Earth plaster questions -- The Mix, and using over tires...?

I've heard a lot about issues with putting impermeable coatings over natural materials (like concrete over adobe or straw bales, creating trapped moisture problems).

What about the other way around -- putting earth plasters over tires?  What do we need to consider in terms of moisture permeability, etc?  The mudding will be 1-1/2 inches thick at the thinnest points.  We do not plan to include a moisture barrier over the tires.  Just tire bale walls, covered with a 1"x2" 14 gau. wire mesh, then lime-stabilized earth plaster on the exterior, and plain earth plaster on the interior.  We aren't sure if it is common to cover earthships with mud plaster... we've seen concrete earthship walls and earth-bermed earthships, but not sure how common it is to have cob/adobe covering the tiles.

Also, we are trying to get our head around the earthen mixture.  We would like to work with on-site materials.  We live on the Colorado Plateau -- ancestral Pueblo lands.  We would think that our dirt would be great for adobe plasters.  From what we can glean from books & online resources most people take their local dirt and then often have to supplement with sand that they can purchase like from a concrete company or something.  We are neophytes, but from our clumsy initial assessments, it seems like our soil is high in sand and does not have enough clay.  Where do you typically get clay to add in to a mixture?  Where can we get our soil tested for construction purposes so we aren't just depending on our own booklearning devoid of experience?  We have reached out locally to hopefully find experienced folks who can mentor us -- of course more difficult in these times.  We do plan to supplement with straw or horse manure.

Thanks in advance for your thoughts!

There are simple tests you can do with your soil. Bottle test , snake test.
Your question perplexes me, because earth is the material mainly used.
Sometimes clay has to be imported from offsite, I hope local knowledge is there to be found.

Thanks, John.  Yes, we've started those types of tests -- I just don't trust mysef to interpret the results as well as someone with lots of experience.  So we'll keep playing with it and maybe we'll start feeling experienced!!  We'll also start doing test patches and see how they turn out, which should help develop our understanding.

Did you mean that most earthships use earth and not concrete?

So here is my confusion/difficulty with the soil tests.

Most resources talk about creating earthen plaster with clay, sand, and straw.  When they talk about using on-site materials, they sometimes use 'clay' interchangeably with on-site dirt.  They talk about testing your local dirt for the ratio of clay/sand/silt.  Then they talk about amending your dirt/"clay" with sand.

It is confusing because of using the words "clay" and on-site "dirt" interchangeably.  I guess it's because the authors come from places where the on-site dirt is mostly clay...?  I've seen various resources that do this; the one I'm using at the center of my current explorations is Earthen Floors by Sukita Reay Crimmel and James Thomson (we will have earthen floors and earthen plaster in our house).

So I did the dirt tests... the jar of water, molding with it, checking the stickiness... and it clearly does not have enough clay (sandy loam) although it does have some clay.

But then once you've determined "good" dirt with the right amount of clay, you are supposed to mix your "clay/soil" with sand in varying ratios.  You do tests with, e.g., "clay/soil":sand ratios of 1:2, 1:3, 1:4

Since our dirt is "bad" due to having too much sand, I went ahead and did the test with our straight soil, hoping that it would be as good as a 1:4 mix, anyway!

I'm trying to attach captioned photos of the tests (I didn't take a pic of all the tests -- like the snake test which showed there wasn't enough clay).  The preview doesn't show them, so I'm not sure if they will come through.

But anyway, it dried to a solid disk that stuck to the board even when I held it vertically.  It was hard and resistant, but I was able to scratch into it and keep scratching material off of it.  I was able to separate it from the board and hold it like a platter.  When I dropped it from hip-height, it shattered.

So yes, I think it shows that it doesn't even have enough clay to be used as a clay&sand mix.  Too bad, since we have a big mound of this stuff stockpiled at the build site!  It seems close.  But I'm not sure how to amend it to work...

We've been working on building and probably won't plaster until the new year (when we'll be under the roof), but we're revisiting our mix.
We found some more clay-like soil available from our local concrete supplier.  They have mounds of dirt and don't vouch for the clay content, but it seems fairly uniform, and they let us just take some to test and play with.

We did the water jar test which showed a higher clay content than ours.  We could make a nice sticky ball, and a snake that wrapped around our fingers.

