- What is Adobe?
- Adobe Construction Techniques
- Traditional Surface Coatings
- Adobe Deterioration
- Sources of Deterioration
- Repairing and Maintaining
Adobe is also known as the sun-dried brick. It is one of the oldest building materials known to man. Originally, sun-dried bricks were never fired. They were consisted of grass or straw, water, clay, gravel, and sand mixed together by hand. This mixture was then placed in special wooden molds and left to dry under the sun. Over the years, the technique suffered a few modifications. One of the most important upgrade to the original method was to kiln fire the bricks. These fired bricks may look similar in size to the unfired ones, but they differ in strength, color, and texture. Additionally, some constructors went even further and changed the composition of the adobe brick by adding asphalt, cement, and bituminous materials. However, just like the previous type, these bricks have different strength and appearance features.
The original adobe construction methods in North America have not varied for more than three and a half centuries. The adobe construction techniques employed in the sixteenth century in the Southwest are still used today. However, the unfired adobe bricks come with a lot of disadvantages. Among the most important drawbacks of unfired adobe bricks is the fact that they never harden. They swell and shrink constantly depending on their water content. For instance, if the water content is high, the brick is lower in strength and highly unstable. Bricks that are low in water content, on the other hand, present an increased level of strength and durability.
Adobe bricks will not permanently bond with stone, wood, or metal simply because they react totally different under the same weather conditions. In other words, adobe bricks exhibit much greater movement than most materials. As a result, the brick may develop twisting and cracking in the area where it interfaces other materials. However, since adobe bricks cannot be the only construction materials used in a building project, connecting them to other materials is inevitable. So, it appears that people have found materials that react less aggressive when connected with adobe bricks. To name a few, cement and lime mortars, wood, and fired bricks are some of the construction materials that are considered safe to use in adobe constructions. For example, wood may be used for an adobe building’s roof, and stone may be used for its foundation. Since these materials are held in place by the compressive weight of the wall above them or by their own weight, the variations and possibilities in design have been strictly limited by the physical constraints of the material.
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Floors: In the historic adobe buildings, flooring materials were placed straight on the ground, most of the time, with no subflooring preparation whatsoever. They have varied from flagstone, to tile, fired brick, wood, or earth, to adobe brick.
Building Foundations: Historically, adobe building foundations varied because of the difference in availability of materials and local construction practices. Some were almost nonexistent, other were large and substantially built. As a construction materials for adobe building foundations, people have used cavity walls, fieldstones, or bricks. The cavity walls were mostly infilled with seashells, fragments, tile, and rubber stone. It is important to mention that adobe structures were very rarely built over crawlspaces or basements.
Walls: Due to the adobe bricks’ weakness, adobe walls tended to be seldom rose over two stories. Additionally, they were massive, and very often, braced with buttresses in order to increase stability. However, since adobe buildings were load-bearing with low structural strength, their height was significantly limited by the physical constraints of the adobe brick. As a matter of fact, all adobe mission churches in the Southwest were approximately thirty five feet high.
In order to even further increase the strength and stability of adobe constructions, a long wooden timber was placed within the last courses of adobe bricks. The main goal of this timber was to distribute the weight of the roof evenly on the walls of the structure. This method was very popular in the Southwest.
The Brick: The adobe brick was molded from clay and sand mixed with water to an homogeneous consistency. In order to increase the flexibility of the brick, grass or straw was added as a binder. Another advantage of adding straw or grass was to help the bricks dry more uniformly. For durability, the most important aspect was the sand-to-clay ratio. After all these materials have been mixed together, the mixture was placed in wooden molds, tamped, and then leveled by hand. Immediately after, the bricks were turned out of their molds in order to dry on a level surface. To prevent the bricks from sticking, grass or straw was placed on the ground. The bricks were left to dry for about five days, and then they were raised on their ends and left for air-curing for about four weeks.
Mortar: Most walls were consisted of adobe bricks laid with mud mortar. This mortar exhibited the same properties as the bricks: deterioration, thermal contraction and expansion, weak to shrinking and swelling, and susceptible to the same rate of moisture absorption. However, despite it’s major drawbacks, this type of mortar was the only bonding material that worked with adobe bricks. Today, lime and cement mortars are used with fired adobe bricks; unfortunately, they are still incompatible with unfired bricks simply because they have different contraction and expansion rates.
