Adobe as Plaster
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Q: With my wife, we have recently established a private company in Ethiopia for an investment in eco-cultural lodge in Lake Tana area. As the lodge will be build out of Adobe (similar to some historical construction in Ethiopia), we are considering various construction technique options. Each lodge will be build like a small Ethiopian Orthodox church with wall decorations and paintings. We only use local material (foundations made out of stones, Adobe wall, and thatched roof). The main concern we have in designing is to stabilize the plaster (mud or other material) in order to preserve the wall frescoes we are planning to produce. Do you have any reference on how best to plaster Adobe/mud houses? What material is mostly recommended for interior plastering, keeping in mind that the lodges need to keep a fairly high standard of comfort.

A: The same mud that will make adobe bricks makes a good plaster if it is screened through a fine mesh of about 1/8th inch.

Exterior Plastering: the rainy season can be very wet and in order to protect wall frescoes on Adobe, stabilization of mud plastering (or other material) is a main concern.

Good overhangs of the thatch is the first defense. There must be something that locals use. Around here there is mashed up and fermented mixes of cactus and manure. Some folks rely on lime-based plasters.

A book on latex concrete technology has recently been produced (Albert Knott and George Nez, "Latex Concrete Habitat", Canada, 2005). Could Latex concrete or latex slurry be used to stabilize mud plaster?

You can add latex to mud plaster. Careful. If it is so waterproof that it does not breath, then water vapor cannot get out and large chunks of the plaster fall off. Experimentation with appropriate ratios for each soil is required. Make test samples of various ratios and plaster a half-inch thick on a board. Let it dry and set them out to get rained on to see what happens.

Moreover, the municipality of Bahar Dar and Amhara regional state in Ethiopia had very recently (19th of June 2005) agreed to lift a ban on mud house construction in urban areas. Yet, they would like to regulate such construction in order to maintain minimum standard for urban development (they don’t want to see slum types of construction in their cities…). Is there any existing standard to which the Municipality and regional government could refer? Bahar Dar enjoys a warm climate with mostly uni-modal rainfall. The wet season can really be wet but it does not last more than 5 months in total.

There are links to the older New Mexico adobe code with an update to the newest code coming soon on QuentinWilson.com.
I think the old one is more appropriate to your location.

Q: Can you make an adobe plaster type mix to cover an old unpainted cement block wall?

A: (Kelly) Yes, adobe does adhere pretty well to a cement block wall. Make sure there is enough clay in it to make it sticky, without causing it to crack too much.

Q: Should I mix anything else with the clay, and what is best to seal it so as it won't fall apart with the many changes of weather like rain, wind, dry heat, sun here in the Las Vegas Desert climate we have?

A: (Kelly) Ideally you want somewhere between 10% and 30% clay, with the rest sand to keep it from shrinking and cracking too much. Sealing it depends partly on how well protected the wall is from the weather by roof eaves. If it is tucked under sufficiently, the straight adobe soil mix will hold up pretty well. Otherwise, you might want to add some Portland cement (maybe 5-10%) to help stabilize it.

Q: My wife and I are considering the purchase of an adobe brick home built in 1981 with a basement style foundation in southern California. We noticed that some of the bricks are eroding from the weather. Can we plaster over parts of the bricks to preserve them? I read somewhere on your cite where you said water would leak through and cause problems for the adobe to dry out. Can or would I have to tar paper behind the plaster or just go right over the adobe?

A: You can certainly plaster over adobe bricks. Some people think that adobe and cement based plasters are not compatible. I used standard cement/lime stucco over adobe homes new and old for 25 years with no problems. I would recommend omitting the use of tar paper or Jumbo Tex between the stucco and the adobes. It is the paper that introduces the barrier that stops the adobes from breathing. Stucco allows moisture and water vapor to pass through. So much has been written that we have ended up with Jumbled Text.

Q: I have a home which needs to new siding. I would like to know if it is possible to use adobe (plaster) to make it look like an adobe home on the outside. If so please tell me what and how to go about doing it. I love the adobe look, I have not found any siding that really pleases me or suits me.

A: Adobe plaster sticks to most anything but you may have trouble finding plasterers to do the job. You can do it yourself but it will take some experimenting. First you might tack 15-pound roofing felt over the siding. Jumbo Tex works also and meets the Code. On top of that nail or screw self-furring stucco netting and then plaster onto that. The best mud will have about 30% clay and 70% sand. Chopped straw can be added but is not absolutely necessary. Two coats might do it. To make it last longer, emulsified asphalt can be added in the amount of about 12-ounces per wheelbarrow. Emulsified asphalt comes from the companies which supply highway construction and repair contractors. Another alternative is to use standard cement/lime/sand plaster which will be familiar to most plasterers in your area. The final color coat can be a nice Santa Fe Brown.

Q: Last year, I followed and old dream of mine and bought an old adobe house in Guaymas/Mexico. It was built in 1860, out of adobe bricks and mud. The walls are about 28 inches thick and are still in pretty good condition. But it is hard to evaluate as sometimes in the past a cement finish was applied on the outside, thus covering the original finish. Some of that outside skin detached completely from the original adobe and rainwater consequently washed some of the soil out, leaving a void between cement and adobe. One part of the house lost its roof a long time ago, thus showing a lot of wall damage. How can I restore these walls, using modern building materials such as cement if possible at all? Can I chemically bind cement to old adobe earth? What other restoration methods are recommended?

A: You are in an enviable position. There are no chemicals that will bond adobe to cement short of spray-on two-part urethane. I don't think that is what you want to do. I recommend that you strip away the cement stucco where it has detached and use repetitive mud plastering to rebuild the missing adobe. Or perhaps parts or whole adobe bricks mortared back into the wall if the depth of the loss is great enough. Then you might re-plaster with cement stucco again. Or mud plaster. If not in Guaymas, somewhere in surrounding villages there has to be a few older folks who have abundant adobe experience. Most of the youngsters have only worked on concrete and clay tile buildings.

C: That is exactly what my contractor in Mexico suggested. Using local dirt, ( una mezcla de tierra, agua, semillas de trigo, cal y un poco de yeso para reparar el adobe), and building it up to original size/shape. At that time, we may use DARAWELD C as a binder for the stucco mix. I had the Daraweld dealer stop by and explain how his binder could be used with adobe.

Q: My son is building a passive adobe home in Willcox AZ. He is an amateur in adobe taking on this project. They are currently laying the block. He thinks the exterior must have stucco but hates the thought of covering up the beautiful block. His block has the cement added. What is the proper overlay for both the interior and exterior block. His whole purpose of using adobe is for conserving energy and plans to use a wood stove only for heat.

A: The New Mexico Code allows adobe to remain unplastered on the exterior or interior. Lots of Arizona homes use exposed adobe. Nearly every Arizona county has its own, if any, adobe code. Plaster is chosen mostly for its look or as a protective or sacrificial coat in areas of greater moisture.