I set up a couple of plaster tests.  On the right is plaster made from the concrete supplier's dirt alone.  It's still wet in the picture, but I'm excited that it started cracking within 1/2 hour!  We're hoping it's pretty strongly clay so we can use lots of our on-site dirt, since we have huge mounds of sandy dirt... plus we would love to have our house covered in our own dirt.  On the left is a mix with 3 parts our dirt (sandy) with 1 part dirt from the concrete supplier.  I'll udpate with the results, including any other mixes we make after seeing how these turn out.

This online course might be useful to you.  There are a few free videos for previewing on this page:

The Mud Home's "The Perfect Earth Plaster" online video course


That lady also has other courses, but I think this is what you are asking about.

Good luck.  I'm at the same step.  Also, another resource is the Canelo Project in Arizona, here is their website's video link.  These are free:

The Canelo Project how to earth plaster videos


They also do phone consulting and classes.

Thank you, Kim!  We coincidentally found this course and have been watching it.  You're right, it is perfect for our needs!  
Her solution to finding clay, though, is to go to a park where there is a pond or river and harvest there... might work in Turkey, but not here!  We may well be buying clay to add...

We'll check out the Canelo project too -- thanks for that!

You might be surprised by someone who knows where to find clay locally if you ask around.  I'm in NM, too, and I learned that a farm out here gleyed their pond with a load of clay.  That's amazing for a couple reasons, one being that the soil here is very sandy and yet the gleying worked well.  They found the clay while out riding around checking on their cattle.  Someone might have a dry  creek with some in the base, or something like that.  Never know!  Might save you a big chunk of money.

The people putting out the Canelo project videos use partially local clay, I believe.  They may have tips on how to find it, too. I'm sure they'll know affordable place to buy from, as well.  I love the video with the oiled clay wall.  My husband and I are intending to do that on one wall of the house we are building.

Wow, Kim, thanks for sharing the Canelo resource -- beautiful videos! I've just watched two so far, but will be watching more.  I am also thrilled by their Japanese influence; I've been thinking of our explorations here as being "Japasazi" -- so politically incorrect, I know -- but we are living here on ancestral Puebloan land and wanting to honor and be good stewards of the land and its history, while also being true to who we are. For me that includes Japanese connections in our food sovereignty efforts, and working with nature, etc. I have been finding so many Japanese connections in unusual places, and the Canelo project turns out to be the most recent example...

Where are you?  We are just north of I-40, 90 minutes west of Albuquerque.  We've been sort of looking for clay-- our local concrete supplier has a pile of dirt that has slightly more clay than we do. We've asked all of the workers who have helped out on our build so far, but no one has known of anywhere yet. Maybe I should post on Facebook marketplace or Craigslist...

Glad you like those videos.  That's the best plasterwork I've seen examples of online or in person, and that's what my husband and I are striving for.  They do classes and also consulting, and I intend to do at least a consult before doing our wall.  

Another option is you could pay one of them to come up and help you do your first section of wall - like a hands on clay plaster formulation and plaster demo. My husband wants us to do that, thinks we will do a lot better with some direct instruction.

I'm down in the bootheel.  There is a great variety of soil types here, we've noticed.

Good luck and hope you can have fun in the process!    I looked at your website - and I can't wait to see the finished product.  Very cool.

I suggest you use on site soil and amend it if you need to. You can always add lime or a very small portion of Portland cement that will really help improve the structure of your plaster. A few shovelsful of Portland will improve the plaster enormously without making it totally impermeable. My friend used soil "straight" after screening out most of the rocks, and 1/10 Portland. It dried nice and hard and has held up well.

David Balt wrote:I suggest you use on site soil and amend it if you need to. You can always add lime or a very small portion of Portland cement that will really help improve the structure of your plaster. A few shovelsful of Portland will improve the plaster enormously without making it totally impermeable. My friend used soil "straight" after screening out most of the rocks, and 1/10 Portland. It dried nice and hard and has held up well.

Thanks, David.  I have been wondering about this.  Does lime/portland make up for having less clay? Or do you need the appropriate clay content and then lime/portland helps strengthen?  I am aware that lime/portland provide some water protection, but wasn't sure if it also acts like a binder (similar to clay)

Kimi Iszikala wrote:I'll udpate with the results, including any other mixes we make after seeing how these turn out.

Belated update:  Even the full "clay soil" sample from the concrete supplier seemed maybe slightly low in clay, or possibly "just right" meaning we'd have to use it straight and haul in many tons, with out using our on-site dirt.

