applied regularly.
- Stucco: This consists of cement, sand and water and is applied with a trowel over a wire mesh nailed to the adobe surface. This material has enjoyed popularity because it requires little maintenance when applied over fired or stabilized adobe brick and because it can be easily painted. Many New Mexican structures appear to be adobe but they’re actually stucco-clad wood or concrete, as their builders use stucco to convey a historic adobe appearance.
- Synthetic Water Repellents: Adobe repellents or sealers are liquid products applied to the surface of adobe to protect adobe bricks from moisture.
Adobe Buildings and Moisture Damage
Adobe structures are extremely vulnerable to the effects of moisture, which are mainly in the forms of rainfall and the local water table. Adobe will lose its structural strength as it becomes saturated, turning into putty and eventually flowing and dripping as a liquid. Rainwater splashes can cause coving, which is the hollowing-out of the wall just above grade level. The dr
ying process after rainfall can create furrows, cracks, deep fissures and pitting. Weakened walls will bulge, deform and eventually collapse under the roof’s weight. For these reasons, the survival of adobe structures depends on keeping them free from excessive moisture.
The destructive effects of moisture on adobe buildings may be substantially halted by the following remedies:
- Slope the land adjacent to the structure so that rainwater does not pool next to its lower walls. Consider creating drainage channels, French drains or swales to direct rainwater away from the building.
- Remove trees, plants and other vegetation from the adobe structure’s foundation and walls. Moisture can be collected by their roots beneath, next to or even on the building’s walls. Roots might also be growing into the structure and physically destroying the walls from within. See our article on Tree Dangers for more information about the problems of intrusive roots.
- Apply hydrophobic coatings to repel water from the walls. In older structures, mud plaster, whitewash or stucco may be used to maintain the historic appearance.
- Slope the roof. Flat roofs will allow rainwater to pool. Parapet roofs are particularly troublesome, as their high walls will allow rainwater to pool, slowly disintegrating the adobe.
- Stabilize the bricks near ground level. Natural untreated bricks should not be used within 4 inches of ground level, where moisture intrusion is most likely.
Adobe Buildings and Earthquake Dangers
Adobe is a heavy yet relatively weak building material, which makes adobe structures particularly vulnerable to earthquakes. The typical mode of collapse is out-of-plane failure of the walls, resulting in the loss of support for the roof.
The scope of this danger can be seen in the devastation caused by the 2003 earthquake in Bam, Iran, where a 6.6 temblor leveled thousands of adobe homes. Along with them fell the 2,500-year-old Bam Citadel, which, at 180,000 square meters, was then the largest adobe structure in the world.
In seismically active Peru, one can further see how not to design earthquake-resistant adobe homes. Traditional construction techniques used there do not call for binding the four walls together, making them vulnerable to even modest seismic shifts in a country that has experienced more than 450 major earthquakes in the last century alone. One Architecture Week reporter commented, “You can see gaps at the corners and between the walls and the roof. That means that when there's an earthquake, the walls just flop outward like cards. And the roofs, which can weigh up to 11 tons, come crashing down, crushing people to death."
Experimental building techniques in Peru have revealed that strips of electro-welded wire mesh may be used to "sew" the house together along the inside and outside seams of the walls, which are then covered with concrete.
Local materials, such as bamboo and sugar cane, have also been used successfully to strengthen houses against earthquakes. These measures do not make the structure indestructible but, rather, allow the occupants extra time to escape before the ceiling comes crashing down.
Building codes related to the structural reinforcement of adobe are more stringent in seismically active regions, and inspectors and adobe homeowners should learn about the earthquake dangers for their region. An engineering analysis should be performed to determine whether and what type of reinforcement may be necessary. According to the IBC, steel reinforcement, if used in adobe, "should be embedded in a cement-based mortar and grout unless there is a positive interlacing of reinforcement around the earthen material." Misguided attempts to secure adobe walls often cause more harm than good, as steel reinforcement inserted vertically into adobe bricks can cause cracking because it prevents the new adobe from naturally shrinking. Steel will also not bond with adobe as easily as it will with fired brick or concrete.
Other Inspection Tips
Inspectors may notice that adobe walls are pitted, bulging or cracked, or the roof may be sagging, but the cause of these problems may not be obvious. Yet, historic and modern adobe structures share common deterioration problems, so it pays to understand the basic vulnerabilities inherent in the material.
Consider the following sources of adobe deterioration:
- Intrusive vegetation: Perhaps not surprisingly, adobe tends to attract vegetation and animals that naturally live in soil. Seeds deposited by animals or blown by the wind will germinate there. Insects, rodents and birds will find adobe walls and foundations very comfortable to nest in. Plant roots will forcefully degrade adobe bricks and retain moisture, undermining the strength of the structure. All plant and animal pests should be removed from the structure unless their removal would cause further damage.
- Sand erosion: Wind-blown sand is a common source of adobe deterioration in desert climates. This damage is generally found at the top half of the wall and at the corners, where it can be distinguished from coving that is caused by rain splashing on the lower portion of the wall. New adobe mud may be applied where sand has damaged the walls or roof. Trees may also be planted as windbreaks, although they should be planted far enough away from the structure so that their roots do not themselves pose a threat.
- Incompatible materials used for repairs: Older adobe structures have periodically been repaired using cement or steel, which may cause the surrounding adobe to crumble. The reason for this is simple: the relatively weaker adobe material is crushed by the newer materials, which expand at different rates due to temperature changes. Watch for the presence of steel doors and wooden lintels. Latex and plastic wall coatings applied to their exterior will not expand with the rest of the structure and will eventually cause portions of the wall to break off.
- Cracks in walls, foundations and roofs: In adobe, cracks are generally quite visible, but their causes may be difficult to diagnose. Some cracking is normal, such as the short hairline cracks that occur during the curing process as the adobe shrinks and continues to dry out. More extensive cracking, however, usually indicates serious structural problems. The IBC states:
No units shall contain more than three shrinkage cracks, and no shrinkage crack shall exceed 2 inches in length or 1/8-inch in width.
- The ground is not compressed or sufficiently tamped before building: Before assembling an adobe structure, the ground should be compressed because the weight of adobe bricks is significantly greater than a conventional frame house. Uncompressed earth may allow sinking of the structure, eventually resulting in wall cracks, among other problems.
In summary, adobe is an ancient and beautiful building material, and there are ways to prevent it from returning to the earth on which it stands. InterNACHI inspectors, as well as green homeowners, can help avert costly mistakes and potential disasters by learning some fundamentals about its characteristics.
Some of the Information and images contained in this article were sourced from the Technical Preservation Services of the National Park Service, ArchitectureWeek, and the International Code Council.