Roofs: Southwest adobe roofs used to be flat with low parapet walls. Logs were placed on top of the walls, which supported wooden poles. These wooden poles supported lathing. The wood that was used the most was cedar, mesquite, or aspen. Shaped squared timbers or lightly dressed logs were spaced on close centers resting on either the decorated cantilevered blocks or horizontal wooden member which topped the adobe wall.
The roughly dressed logs often projected through the facade defining the historic adobe construction detail. In order to push the aesthetic features of the building even further, hand split planks or wooden poles were then laid across the top of the roughly dressed logs. In the West Tucson and Texas areas, cactus ribs (saguaro) were used to span between the logs. After railroad transpiration arrived, planks and sawn boards were often used instead. This usually happened in the late 19th and early 20th century.
Next, fabric, plant fibers, or cedar twigs were placed on top of the planks or poles. Adobe earth was then compacted on top of the lathing. Twigs were necessary only if planks were used. Finally, a coating of adobe mud was applied over the entire roof. Although there were called flat roof, a certain slope was usually applied in order to direct the rainwater towards the parapet walls. Sometimes, sheet metal, tile, or hollowed logs (called “gargolas” or “canales”) were used as drains.
Hipped and gable roofs became highly popular in adobe constructions in the 19th and 20th centuries. They are also called “territorial” preferences and styles for certain materials developed. For instance, tiles were very popular in southern California. In New Mexico, the prevalent material for roofs was sheet metal.
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One of the main problems with adobe surfaces is that they need regular maintenance. So, in order to protect both the interior and the exterior surfaces of new walls, surface coatings like stucco, whitewash, lime plaster, and mud plaster have been used. When applied on the exterior surface of an adobe wall, these coatings prevent surface deterioration. In the past, these techniques have been readily available and relatively inexpensive for the adobe owner. However, recent changes in socioeconomic and cultural values plus the increase in labor costs have forced many owners to seek more lasting materials as alternatives to these historic and once relatively cheap surface coatings.
Cement Stucco: In the States, cement stucco was used as an adobe surface coating in the early 20th century, mostly for the revival styles from the Southwest adobe architecture. Cement stucco consists of sand, cement, and water. In order for the mixture to adhere to the surface, a wire mesh has to be installed in advance. Using a trowel, the mixture is then applied in one to three coats. The main benefits of cement stucco are ease of maintenance and the possibility to be painted. The only drawback is that it cannot be applied over unfired adobe bricks. Cement stucco itself does not bond with either unfired or fired adobe. However, it stays in place because of the wire mesh that is nailed to the wall. Unfortunately, nails tend to rust due to the high water content of adobe and become loose. Consequently, the stucco layer will become loose from the adobe as well.
Lime Plaster: The main adversary of lime plaster was mud plaster. However, the latter had much bigger problems than lime plaster. Lime plaster was widely used in the 19th century as both an interior and exterior coating. It’s stronger points are also its weakest. Since lime plaster is a much harder material than mud plaster, it tends to crack pretty easy. It consists of sand, lime, and water and is usually applied in heavy coats. In order to make the lime plaster adhere to adobe, walls were usually scored diagonally with hatches, making grooves of about one and a half inch deep. The grooves were then filled with a mixture of small chips of broken roof tiles or stone and lime mortar. The wall was then covered heavily with lime plaster.
Mud Plaster: When it comes to adobe, mud plaster is perhaps the most compatible surface coating. This is because of its very own consistency. Mud plaster is composed of sand, clay, grass or straw, and water, which is the exact mixture used to create adobe bricks. Therefore, mud plaster adheres very good to adobe. Unfortunately, applying mud plaster is a laborious and time consuming job. Once the mud plaster is applied on the wall, its surface has to be smoothed by hand. Often times, rounds stones, sheepskins, or deerskins were used in the process. In order to add color to the final layer, ocher or pink pigments were added to the mixture.
Whitewash: This type of surface coating has been used on adobe buildings before recorded history. It consists of clay, water, and ground gypsum rock. Whitewash acts as a sealer for the adobe bricks, which can be either applied with large pieces of coarse fabric or simply brushed on the wall.