Now we bought a 10 lb sample bag of the cheapest clay we could find from a nearby pottery supply shop.  We are hoping that a small addition will make our on-site soil usable.  We should have tested it by now, but it's been a long cold snap and I just want to sit in the tiny camper with the propane on and eat, and I am not in the mood to mix up a watery mix to test...

Kimi Iszikala wrote:Thanks, David.  I have been wondering about this.  Does lime/portland make up for having less clay? Or do you need the appropriate clay content and then lime/portland helps strengthen?  I am aware that lime/portland provide some water protection, but wasn't sure if it also acts like a binder (similar to clay)

Yes, lime and cement is a binder. Clay, lime, cement, and asphalt emulsion are the primary ways to make a bunch of rock and sand stick together. They all have pros and cons, which is a whole different topic. If you're doing earthen construction there are arguments for and against all of them, but if you're looking for a breathable plaster then you may want to stick to things like lime and clay as they are generally used in that regard. Two questions I have for you though:

1) Have you contacted any quarries or aggregate suppliers? I was able to find a pretty dead-on mix from their reject sand pile, sometimes known as dug sand or fill sand, it's a uniform product that's high in clay and no one wants it. It's what the crushers spit out after they've screened the more valuable products (gravel, typically). It's amazingly cheap and you'll pay only for the delivery, plus maybe a couple of bucks a ton. But if you can secure that you'll have a perfectly uniform pile of material sitting on your building site ready to go and you won't need to futz around with other ingredients (unless there's something else you want to add for some other reason).

Edit: I went back and re-read, it looks like you did find something like this. Personally I think this is the way to go. The value in having shovel-ready materials on hand, pre-screened, is immense. Earthen building is heavy on labor and minimizing the amount of times you need to shovel/lift/stir is huge.

2) Have you considered using a cement mortar on the exterior and a clay/lime on the interior? I don't know your area, but the choices are (a) cement based if you live somewhere that gets a lot of rain (b) lime based if you live somewhere that doesn't. It's talked about here, halfway down: http://earthbagbuilding.com/faqs/stucco.htm . Having the interior be more breathable is a compromise you may need to make if you're forced into using cement on the exterior, or you could put sufficient roofing over the walls that even in a rainy climate you're not going to really see much of that moisture hit the walls so clay or lime would be fine.

Tony has some really good advice generally, but I would caution against putting portland cement on either side of an earthen wall. Breathability is in both directions and if you block the exit path of moisture, it accumulates in the wall material. In wet climates, a lime plaster with sufficient roof overhang is a good way to roll. Houses in England and Europe have used lime render for hundreds of years.

Phil Stevens wrote:Tony has some really good advice generally, but I would caution against putting portland cement on either side of an earthen wall. Breathability is in both directions and if you block the exit path of moisture, it accumulates in the wall material. In wet climates, a lime plaster with sufficient roof overhang is a good way to roll. Houses in England and Europe have used lime render for hundreds of years.

Yeah no argument from me on that one. My only suggestion of using cement on the exterior is if the building roofing doesn't shield the walls sufficiently and you're in a very rainy place. Even where I'm at, which gets ~6" of rain per year and maybe 1' of snow per year, lime with overhangs is the way to go.

Update... Just to close the loop in case this is of interest to others who happen upon this thread...

We ended up using shotcrete over our tire bales. Just too many voids and too wonky of a structure to feel confident getting a robust, critter-proof envelope using earthen materials ourselves, and it would have taken us back-breaking ages. Shotcrete took 2 days (and a lot of money.)

We used lime plaster on the exterior of our earthbag walls (triangle walls between the tire bale bond beam and sloped roof). We will use earth plaster on the interior of those walls.

We used cement/lime stucco on the most exposed portion of our stick-built wall... the south-facing front wall with a small overhang (sized for passive solar rather than for surface protection). The stucco is above our full-length porch roof.

Under the porch we are using earth/straw plaster with our concrete supplier's fill dirt as the earth. We did go to a fantastic Canelo workshop, and followed their advice for earth plaster over OSB: plastering over high-quality concrete bonding glue while the glue is still wet. No lath at all, except some strips of fiberglass lathe around windows and doors for added tensile support. It seems to be working well. After occupancy permit we will probably add an earth/sand finish coat just to dress it up a bit.

Details of most of the above are on our blog, brownkawa.com although I haven't yet written up the front stick-built wall plaster... we are still finishing that job.