Adobe as Plaster

Quentin Wilson and Associates, specializes in solar adobe design and construction. He grew up in the South Valley of Albuquerque, New Mexico where he watched adobe bricks being made. In the fifth grade, he made miniature adobes on cookie sheets in his mother’s oven in order to construct house models for a class assignment. By age thirteen he made full-sized adobes in the back yard and ruined the grass. Later, he traveled a bit, went through the Army, and graduated eventually from the University of New Mexico with a major in physics, minors in math, chemistry, and education in 1970. After teaching high school two years and community college math for three more, Quentin moved into professional solar adobe construction in 1976 as the Project Manager and Instructor for the Sundwellings Demonstration Project at Ghost Ranch, Abiquiu, NM. He became a licensed general contractor in the State of New Mexico in 1982. He has been building homes and teaching seminars and workshops ever since. In the fall of 1995 he established and taught the full-time Adobe Construction Program at Northern New Mexico Community College. His website, quentinwilson.com, lists the course schedule and many other resources related to working with adobe.

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.

Q: We live in Panama Central America and are building a house with clay bricks. It rains a lot here and for the plastering we are going to use our own soil, but what do you think we should mix it with for the outdoor plastering of the house and what should we mix the soil with for the indoor plastering of the house.

A: Indoor plaster needs nothing added. The outdoor plaster can be waterproofed with emulsified asphalt which is used for cold process highway repairs. (12 to 16 ounces per wheelbarrow.) You might also be able to find some latex/acrylic at a paint manufacturing facility. Elmer's yellow, waterproof glue will work if you can find it. About a pint per wheelbarrow. Fish oil emulsion is used in Chile. If you use too much of any waterproofing material, the plaster will be unable to breath out any water vapor that gets into the wall and then the plaster looses its bond with the bricks. If you have a good old Panamanian roof of thatch with a lot of overhang, your problem is already solved. You might also look around to see what the local populations have developed where there is still earthen construction. Panama experiences more rain than most other locations.

Q: Can you seal adobe brick. If so with what? I live in a adobe house the brick wash away every time it rains or it gets windy out.

A: There is a product called OKON. It is formulated in w-1 and w-2. I can never remember which is the right one but the company or a distributor should be able to advise. OKON is invisible and breathes so water vapor is not trapped inside the adobe bricks. They were located in Colorado and could be found at www.okon.com . The company has been bought up by a chemical giant but the product should be available. There is no magic bullet other than mud plaster that is renewed every year or so or a cement/lime/sand standard stucco or a lime/sand plaster.

Q: We live in North Carolina - plenty of red clay soil. Can I use this soil mixed 3:1 with Portland cement and plastered on (ca. 5 in thick) chicken wire to cover an adobe bread making oven.

A: I don't think that Portland cement is the right thing to add. Portland and clay are not compatible and work at ruining each other. Lime might work, emulsified asphalt might work. If you can find the highest clay content that will not crack that will give the best natural water resistance. As sand increases in a plaster mix, there will be less or no cracking but the water resistance begins to decrease. It's a balancing act.

Also, the chicken wire would most likely be a problem with high temperature changes that will take place when the oven is fired. The metal expands at a much higher rate than the plaster. When we plaster an horno we rely on the ability of the earthen plaster to stick to the mud bricks. Some hornos have massive cracking take place when we get them really hot. They return to normal upon cooling.

The full truth of the matter is that so far, no great material has appeared that stands up to moisture and heat on an oven. We just replaster the ovens every year or two. That might not sound too encouraging. British and New England ovens end up with a small roof built over them about  a foot above the highest part of the oven.

This is hardly a definitive solution for you. Usually a person just has to fool around with the local soil to find the best answer. You might report back if you figure it out.

Q: How do you make the mixture of drywall mud/joint compound, to cover the inside walls in an adobe brick home? Do have any other ideas to finish the interior walls, and still see the brick affect?

A: I have never mixed mud with drywall compound. I just make the good old 30 clay 70 sand mixture and run the components through a 1/4 in screen. It is a fine plaster for adobe wall. Maybe run through a 1/8 screen and wash it on with a sponge from the tile industry, a sheepskin or towel. Or just start washing the wall with any of those things and enough material will be removed from the high spots to partially cover the low spots.
Adobe is easier than anyone might think.

