Creating an Earth Plaster Recipe From Your Soil –

Written by Andrew Morrison

plastering straw bale houseEarth plasters are hard to work with. That’s not something that most people expect to hear when talking earth plaster. Most people expect that the natural plaster will be easy to work with and more fun because it is basically just glorified mud. Unfortunately, this is not the case. Earth plaster is an art form and a learned skill rolled into one. With all other plasters, your materials are consistent; however, with earth plaster, the ingredients can change from one square foot of your land to another. You have to fully understand the materials you are working with and how they fit the bigger picture of the overall mix.

There will be lots of trial and error learning when creating an earth plaster. For this reason, I suggest you build a small bale wall on which to test your mixes. This can be as simple as a stack of bales that will accept plaster. Make sure that you can put some pressure on the bales as you apply the mud without them falling over. You want a realistic test of the material. If you find that the plaster is too crumbly, then you don’t have enough clay in the mix. If it is cracking like crazy, you have too much clay in the mix.
measuring plaster
A Mason Jar Test: Fill a mason jar half way with soil you think would work for your plaster and the rest with water. Shake the jar vigorously for a few minutes to mix everything up. Set the jar down and wait for it to settle out. The sand will settle out in the first few minutes. The silt will be next and will settle out in a few hours, maybe as long as 10. The clay will take a couple days to fully settle. Measure the sand layer, the silt layer and the clay layer. You can determine the percentage of each based on the relationship to each other displayed in the test. You want a soil that is roughly 80% sand and 20% clay and silts.

Testing your soil is a must. Find a large area that you are willing to dig up and use for your plaster. One option is to use the soil removed from any excavation for the house or structure. Screen it through several grades of screening starting at about 1/2″, moving to 1/4″ and ending at 1/8″. This will give you a quality material to work with when plastering with no rocks or major lumps. With the material screened you can run mason jar tests on the final batching of material as before, this time with an actual finished material that is all mixed together. If your testing shows weakness by either too much or too little clay, make adjustments with bag sand or bag clay.

sureflexYou will want to stabilize the plaster as well. The most common stabilizers are cement, lime, and asphalt emulsion. Each should be added to your test mix until you have enough to resist a hose spraying on a dry test patch. The hose should not erode the plaster at all. Once you meet this grade, you are ready to plaster. You can add horse hair or chopped straw to the mix as well to give it more tensile strength. This does not effect the ratios of the materials, but it will require that you add more water to the plaster.

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28 Responses

  1. Question related to the Mason Jar test.

    Half with soil and the other half with water, I guess?

    Thanks for the good info. Will start testing.

  2. Hi,

    I have a question about plastic stucco mesh that has been mentioned in at least one book I have researched. Any experience or comments about plastic vs metal? Also, if plastic is viable, any resources? Thanks.


  3. Hey …thanks so very much for doin what you do best and love…educatin joe public…i love…all that i read…know about this stuff for over 15yrs…and now…i found you… bless you brother…for your efforts…i have read so much stuff…where is that 4,ooo dollar……sorry i am blonde when it comes to computer stuff..I would like to start small…and go from [email protected]…I have many family memeber..lookin at regular homes…now i can teach them a better way…thanks ken luquette

  4. I have used plastic mesh by Tenax and have been relatively happy with it. I personally prefer using the metal mesh as it holds its shape better. The plastic mesh is easy to work with, less expensive, and cuts with a utility blade. That’s about it for advantage. Again, I prefer the 2×2 14 gauge welded wire mesh myself.

  5. I have also used Tenax. It comes in eight-foot or 10-foot rolls so you can literally wrap each level of the building with it in one shot. Of course your mesh should be part of your design based on strength requirements, among other things, as mentioned here:

    Remember, choosing your plaster system is not as eay as finding the cheapest materials.


  6. Hi,

    Thanks to both Andrew and Jeff for your help on the stucco mesh. I am shopping around and have found that the metal is very expensive. I may do a practice run with the plastic, if I have time. My windows are taking a lot longer than I had anticipated. I have 6 large triangular windows which are not equal on any side and most are different dimensions from each other. I am having fun with the challenge of it, though!

  7. Hi Tracy. Keep in mind that the plastic offers very little shear resistance and the metal (welded wire mesh) offers a great deal. If you plan to rely on the mesh for your shear system, you will most likely need to use the metal. It is more expensive, but it is a better option when you consider the shear value. If you don’t need the shear value, then the plastic is fine.

  8. I think i am the first in my country who build with straw.
    I need to know how to trim the bales because are not straight.I choose a slip bath before stacking.So how to make the wall straight? After the slip bath should I attach the lath ?Is ok a 19 gauge lath?