Other Types of Surface Coatings: Among the most popular types of surface coatings, we can count coatings of plant extracts, Portland cement washes, paints (emulsion, resin, or oil-based), and even coatings of fresh animal blood. Most of these coatings are easy to apply and relatively inexpensive to procure.
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When rehabilitation or preservation is contemplated for an adobe building, it is usually because the roof walls have deteriorated in some way. The roof may be sagging and the walls may be bulging, pitted, eroded, or cracked. Before starting any physical work in the attempt to restore the adobe building, a plan must be developed. Here are several factors that should be considered when planning the repair and stabilization of an adobe construction.
- determining the nature of the deterioration;
- identifying and correcting the source of issue that is causing the deterioration;
- developing restoration and rehabilitation plans that are sensitive to the integrity of the adobe construction;
- developing a proper maintenance plan as soon as the restoration and rehabilitation is completed.
As a general advice, we recommend you follow these principles when restoring an adobe building.
- You should always seek professional assistance when restoring an adobe structure. The advice or services of a professional architect or other preservationist proficient in adobe stabilization and preservation will be of great help to you. Although this may be a bit more expensive than to do the work yourself, you will recover your investment in the long run. Never underestimate the difficulty level of the job. Irreparable damage may be done by inexperienced “preservationists”. Additionally, you may need the services of a professional in order to interpret the local code requirements.
- Before any physical work begins, you have to make sure that the source of the deterioration has been found, analyzed, and corrected. For example, bulging or sagging walls may be the result of excessive roof loads and/or an issue called “rising damp”. Since adobe deterioration is most of the time the result of a combination of problems, it takes an experienced preservationist to analyze the deterioration, identify the source of the problem, and stop the deterioration before full restoration begins.
- In order for the restoration process to be a success, you must use the same construction methods and replace or repair adobe building materials with the same type of materials that were originally used. The best way to do this is to use only traditional building materials. Replace or repair rotted wooden lintels with similar wooden lintels. Replace or repair deteriorated adobe bricks with similar adobe bricks. Introducing different types of materials in the adobe construction may cause problems far exceeding those which deteriorated the building in the first place.
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The following are some of the most common sources and signs of adobe deterioration as well as several solutions for the problem. As we said before, adobe deterioration is often the result of more than one problems. This means that the remedy of only one issue will not stop the deterioration if other problems are left untreated.
Structural Damage: There are a few common structural issues in adobe buildings, and although the results of these issues are easy to see, their causes are not. Most of these problems originate from inadequate or weak materials, insufficient foundation, improper design or construction, or the effects of external forces such as earthquakes, snow, water, or wind. Whatever the case may be, the services or a professional soil engineer are necessary to evaluate the issue. Solutions may involve replacing or repairing badly deteriorated roof structures, inserting new door and window lintels, buttressing walls, realigning bulging and leaning walls, or repairing foundations.
There are many signs of structural deterioration in adobe buildings, the most common being cracks in roofs, foundations, and walls. Although the cracks may be easy to spot, their causes may be more difficult to diagnose. The presence of small cracks in adobe buildings is pretty normal. However, if the cracks are significantly deep, then the building may have severe structural problems. In any case, all structural problems must be examined by a professional who can make a recommendation for the repair.
Water-Related Problems: Typically, adobe constructions deteriorate because of moisture, excessive ground water or rainwater. Therefore, successful restoration, stabilization, and the ultimate survival of an adobe construction depends on how good the structure sheds water. You should never underestimate the importance of keeping an adobe building free from excessive moisture.
The erosive action of rainwater and the subsequent drying out of wall surfaces, parapet walls, and roofs can cause pitted surfaces deep fissures, cracks and furrows to form. Over-saturated adobe loses its strengthm and consequently sloughs off forming rounded parapets and corners. If left untreated, moisture damage can eventually destroy adobe roofs and wall, causing their deterioration and ultimate collapse. Another problem constitutes standing rainwater that accumulates at foundation level.
Ground water may be present due to changes in grade, excessive plant watering, seasonal water fluctuations, improper drainage, and a high water table. Due to the capillary effect, water from the ground raises into the wall, causing the adobe to cove, bulge, and erode. Coving is also caused by spalling due to freeze-thaw cycles.