Q: I am building strawbale homes and homes that use recycled materials in Old Australia. I have recently been asked to seal the domed roof of a mud brick house. The weather in this particular area is at times very hot and also there can be high rainfall. There have been materials applied to the dome (Cement render) but it has cracked and leaks have been there for some time. Is there any advice that you may be able to share with me.

A: (Kelly) This is a pervasive problem with domes and vaults, and my best advise is to first repair any large cracks as best you can, or perhaps even replaster the entire dome using fiber-reinforced stucco. And then paint the dome with several coats of a special roof sealer, similar to elastomeric paint, that is a light color to reflect the heat.

Q: My family and I just built a greenhouse attached to our home and we would like to cover it with adobe plaster. We checked the soil and think we have at least 10% clay. We screened the dirt, mixed it with water, and did a few test strips with roofing felt, chicken wire, and scrap wood. They keep cracking and we don't know what the next step should be! We live in Colorado and it's getting cold at night, can that be part of the problem? What do we need to add to the mix?

A: You must have too much clay if you are getting cracking. Try adding one part of sand, the coarser the better, to one part of your dirt. Then if it still cracks, try two parts sand to one dirt. Even if it freezes at night, you have a few more weeks to work as long as you stop plastering about 2pm. Then the plaster dries enough so there is not enough moisture to freeze. Freezing makes the plaster spall or burst. It does not cause cracks.

Will try the sand. Do you think it will have a better looking finish than straw? Tried straw before I got your e-mail and it helped with the cracking, but it is not the adobe look that I had in my mind. Any advice for an aesthetically pleasing and water resistant finish would greatly be appreciated!!

Straw, too, is a fine and traditional way to control cracking. If it is available and you like the look, by all means use it. The higher the clay content in your plaster, the more water resistant it will be. I just hate to go chasing all over the place to find straw so tend towards sand.

Q: I have a question concerning applying an adobe or super-adobe mix over fired or non-fired adobe brick? Can this be done in either case, and if so, what techniques should be used to insure long-term surface adherence?

A: Standard adobe sticks to most any substrate. It works great on adobe and usually on fired adobe but sometimes you have to experiment as to whether to wet the fired adobes or not before application. Over 18 years, I worked side by side with Michael Valdez doing interior mud plaster on adobe walls. He always wet the wall, I never did. Aside from his superior smoothness, I can see no difference in the walls when I visit past projects.

As for super adobe I have no experience, but since it contains gypsum, at least in the product I am thinking of, it should do as well as adobe plaster. Gypsum generally increases the stickytivness. Again, with zero super adobe experience, that is a projection.

Q: I live in cape Town, South Africa. I have been given sun dried clay/sand/straw blocks which I want to use to add to my existing fired brick, cement plastered house. Two places have been built in my neighbourhood with adobe like this from the same source. Both have been plastered with fine sifted clay, rubbed into the walls as a plaster and painted with a linseed oil/turps mix (yikes, we live in a veld fire prone area - another concern). I am concerned that with our driving rains in strong wind, any extended eaves wouldn't protect the walls. What alternative plaster could I use that would be lower maintenance and stable? Can I use a standard cement/sand mix with waterproofing additive? Will it bind with the clay walls and not crack due to clay breathing?

A: A lime plaster would be great if you can find someone in your area to advise you on the technique. It will stick to adobe and breathes well. A cement/lime/sand standard stucco will also work well as long as you plaster directly on the adobe wall. Wire stucco netting or chicken wire as some call it, might help mechanically bond the stucco to the wall. Use no paper or other moisture/vapor barrier under the stucco and it will breathe sufficiently to work with adobe. Stucco has published values of 5 to 6 perms. No one has measured the permeability of adobe yet.

Q: I'm about to run electrical cables in my adobe walls by cutting a channel/groove a couple of inches into the wall. What is your recommendation for filling the channels? I've read that I could use a sand/cement mix of 10 to 1 which will be 'soft' enough not to break away from the adobe brick. Do you think this is a suitable material or should I use something else - I'm worried that the structural integrity of the walls will be compromised if I don't get it right.