  9. Hi Adrian. Congratulations on being the first! I use a weed whacker to clean up the face of the bales after they have been straightened by pushing them in or out of the building according to a straight edge. You can learn more about this on my DVD available at or by clicking the links to the left. 19 gauge lath is very thin. I use 14 gauge. At the very least, I would suggest 16 gauge. The stiffer the mesh the easier it is to work with as it will maintain its shape better. Good luck.

  10. Hi,

    I am on the stucco mesh subject again. The structural engineer spec’d shear walls at each corner and at the garage door opening, with 10 hold downs in the footing and I am wondering if this is sufficient enough to go with he less expensive option. I am concerned about budget right now. I plan to drywall the interior (with a lime water coat first) and lime plaster on the exterior with a straw facade and plaster treatment on all the exposed exterior plywood. There is no reference to stucco mesh on architectural or structural drawings. Thanks. PS I received your plaster dvd, thank you so much for you info.

  11. Hi Tracy. If you have designed shear panels, then you can use whatever mesh you like. There is no required shear value in the mesh with the panels already in place.

    Drywall on the interior? How do you plan to complete the window and door openings? I imagine how that can work, but am curious as to your plans. It seems like it would be quite angular. That may be your intention, so I am not judging, just curious.

  12. Howdy,

    Yes, drywall, because I believe it will be cleaner with sawdust, Sandy Valley dust and dirt, among other concerns. I do plan the lime water wash first, to seal the straw. I want to drywall the window openings and will need to custom cut the triangles, but I think drywall window sills and on the wall surrounding the windows on the inside will bounce some more light into the shop. My sister, Darby, has a great deal of drywall experience and I hope to have her come for the big push at that point. Baseboards, some sort of trim work on the top of the walls and at corners, but I am not sure yet. The ceiling will be open, so drywall to the roof plate will need some sort of finish that won’t look unfinished. Again, this is my practice building, so I am learning and whole-heartedly open to suggestions. Thanks.

  13. Okay. I would suggest that you place framing at the corners of the window well.s IN fact, you might consider building full bucks with angled plywood that would support the drywall over the entire surface.

  14. Hi,

    I have built the bucks. I posted a few pics on the Strawbale Enthusiasts Group page on Facebook. I built each window buck on the ground for a dry fit & then placed them, as each was a different sized opening and each window was slightly different. They turned out OK, but need some finish work to be what I have in mind.While the weather is somewhat cooler, I have moved on to completing the roof and then finishing off the exposed plywood. Slowly, but surely, I am getting to the point of plastering. Still have some exterior electrical prep to do, flashing, etc. If you see the window bucks on the FB page, can you tell me if that is what you were describing? I still have to fill in the two sides with straw, so only the windows are exposed. Thank you for your input.

  15. Tracy,
    That will work. It looks like you plan to have the windows inset from the exterior face of the bales. Be sure to fully flash the transition from the window to the bales as a failure there will go undetected for years as water leaks into the bales under the window. This detail must be really well handled.

  16. Hi Tracy and Andrew
    Just one question, how does drywall work together with strawbale and clay contest ? What I ment is, that drywall are not breathable material isn’t it ? Can you use Fermacell instead ? It is stronger then drywall, breathable and fire resistant then drywall. I’m using a lot Fermacell in my remodeling projects and just wondering about strawbale and Faermacell association to each other.



  17. Hi Kolekoll,

    I am not familiar with the product you are suggesting, “Fermacell”. I’ll google i and do some reading. In the meantime, I had planned to drywall with a lime water wash on the interior bales 1st. I can’t give you any info about the “breath-ability”, although, I have no plans for a sealer-type finish. I just want a clean look for the shop. The more surfaces, as a plaster would provide, the more spots for sawdust and, just plain sand to rest and make it difficult to maintain. Thanks for your interest and I WILL research your suggestion regarding Fermacell.