As ground water raises into the wall, it breaks the bond between the clay particles in the adobe bricks. Furthermore, salts and minerals brought up from the soil can be deposited or the wall when the water evaporates. If these deposits become heavily concentrated, they can and will deteriorate the adobe foundation.
The best way to protect an adobe structure from rainfall erosion is to ensure a water-tight roof. Roof and wall structures that are properly maintained with surface coatings or traditional tiles generally resist to destructive effects of rainfall. However, the roof drains must be in very good condition in order to ensure rapid run-off. Another way to protect adobe from rainfall erosion is to cap parapet walls with fired bricks. This technique was widely used by 19th century builders in their attempt to halt the destruction effects of rainwater over adobe.
Rainwater that accumulates at the base of adobe buildings should be diverted as soon as possible. This can be done by building stone, tile, or brick gutters. Gravel-filled trenches can be used also. Another highly effective method of diverting rainwater away from the adobe is regrading.
Rain splash can also cause damage to an adobe construction. This negative effect of rainwater over adobe walls is known under the name of “coving”. The best way to repair coving is to use adobe bricks stabilized with soil cement. Curb-like buttresses, cement stucco, or concrete patches, are not recommended because moisture may be trapped behind the concrete.
Cement patches and cement stucco can cause severe adobe deterioration. This is mostly because the expansion coefficient of cement stucco is 3 to 9 times bigger than that of adobe. This usually leads to cracks which in turn lead to moisture problems. Moisture can cause severe damage to an adobe building. The problem is that the problems won’t be viewed for as long as cement stucco covers the wall. However, the building may collapse under its own weight if actions are not taken.
Remedies for Preventing the Destructive Effects of Moisture on Adobe Buildings
Installing Footing Drains: Trenches about several feet deep and two to three feet wide are dug around the building at the base of the foundation. If the soil is weak, you can slope the walls of the trench so that the water and moisture won’t cave in. Next, a polyethylene moisture barrier should be installed over the walls and bottom of the trenches. This will prevent the collected water from saturating the surrounding soil and adobe foundation. Plastic pipe or clay tile are then laid in the bottom of the trench. A 6 inch layer of gravel is installed and the rest of the excavation is then filled with porous soil.
Regrading: The ground adjacent to the wall or foundation should not be leveled. In order to avoid water draining towards the walls or foundation, regrading should be considered. This involves sloping the ground adjacent to the walls away from the building.
Plant Removal: Trees, shrubs, and other plantings may be causing physical damage to the adobe building. This is mostly because their roots can grow into the adobe wall or foundation; plus , they may be trapping excessive moisture.
A Word of Caution: Trenching, regrading, or plant removal may be potentially destructive to archeological remains associated with historic adobe construction sites. Therefore, any disturbance of the ground should be undertaken with careful planning and prudence.
Wind Erosion: Another common cause of adobe problems is windblown sand. Unfortunately, identifying the effects of windblown sand on adobe buildings may pose a challenge. This is usually because the effects of windblown sand and rainfall erosion are pretty similar. However, furrowing caused by wind is typically more obvious at the corners and upper half of the wall, while ground water erosion is usually present at the lower third of the wall.
The best way to mitigate the destructive effects of wind erosion is regular maintenance. Adobe mud should be used to repair damage created by windblown sand. If high winds are a serious problem, a wind breaker or screen can be built using trees and fencing. Care should be taken when planting trees around adobe building. As we have said before, roots can cause damage to the foundation by attracting moisture.
Vermin, Insects, and Vegetation: Pest and vegetation can create damage to adobe structures. Seeds deposited by animals or wind may germinate inside the adobe wall. The action of plant roots will break down adobe bricks or retain moisture. This will eventually lead to severe damage of adobe walls and roofs. Insects, birds, and animals can live in adobe buildings, nesting and burrowing in foundations and walls. These pests can cause severe damage to the internal structure of the building. Since termites can travel through adobe walls as they do through natural soil, their presence shouldn’t be underestimated.