A: The same mix that was used for your adobes or the mortar between them will work. Sometimes it is wise to have a slightly damp mud mortar and dry pack it in with a board of the right width about 8 inches long. You can pound it in with moderate taps from a rubber mallet. Standard cement/lime/sand mortar mix will work and so will gypsum/sand scratch coat plaster mix. I have never used a 1/10 mix but it might work. Basically, with adobe everything works.

Next time, consider running most of your horizontal runs in the mortar joint as the walls go up. You can use type NMC which is a hard to find form of Romex. Otherwise UF cable works fine. Bury them in the middle of the wall and you can be assured that only the longest nails or screws will ever have a chance to hit them. The outlet and junction boxes can be set at this time so much easier than trying to anchor in an existing wall. Vertical runs can be made on the back side of door and window rough bucks which would, of course, be in place before the walls proceed upward.

Q: Almost finished with an earthship in NC. Our contractor had health issues and was unable to finish the adobe on our back wall in prep for waterproofing. It was packed out most of the way but not finished before winter. The outer surface area froze and fell off. Since the contractor could not finish it, my husband and nephew stepped in. The wall is nearly flush, scheduled to have the spray on water proofing next week followed by drying and then 'dimple board' ...then backfill. The contractor said he was backing out of the job entirely due to liability because my husband and nephew used more concrete in their adobe mix than he recommended. Are there rules about the concrete to dirt mix in the adobe for a back wall? Reasons that you know of that more concrete vs dirt would be bad?

A: This is a complicated subject. Basically cement and clay are not compatible and spend their energy destroying each other until one overwhelms the other. The entire handbook "Soil Cement its Use in Building" was put together by the UN and used by the US Peace Corps to figure out how to make Compressed Earth Blocks with soil and cement.

If your plaster holds up and makes a thump, thump sound when you whack it, it's okay. If it just goes pleth, pleth then it is friable and may not hold up. In any case if you are going to intimately back fill against it, it's not going anywhere. A crummy built Earthship will probably only last 400 years and you won't get your money's worth. Well built it should be good for a thousand or so. Either way, I think you will have a fine home.

Q: I am currently working on an adobe project that was built in the 1900's. There are visible deteriorated adobe bricks. Once these blocks are replaced, can Dow foam panels be installed over the exterior walls and then stuccoed?

A: Yes, nails supporting the stucco netting which will support the stucco should be substantial. We often as not used 5-1/2" Pole Barn Nails with ring shanks. Some folks use long deck screws. Either way, the supports should be 16" on center horizontally and vertically and should penetrate into the adobe wall 3-1/2", OR MORE.

Q: What is the best way to repair cracks in/on exterior cement covering on 1930s unstabilized adobe? The stucco/cement layer is 3/4" + thick. I was considering angle grinding a key along cracks then filling with concrete mixture? What method, material, ratio, process might you suggest? Would a concrete bonder be used?

A: If you can angle grind a channel that is wider on the inside than on the outside surface of the stucco that will be the best solution and may be just what you mean by a key. That procedure will not require a stucco bonder but it might not hurt to use it. Most of the fill can be standard cement/lime/sand stucco which you mix yourself or in some areas buy by the bag at a lumber yard that stocks El Rey Stucco or Quick Crete products. Pre-mixed Masons Mortar will also work. The final coat should be pre-mixed stucco from El Rey or Oriental stucco in the closest color you can find to match the original. El Rey makes a product called Fog Kote that comes in powder form in various colors in a 6-pound tub and after mixing with water is sprayed on afterward to blend new with old work. It is not always 100% effective in obscuring the joint.

Q: I have a fired adobe brick home built in 1960. The east facing wall receives the brunt of rainstorms and sometimes freezing temperatures. Some of the mortar was replaced on this wall before we bought the home and it is starting to come out again in the same place. Also the adobe has some small air pockets that I think could be bad if water gets in there and freezes. Should we fill the air pockets with mortar? What kind of mortar should we use? Should we try to seal this wall or stucco over it? The other walls do not have this kind of problem other than step cracks or hairline cracks. The chimney is built of adobe and has never been repaired, small chunks have dropped out. Should we stucco this?