  18. Hello Andrew,

    I want to start off by thanking you for being such a great resource for people who are trying to build naturally and responsibly. I purchased the three DVD’s you sell and was very pleased with two of them: building with bales and monolithic slab foundation. However, I was unpleased with your plaster DVD. First, I thought it was short and you did not go into very much detail about the different types of lime plasters or tips on how to mix pigments into the plaster. I was also disappointed that you prefer to use lime plaster exclusively for base and finish coat inside and out. Although lime plaster is great, it has a large amount of embodied energy both because of the manufacturing and shipping (most if not all lime plasters made in Europe). I was taught to use site made earthen plaster as both first and second coat then lime plaster as a finish coat on exteriors and American Clay pigmented plasters for interiors. To me this makes more sense since site made earthen plasters take a small amount of embodied energy to produce. I was also disappointed that you started this post with “earthen plasters are hard to work with” as that is not my experience. As long as you mix your own plaster with the right ratio of good clay, sand, water and straw I don’t see much difficulty. I was also taught to first spray the straw with clay slip then use a first coat with higher clay content (for adhesion purposes), then increase sand and decrease clay (to prevent cracking) for the second coat. I am curious what you think about this. Thanks

  19. Hi Marlon. Thanks for the feedback. I appreciate you taking the time to share your thoughts. Many people have used the methodology that you suggest of earth plaster underneath a finish coat of lime plaster. In the last year, there has been a significant amount of data showing that this is a bad idea. It is extremely difficult to get a lime plaster finish coat to properly adhere to earthen base coats and the result is delaminating plaster. It works for the short term, but does not hold up in the long run.

    The “hard to work with” comment relates to the difficulty in creating quality earth plasters from locally harvested soils. As a geologist and from my own experience with plastering, I can say that soils can vary greatly from one test hole of supply to another. I have literally seen holes 3′ apart that are 100% different in composition.

    Consistency is absolutely key when it comes to plaster creation. If you cannot ensure consistency in your plaster from one batch to the next, you run the risk of poor adhesion and, once again, delamination. If you can manage and maintain the consistency of the plaster, then earthen plasters are fantastic; however, that is a scientifically difficult thing to do.

    When choosing a plaster, there are a lot of considerations. Price, embodied energy, ease of application, durability, long term maintenance, and so on. I think this is a very personal decision and the level of input from each aspect will affect each individual and their decision differently. I do agree with you that if one can use an earthen plaster (under the parameters set out above) that it is a great choice and one with less impact on our planet.

    It’s important that your coats of plaster get weaker as they move out. In other words, there should always be more sand in the mix as you get closer to the surface. This is important because if you get failure in the first coat or second coat, you want to know that as soon as it happens so that you can fix the problem before it escalates. If you have a stronger plaster on top of a weaker one, you will miss the cues and delamination or other catastrophic failure may take place. A finish coat of lime is just such a scenario. It is harder than the clay and thus will hide any failures in the base coats until it is too late.

    I believe that major attention to detail and using proven materials in plaster is important. It is no fun removing a failed plaster coat and replastering an entire home. I may be a bit conservative on this, but I don’t think it’s a place to take chances.

  20. Hi:

    I appreciate the great information you are providing. One comment about soils.

    Rather than reccomending folks to use the mason jar test, why not just recommend that folks send in a couple of different samples to a a state or extension soils lab and ask for a soil texture or particle size determination. In most states these only cost $5-$10 a sample and that will give you the percent sand silt and clay.

    Then you can use that to calibrate your other samples either doing the mason jar test or the field method for soil texture (google “field soil texture” and some guides should come up.

    In addition, all private land in the lower 48 has a completed soil survey. You can zoom into your property and get soils data (including sand silt and clay). It is not always accurate for small areas (such as building sites since minmum sized mapping areas are about 2 acres) but usually is accurate about 80% of the time.

  21. I would always add the hair or straw to the earthen mix as it is pretty weak on its own in terms of tensile strength. That said, I am a lime plaster guy first and foremost, so I don’t have the expertise on clay plasters to know the intricate details. I would recommend speaking with Bill and Athena Steen at the Canelo Project ( as they do beautiful earthen plaster work and have much more expertise on that subject than I do.

  22. Excellent tutorial and quick easy to understand steps. That makes it so much easier on me doing the research for my site. I already do the jar testing for an earthbag home I am planning, which the ratios are just different from the bags to the plaster. But do you have any experience with the earthen floor? I’m curious to know if it’s going to be the same as the plaster or am i talking a whole different ballgame?

  23. Hey there,
    Thank you for this helpful tutorial!
    I have two questions that seem to linger
    1. I am wondering if we can add sand or clay to the mix after we do the mason jar test – depending on what ratio we need and
    2. What do we do about silt?
    Thank you so so so much for your help!

  24. Hi Jem. Yes. The mason jar test tells you what you have and then you can augment as necessary to get to the “perfect recipe”. In terms on the silt, there’s not much to be done on most cases as the percentage is usually quite low. You don’t want a very high content of silt, but for most plasters in most soils, you’ll be fine. I should caveat that response by saying that my expertise is in using lime plaster, not earthen plaster. Some really experts would be Athena and Bill Stein from the Canelo Project. They can help you much better than I can with earthen plaster details. Best of success to you!

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