In order to protect the integrity of adobe structures, it’s important to deter any insects, animal, and plants that may pose a danger to adobe walls. Furthermore, preventive measures should be taken against their return. If large trees are planted close to adobe buildings, their removal should be undertaken with care so that their roots will not dislodge adobe material. If chemicals are being used as a pest control method, the effects they may have over the adobe structure should be carefully examined by professionals
Material Incompatibilities: Since adobe structures are continuously shrinking and swelling, it is likely that repairs and maintenance work have been already carried our during the life of the structure. This means that the composition of adobe may have suffered modifications on the way, and this is a factor that should be highly considered when restoring adobe. Unfortunately, until recently, adobe walls have been sprayed with latex or plastic surface coatings; deteriorated wooden lintels have been replaced with steel ones; and adobe bricks have been re-pointed with Portland cement. Adobe has rendered these methods ineffective, and more important, destructive. Latex and plastic surface coatings prevent adobe walls from expanding and contracting with temperature variations, and this causes them to crack and crumble. Steel lintels don’t react to temperature variations like adobe does. This means that cracks and holes will develop at the area where the steel lintel meets the adobe material. Finally, Portland cement causes the adobe wall to crumble due to the cement’s high strength.
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Once the deterioration process is stopped and any resulting damage is repaired, the restoration process can begin. As we’ve mentioned in many occasions, careful consideration should be given to the use of traditional adobe construction materials during the restoration process.
Patching and Replacing Surface Coatings: Typically, almost every adobe structure was coated. Since these coatings deteriorate pretty fast, they need to be replaced frequently. In order to ensure the preservation of the historic adobe building, every effort should be made to use the same material as the one that originally coated the surface.
Buildings that have been covered with mud plaster should be re-coated with the same type of material. However, before coating adobe with mud plaster, the old coating must be scrapped off first. The technique used in the re-coating process must be as similar to the original as possible. Although mud plaster may require more maintenance than other surface covering materials, it is the only way to do it.
If lime plaster or cement stucco has been previously used on the adobe building, then the restoration process becomes more complex. First of all, the current coating has to be removed as careful as possible in order not to deteriorate the adobe brick fabric underneath. Once the coating has been removed, a professional must assess the damage. In most cases, the adobe structure had suffered severe damage due to the hard-like coating applied on top. Whatever problem is found, it must be fixed before proceeding with the restoration process.
Patching and Replacing Mortar: Care should always be taken to match the original, texture, color and material when repairing deteriorated or loose adobe mortar. Additionally, never use Portland cement or lime mortar to replace adobe mud mortar. These materials have different thermal expansion rates. With the continual contraction and expansion of adobe bricks, lime mortars and Portland cement will cause the brick to crumble, crack, and eventually deteriorate.
There may cases when the adobe building had Portland cement or lime mortars in initial construction. If this is the case, replacing these materials with adobe mud mortar is not advisable. The process may cause irreparable damage to the adobe bricks.
Repairing adobe cracks may involve a technique often used in re-pointing masonry joints. The cracks should be raked out to a depth of two or three times the width of a mortar joint. This way, the mortar will have a good mechanical bond with the bricks. Water should be sprayed over the bricks in order to increase the cohesive bond. A large grout gun or a trowel with new adobe mud mortar may then be used to fill the cracks.
Repairing and Replacing Wooden Members: Termite infested or rotted wood members such as flooring, wall braces, lintels, savinos, or vigas should be repaired or replaced. As a general rule, wood should always be replaced with wood. The exceptions are carved corbels where low-strength epoxy compounds may be used to make repairs. This approach will save the original craftsmanship.
Patching and Repairing Adobe Brick: When repairing adobe brick, every effort should be made to find clay that is similar in texture and color to the original fabric. If an adobe brick is only partially disintegrated, it can be repaired on the spot. The damaged material may be scraped off and replaced with new adobe mud mortar. It is a good idea to use new adobe mud every time a repair is attempted. Although fragments from the disintegrated adobe brick may be mixed with water and create adobe mud, these fragments may contain high concentration of salts which will affect the final result.
If a large area of the adobe brick has been spall or destroyed, commercially made adobe bricks can be used instead. Another option is to create the bricks at the site. These bricks are usually three to four inches thick and are composed of adobe mud. The damaged adobe brick should be scrapped off and the new brick inserted in its place. Often times, it may be necessary to cut back into good brick only to achieve a flash fit. For a better bond, spray the brick and the hole with water.