A: For a fired brick wall, the simplest repair method would be to use standard cement/lime/sand mortar which is most likely what was originally used. You can buy it premixed in a bag from Quickcrete which is the most widely distributed in the West these days. As a rule of thumb, mortar should be replaced to a depth about twice the width of the joint so that the new mortar does not pop out of the joint. If you go in more than an inch it is better to do it in two steps filling half the depth and about four hours later the outer half to avoid cracking. If there is a lot to do, a hawk and pointing or margin trowels will be useful. Otherwise, a hand in a latex or nitrile glove will work just fine if you can achieve the desired surface shape of the mortar joint. In recent years, some masonry repair materials have appeared in caulk tubes to be applied with a caulk gun. For a small job that should work, but I have no experience with them myself.

Q: I am based in India, where there are many beautiful variations of natural plasters, most of which include cow dung (fresh and/or dry). I am doing the interiors of a ready made apartment, but really wanted to coat the walls with an earthen plaster. The general recipe here is sand, fresh cow manure, clay, and straw. Do you think this would be problematic? Also, would it still help cool the room if the walls are made of brick and lime. People here mimic the earthen plaster look by mixing plaster of paris and glue with paint...but I want to use natural materials for their cooling and antiseptic properties as well. Please advise.

A: As far as I know, the use of cow dung is for the purpose of making earthen plasters more resistance to water damage. That should not happen on an interior wall so I think that you can easily omit the cow dung for interior plasters. That will eliminate the possible source of bacteria that might be in the dung. Outside plasters soon get baked by the heat of the sun and the ultraviolet and experience natural purification. Sand, clay and straw make a fine plaster once you get enough sand and/or straw to keep the clay component from cracking. Usually, 70% sand and 30% clay makes a perfect plaster. Straw allows a higher percentage of clay to be used.

Certainly the use of earthen materials will contribute to the interior comfort. The plaster will most likely not be more than one- to two-centimeters in thickness so the thermal contribution will not be high. Besides the ability of earthen materials to moderate temperature changes, the plaster will participate with the moisture in the air and will also moderate humidity changes in the interior space. I wish you the best of experiences as you develop your faux plaster of paris/paint/glue. Earth plasters are making a big impact on interior surfaces in Europe right now, especially Germany. It is a great way to cover up the somewhat toxic surfaces produced the past several decades.

Q: What about using cement plaster over adobe?

A: Certainly we need to find ways to reduce the amount of cement used worldwide in the construction of houses. The proposed cement plaster appears to be very thick and it is on both sides of the adobe wall. Wire ties are not needed through the wall. It is a commonplace technique to simply nail the stucco mesh to the wall with sixteen-penny (8.5cm or 3.5-inches long) nails or with thirty-penny pole barn nails, (14 to 15cm or 5.5 -inches long with ring shanks to improve holding) or 6 to 8cm (2.5 to 3.5-inch long) deck screws. This way, cement plaster is only needed on one side of the wall, the exterior. The interior can be:
1.) Left plain with the bricks or poured mud visible,
2.) Washed lightly to soften the lines but still allow them to be seen, or
3.) Plastered with mud plaster of the same composition of the wall itself.
Plastering the exterior with the industry standard cement stucco to a thickness of 1.5 to 2.0cm (+/- 0.75inch) would use about 1/3 of the volume of cement plaster that I see used in the photos which appear to be 3 to 4cm.

Interior cement walls are absolutely no fun to live with. Modifications are difficult, it is hard to drive a nail into the wall to hang a picture or a mirror or a shelf. It is difficult to carve niches to place belongings or create shrines.

Standard cement/lime/sand stucco has a perm rating of 5 to 6perms. That is not zero. It allows water vapor to escape from the wall. However, in the USA, Europe, parts of Latin America, and certainly parts of the world with which I have no construction knowledge, standard industry practices specify the use of double ply asphalt saturated Kraft building paper to wrap the wall before the application of stucco. Depending on the manufacturer this barrier might be close to zero perms or as high as 30 perms. A low perm wrap would be bad for earthen walls or any walls, for that matter. A high perm wrap would be much better. However, thousands of adobe homes -twenty-five built by me - in the American Southwest and Germany have exterior cement/lime/sand stucco applied directly to the wall with no wrap since they were built before the code required wrap. This may be the best method since with no wrap, the stucco is able to penetrate the wall anywhere there are voids in the mortar and that increases the attachment of the stucco to the wall.

It is useful to note at this point that with all the talk about breathability which is the down home term for water vapor permeability/transmission, no one has measured the permeability of mud walls. We don't know if mud is 0.21256 or 6 or 60 or 600perms. Until someone goes through the ASTM E-96(A) protocol to test mud/adobe/rammed earth/compressed earth block for water vapor transmission, then we really have no idea if any wall coating is actually slowing down the moisture vapor transmission from an earth wall.

We do know that anywhere there is excess moisture in a plastered adobe wall it is announced as if by a neon sign on the exterior of the plaster by dark patches followed by efflorescence - the deposit of white crystals on the outer surface of the plaster as the water evaporates. This can be seen on stuccoed frame walls, concrete block walls or any form of masonry. It is when these signs are ignored that plaster detaches or adobe dissolves. The equivalent in wood buildings is rot or peeling paint. The solution is to find and eliminate the source of moisture.

Q: I am thinking of putting a stone facade up to four feet on an adobe wall. Will this stop breathability of the adobe, or will it just stop where the facade is? I will be putting a lime plaster on the rest of the wall.

A: The stone facade will stop or restrict the breathability only where it is in contact with the wall. Cement and lime based mortars around the stone allow some moisture vapor transmission but not much so there will still be some breathability.

Q: We have an adobe house in CA, over 50 yrs old. We are redoing the bathroom. And want the best way to fasten tile to adobe if possible. Or would we need to make a frame with backer board to tile on. The shower had tile about 1/2 to 3" thick of thin set with black paper and a wire mesh on the Adobe.

A: Lots of folks do a frame wall with backer board. I almost always did standard cement/lime/sand stucco mix on bathroom walls. We used wire mesh but no black paper so that the plaster stuck directly to the adobe. We then used acrylic based adhesive always looking for the one that proclaimed the highest resistance to water incursion. You might get the best advice on selecting the adhesive from a high-end tile supplier rather than a big box store. Showers with tile are always a challenge for any type of construction.

Q:  I have inherited an Adobe home near Taos and I want to finish it. It has a scratch coat over Adobe blocks. I know it will need a brown coat before the color coat. I would like to roll on the color coat, but a contractor is saying you can't roll on color coat over Adobe. The scratch coat and I believe the brown coat is a cement sand mix so won't that bond to a roll on latex or acrylic color coat?

A: There are a number of paints available specifically to roll on to cement-based walls - stucco, concrete, cement or pumice blocks. Don't do it. It would save you money but once you paint stucco you can only go back to stucco after sandblasting, treating with a stucco bonder or hand scraping. Good stucco in Taos can last 60, 80 years. Paint will begin to degrade from ultraviolet impact in ten years. At twenty years it will need to be wire brushed and re-painted. Should your interest in paint continue, the stucco must be fully cured so once a brown coat is applied it needs 28 days minimum curing.

Adobe walls always find a way to pick up some moisture whether from the ground - called "rising damp" in Europe and other parts of the world - or from wind-driven rain striking walls or cracks in wall/parapet plaster or roof leaks or elevated air moisture from internal sources within the house. Cement/lime/sand stucco has a moisture vapor permeability of 5- to 6-perms and that's enough to allow moisture to escape as a vapor. At the same time we expect stucco and other wall coverings to keep liquid moisture from moving into the wall during a rainstorm. Elastomeric stuccos that come out of a bucket usually have about zero moisture vapor permeability. Some exterior masonry paints will also be around zero. The late, great Wellborn Paint Company of New Mexico addressed this with a masonry paint formulated to be somewhat moisture vapor permeable. So, if you do paint, make sure that the manufacturer states that the paint is not zero perms.

Q: My home is in Salt Lake City. Built in 1887 from sun baked adobe. Exterior looks nice. Interior needs some mortar help and then I’d like to re-plaster the walls. Most of the plaster on the lower portions simply fell off. I read your question and answer page. To re-mortar I need the same material as the adobe itself you say. How do I find the actual makeup of the adobe? And for the plaster it seems a standard cement/sand/lime mixture will be fine. I couldn’t confirm whether or not I needed wire mesh or other prep work before plastering. Also. One wall is bowed. Any way to fix that or is it best left alone.