If commercially made bricks are used in the restoration process, caution should be exercised when purchasing them. Most manufacturers today use stabilizing agent such as emulsified asphalt, lime, and Portland cement in the brick composition. Although this is considered a technical advancement (the brick is more durable), it will not benefit the historic adobe building.
Roofs: Flat roofs should be restored with their original materials and form. However, if the roof has been previously modified into a gable roof with sheet metal, then it should not be restored back into a flat roof.
Considering the roof has not suffered any modification to its form in the past, caution should be taken when applying a new layer of adobe mud over it. The first safety measure that should be taken is to temporarily support the roof because wet adobe is much heavier than cured adobe. Failing to add additional support to the roof’s structure may lead to deflection and permanent damage. If the wooden roof supports sag during the process, drainage will be affected.
It is not advisable to build a heavier roof than the original roof. If the adobe foundation or walls have moisture problems, the added weight will accelerate the deterioration process, leading to bulging. Even if the walls don’t have a moisture problem, the added weight will still affect their strength and durability.
Doors, Windows, Floor, Etc.: All original details of the historic adobe building should be retained whenever possible. However, it is understandable if the demands of modern living force owners to change some of these features.
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The nature of adobe constructions makes them to deteriorate in time. The only key to adobe survival is constant maintenance. It is a good idea to have a professional draft a maintenance plan as soon as the restoration process is completed. All changes in the building should be noted and the signs of adobe damage should be closely monitored. Since moisture is adobe’s biggest enemy, the moisture levels of the walls should be monitored constantly. Pest control is a great way to keep pests out of adobe walls. Finally, the roof must be inspected in a regular basis. Ensuring regular maintenance for adobe buildings cannot be over emphasized.
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The bottom line is that attempting the preservation of an adobe structure is almost a contradiction in terms. Adobe is a weak and predisposed to deterioration construction material, not stronger than the soil itself. Its very nature is to deteriorate in time. This is why the preservation of historic adobe buildings is a far more complex problem than people may have imagined. Although it is desirable to halt the deterioration of adobe buildings, no satisfactory techniques has yet been found.
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Kurt, Baer; and Rudinger, Huge. Architecture of the California Missions. Los Angeles: University of California Press. 1958.
Bunting, Bainbridge. Early Architecture in New Mexico. Albuquerque: University of New Mexico Press, 1976.
Boundreau. E. H. Making the Adobe Brick. Berkeley, Calif.: Fifth Street Press, 1971.
Of Earth and Timbers Made: New Mexico Architecture. Albuquerque: University of New Mexico Press. 1974
Clifton, James R. Preservation of Historic Adobe Structures: A Status Report. Washington, D.C.: National Bureau of Standards Technical Note 934, US Government Printing Office, Stock No. 003-00301740-0, Feb. 1977.
McHenry, Paul Graham, Jr. Adobe–Build It Yourself. Tucson, Ariz.: University of Arizona Press, 1973.
Phillips, Morgan W.; and Selwyn, Judith E. Epoxies for Wood Repairs in Historic Buildings. Washington, D.C.: Heritage Conservation and Recreation Service, 1978.
Articles, Periodicals, and Bibliographies:
“Adobe, Past and Present.” Reprinted from El Palacio. Vol. 77, no. 4 (1971).
“An Architectural Guide to Northern New Mexico.” New Mexico Architecture. Vol. 12, nos. 9 and 10 (Sept.Oct. 1970).
Adobe News. Los Lunas, New Mexico. Published bimonthly.
Barnes, Mark R. “Adobe Bibliography.” The Association for Preservation Technology Bulletin. Vol. 7, no. 1 (1975).
Eyre, T. A. “The Physical Properties of Adobe Used as a Building Material.” The University of New Mexico Bulletin. Engineering Series. Vol. 1, no. 3 (1935).
George, Eugene. “Adobe Bibliography.” The Association for Preservation Technology Bulletin. Vol. 5, no. 4 (1974).
Haapala, K. V. “Stabilizing and Restoring Old Adobe Structures in California.” Newsletter of the National Association of Restoration Specialists. Murphy, Calif., June 1972.
Hooker, Van Dorn. “To Hand Plaster or Not?” New Mexico Architecture. Vol. 19, no. 5 (Sept.Oct. 1977).