A: You have a real treasure. If the house was built in 1887 it is almost certain that the adobe bricks were made nearby and the mortar would have been of the same material. There just might be some dirt available in your neighborhood and it will make fine mortar. As an alternative in modern times you can often find dirt that will work at a sand and gravel operation. They might refer to it as crusher trash and it will have clay in it from their cleaning process. If it is sticky when wet, it will work. You might have to run it through a screen of 1/4-inch hardware cloth attached to a wood frame that fits over a wheelbarrow or stands alone with braces.

Most any dirt with around 30% clay and 70% sand will work. By "same material" we are cautioning against the use of cement/lime/sand mortars or heavily lime or asphalt stabilized mortars unless that is the make-up of the bricks. Wire mesh, lath, chicken wire, stucco net, nailed on with 16d nails at 16" OC horizontally and vertically is appropriate for exterior plaster/stucco. It is not needed for interior plaster unless for some reason the plaster just does not adhere well to the wall. Sometimes expanded, galvanized metal lath is helpful at corners, around windows and transitions.

A bowed wall can be tolerated up to 2" out of plane without any worries of structural failure. If it's more than that or if there is an aesthetic judgment against it it can be plastered on the concave side and shaved off on the convex side to get it straight again. Worst case is to shore up the ceiling, take down the wall and build it up again. Sometimes a new foundation is in order.

Q:  I am building an adobe house with my kids. I would like to know how I can achieve a smooth wall; it is uneven with the bricks and it doesn’t look very nice. Can I use the same mixture I used for the bricks and mortar to put on the surface once the wall is dry already? Do I need to wet the wall for the mix to stick? The next question is about the finishing. I can’t find instructions on how to mix horse manure to finish the wall before painting. (I will use hydrated lime to seal and waterproof). Can you provide instructions or a link to directions on using horse manure to finish the wall? And if I can use manure to smooth the wall, is it really necessary to use the adobe mix underneath to make the wall smooth?

A: I have not used horse manure for plastering. I once got involved in using cow manure. It had to be fresh and green according to the Australian expert who demonstrated at the Pojoaque Pueblo Cultural Center. Over the next few years my observation was that the manure plaster did not hold up any better than the surrounding just plain mud manure. Horse manure was often used in making adobe bricks. Some of us suspect that the great contribution came from the undigested straw that was finely chopped by the horse as he/she ate.

You can and should use the same mixture that you used for bricks and mortar. It should stick to the wall just fine. If you are in doubt you can add a pint of Elmer's Yellow Carpenters glue to each wheelbarrow. Purchased by the gallon, that will add about three dollars per barrow full. Finely chopped straw of wheat or barley will help tie it together. If you are going to use hydrated lime to seal and waterproof, the Carpenters glue will only help the plaster stick to the wall but it should do that well if it is the same material as the substrate. Every mixture of sand and clay is different in how it performs so you just have to experiment a bit and turn yourself into the world expert on your materials. Just in the past week I read of two clays I had never heard of before so it's a big world out there.

The purpose of plaster is to smooth walls to an acceptable smoothness and flatness. Whether or not you need to wet the wall before plastering is another question to be determined by doing it. I worked shoulder to shoulder with Michael Valdez for 18 years. He always wet the wall and I never did. Thirty years later there is no visible way to determine if either was was better. With a wet wall, you have a bit more time before the plaster begins to set and it can be troweled a bit more to smoothness. With adobe plaster on adobe walls, however, it is good to work quickly and minimize troweling. Too much troweling and the plaster will fall off especially where it is thicker. We always worked with hawks and trowels which was much faster than applying and smoothing by hand. However, we were in the business and had to deliver a product at a decent price and could not lavish the time on the walls that some owner/builders do. Hand application and smoothing of plaster works fine - better in the minds of some traditional plasterers - but it is all a matter of practice.

The most extensive sources of information start with Bill and Athena Steen and the Canela Project.  www.caneloproject.com Their son has one if not more of the most stunning Youtube presentations on the earthen building spectrum.

Q:  I'm planning to build an adobe courtyard wall in my front yard in Albuquerque. If I buy semi-stabilized bricks and cover the wall in a mud plaster of the same semi-stabilized mix, would a lime wash be compatible and would this combination be durable. Or would I be better off using fully stabilized bricks and plaster and some other product on the surface?

A: If the semi-stabilized bricks you purchase are stabilized with emulsified asphalt and you use the same for the mud plaster, the lime wash should adhere nicely. Too much asphalt such as in fully stabilized adobe bricks and the lime may not adhere as well. In short, there is little experience with lime washes on asphalt stabilized surfaces. If you use fully stabilized bricks and fully stabilized plaster that would be a durable surface itself.

Q: If I use fully stabilized Adobe's for the first 2 courses and semi stabilized for the remainder, can I use semi stabilized mud plaster over the entire wall...will it adhere over the bottom 2 courses? My current plan is to lime wash over the mud plaster.

A: Semi-stabilized plaster will stick well to fully-stabilized bricks. Lime wash should be able to get a grip on the semi-stabilized plaster.

Q: I have been building my own earthbag home near Abiquiu, NM for the last few years. I am currently in the process of plastering the outside of my home with adobe. I know that some people do not recommend this but I really wanted to try. I have been using asphalt emulsion in my mix (about 12 oz to a wheelbarrow full) , to attempt to waterproof, but my plaster seems to be eroding pretty fast already. I'm wondering if I am not using enough? or too much? Would it be better to give the plaster a top coat of linseed oil? I seem to find mixed reviews about whether lime wash on top is good. And since I've already started with the asphalt emulsion, is it too late to try other kinds of waterproofing?

A: Most soils around Abiquiui have a lot of silt. Clay is good because it is the glue of adobe. Sand is good because it is the strength of adobe. Silt just keeps the clay and sand apart. If your soil is the lovely Abuiqui Red then it is not expected to have a long life as an exterior plaster. Twelve ounces of asphalt emulsion per wheelbarrow is just the right amount to start out with. But that Abiuquiu Red silt may be eating it up and countering it's properties. Try doubling the asphalt amount. Top coats of linseed oil never hurt unless overdone. You might try one boiled linseed to three parts of thinner (Paint thinner, turpentine or citrus oil thinner) That will not overwhelm the breathability of the plaster. Three coats of straight linseed just might overwhelm the breathability. If that happens you will know when chunks of plaster fall off instead of erosion of the surface. Lime wash can't hurt. You get mixed reviews since no one really knows all the details of your local soil. Until a soil's composition - clay type, silt, sand (course or fine, round or sharp) is known along with the percentages of each it's hard to predict. Even then, the best of the testing labs or university studies always end up saying "make test samples to verify workability and longevity." You can mix and match most types of earth based plasters. As my old pal Georgia O'Keefe used to say, "In order to spell Abiuquiuie you have to know when to stop." Georgia was able to make many different local soil colors work as plasters but she stuck to interior surfaces.

Q: I'm in the process of building my adobe courtyard wall in Albuquerque, almost half way. I am using semi-stabilized adobes (except the bottom 2 courses which are fully stabilized) . My plan is to do mud plaster and then a multi-coat lime wash on top to protect them. I am not going to be finished before the summer monsoons start. I am wondering if I need to take any measures to protect the wall and the remaining palates of adobes from the rain before I am finished. The company that sold me the adobes said they should be fine, but I don't want to take any chances. Minor deterioration would be OK, but I would feel kind of sad if I wake up one morning to a pile of mud in the front yard :)

A: In all likelihood you can count on your adobe suppliers statements. However, if you want additional assurance, its easy enough to wrap adobe bricks on a pallet using one of those larger rolls of shrink wrap found at U-Haul or even Home Depot. Put a chunk of plastic on the top and drape it over the sides and then wrap around the sides with the shrink wrap. Just keep the bottom of the plastic up off the ground or it will act like a suction  cup and bring standing water up into the adobes.

Q: I am almost done laying adobes for my courtyard wall in Albuquerque. I have a couple of questions about my next step, which is applying adobe mud plaster to the wall (afterwards I am planning to apply a lime wash). I am planning to screen the soil mix through an 1/8 inch screen but otherwise mix it the same as I did for the mortar, semi-stabilized with emulsified asphalt. First, what is the benefit of adding straw to the plaster, and if I choose to do so, how much, what kind and how finely chopped should the straw be (and how to go about chopping it)? Second, is there a minimum nighttime temperature below which I should stop plastering and take it up next spring?

A: Minimum temperature is 35 degrees for the sides of the walls, 45 degrees for the top of the wall. Horizontal surfaces facing the dark zenith of the night sky will freeze ten to fifteen degrees before the ambient air temperature gets to 32. While that is especially true for car hoods and roofs, it is not always true for a surface in contact with the thermal mass of the wall it rests on. But for insurance purposes, stick with 35/45 if you can.

When you screen the soil, you are removing part of the sand/aggregate component and the clay content will be higher. Straw helps hold adobe bricks, plaster and floors together when they might otherwise crack. I use a 1/4-inch screen since any coats of plaster go on thicker than that particle size. Wheat straw is the most common but if you can find barley straw it is a bit tougher. I got a bale of a cubic foot or so of wheat straw chopped to about 3/4-inch at a garden supply place. It even has a bit of a binder mixed in to help the gardner keep it in place. That won't hurt the plaster. Straw can be chopped with a hatchet on a tree stump, run through a homeowner's shredder/mulcher, worked over with a string trimmer in a large bucket - farm size with two handles. You can also follow a horse around and gather the manure which has lots of finely chopped straw.Start with two batches of whatever you can grab with two handsper wheel barrow full. That's straw. Manure, perhaps a shovel full.

Q: My courtyard adobe wall is having some problems. The wall is semi-asphalt-stabilized adobe except for the bottom two courses and a rounding over course on the top---these are fully stabilized with asphalt. I painted the entire wall last year with 3-4 coats of lime wash, which adhered great to the semi-stabilized adobe, but over time is peeling off badly from the fully stabilized adobe, especially on the top after rainfall. Is there something I can use to paint the whole wall, something durable to protect it and not have to repaint year after year. Can I use an exterior acrylic latex paint? Or is there another option?

A: You are after the Holy Grail of Adobe. So far there is nothing that works absolutely. Any material on adobe, or any masonry wall, needs to be moisture vapor permeable and mostly liquid water impermeable. Any vapor impermeable surface material eventually gets some moisture behind it and then it loses its grip on the bricks. OKON w-1 or w-2 might work. There are some specific paint formulations for masonry walls which claim vapor permeability. Ultraviolet in New Mexico boils off most any chemical on the surface of a wall or top of a parapet.

What I do is mud plaster walls. A wheelbarrow of plaster of the same mix as the bricks can be enhanced a bit with emulsified asphalt. I blend in about 12- maybe 16-ounces of asphalt per wheelbarrow. Any more than that and the risk of moisture vapor impermeability raises its head. That mix will get me four, maybe seven years before I am shamed into plastering again. There is a wall on the San Juan Nepumoceno Church in El Rito, NM that has this blend holding up over twenty years. It has been repaired a time or two at the bottom.

You might try Stucco Bonder on the fully stabilized courses and top. Follow it while tacky with a coat or two of your lime wash. Maybe even mix in a little bit of Stucco Bonder in the lime wash.

Q: I have an adobe casita in Albuquerque, built in the 70’s. I’m sure in that time that the exterior surface must have had additional coatings of adobe applied. I am due to have this done again Currently, the adobe has lots of wet looking streaks on it that make it look like there has been a rain storm. This does not change with extended periods of dry weather. Could you please tell me what causes this and how we can avoid this with the new coating that is to be applied? (BTW, the roof is only a couple years old and I am told the perimeter seal looks good but there was an interior leak prior to its being replaced.)

A: What you are calling adobe plaster is likely cement/lime/sand based exterior stucco. This cement stucco will show streaks after repeated wettings. If it were actually adobe mud plaster, I would not expect it to show streaks once it thoroughly dries out. The fix is to find where the moisture comes from and put a stop to its incursion. This is often difficult but most often comes from roof leaks. If you do have a cement based stucco continue with that and do not let anyone talk you into using elastomeric stucco.

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