DIY Soil Cement Sectional Floor

I read an article about making soil cement decades ago, possibly from an issue of Mother Earth News. I thought I might want to try it someday. The wood-framed panelized yurt that we built recently was the perfect place for the experiment. The idea of a “dirt floor” seemed to go well with the setting and the unique building. Plus, I could form it in twelve sections, just as the yurt is 12-sided.

Note that this is not the commercial soil-cement used by some highway departments, which uses sand and crushed stone with the soil and cement mix, making it basically concrete with soil added. They might use other waste materials as well, such as cinders and fly ash.

Since I had discarded the article, saving only the “recipe,” I did a couple of tests first. The mix ratio from the recipe was different from what I settled on.  The original recipe called for less cement by volume (5:1) and the amount of water was less. The calculated volume of material for one of the 2-1/2 inch thick wedges was 3 cubic feet. This proved to be inaccurate when using our soil. The loose, crumbly clay soil turned out to need four cubic feet to allow for all the air pockets. So, adjustments were made, settling on a mix of four cubic feet of soil to 1 cubic foot of cement for one wedge.

We divided each wedge pour into 1/4 cubic foot of cement to each cubic foot of clay soil, making a total of four pours per wedge. The water mix would be whatever made it workable, but originally was figured to be less than two gallons.

My first test sample, a very small amount comparable to the 4:1 mix with the water equivalent to the original recipe turned out fine in the test, but proved to be way too dry in the first wedge we mixed and poured. So, the amount of water became an evolving test as we mixed and poured the subsequent wedges. Generally, two gallons per mix worked with adjustments made for damp soils or dryer soils. Even though all the soil came from one pile, the moisture content varied.

I made two tests. I painted one of them with concrete stain just to see how we liked that. We didn’t. The natural color was just the look we wanted for the yurt floor.

Materials needed

This is a list of all the materials and tools we can remember using. The amounts of soil, cement and water needed are detailed in the instructions.

Soil

Cement

Water

Vessels for storing sifted soil

Wheelbarrow for mixing

Shovel

Hoe

Small shovel for scooping and cleaning out last bits

Strong bucket

A variety of concrete trowels, i.e. margin, brick, rounded finish, long finish

Tamper

Fence wire

Plastic sheeting

Tape

Wood and hardware for forms

Step 1 Build the Form

The first step was to make forms. My plan was to pour the wedges from the back of the yurt to the front, every other one. The forms were simply screwed to the yurt deck. They could be easily moved for each subsequent pour. Once six were done and cured enough, then we’d just fill in the final six wedges. The middle was left to put in something fun for a center focal point.

The center was six-sided while the yurt was 12-sided. Due to this, the forms would meet at a corner point, and the center of one of the six sides, thus being different lengths and angles. I made each a little short and used wedge shims to bring them tight. This also made it easier to remove when the pour was cured.

This photo shows how the pours progressed and the form material was moved.

You could use this form system to pour a square or rectangular form by deciding how large a pour you wanted to do for each section.

Step 2 Bring in the Dirt

I brought in what I estimated to be more than enough sandy clay soil for the project, about 2-1/2 cubic yards. I dug it up on our property, but dirt can be found where excavation projects are going on. Just about any kind of earth can be used, as long as it contains clay and sand. It should be free of organic material and debris.

I tested my soil by filling a jar half full of sifted earth and adding an equal amount of water. I covered the jar and shook it for a few minutes then allowed it to stand undisturbed for an hour or so. Once settled, the soil will have separated into layers of sand on the bottom, clay in the middle and silt on the top. The ideal mix is when there is about 75% sand and 15-20% clay. I did several tests and got different results each time. In this photo there is about 50% sand and 50% clay and silt.

Step 3 Sift It, Sift It Good

Once we carved some time between other summer/early fall projects, Robin started sifting. She started by just chopping at the dirt with a hoe, but then figured out one of her plastic greenhouse trays would do the job and give us a more consistent product. After the first pour, we determined that each wedge would require almost 4 cubic feet of dirt. I had built a 12x12x12 inch (a cubic foot) box to measure the volumes. Robin got the four batches ready in advance of each pour by just filling the box with sifted dirt and storing in various holding containers.

Step 4 Mix It Up

The soil went into the wheelbarrow first, then the cement was added in furrows I made in the soil. I mixed that up using a hoe, chopping at it about 2 inches at a time from front to back so that the cement was well dispersed throughout the soil. Dry mixing continued until I couldn’t see any “orange” clay or “grey” cement bits.

Then I added two gallons of water and mixed until we had a batch that wasn’t soupy, but not too dry. We found that the wetter the mix, the better.

Step 5 Get Down and Get Dirty

Since the yurt has a few steps up to it and neither of us have terrific knees, Robin filled the bucket with about three shovel scoops and hauled it up to me inside the yurt. I slopped down the glop and handed back the bucket. One wedge, which was four batches of mix, took about two hours including a break after the wire reinforcement was tamped in after the second pour.

Since we’d have four pours for one wedge, I worked from the back to the front. After two pours, I spread it all out evenly, using a brick trowel to move big blops around and a margin trowel to semi-smooth it all.

Step 6 Start Spreading the Mud

Even though we used the same proportions of dirt, cement, and water for each pour, the consistency varied wildly. After the first couple of wedges, we decided we liked the wetter consistency and I started adjusting the amount of water depending on what happened with the mix. The wetter blend, about the consistency of Sloppy Joe mix, was easier to shovel and work with, plus those wedges were not as prone to cracking, which did happen on several early pours.

Step 7 Add Wire Reinforcement

I had no idea if soil cement cracks the way concrete can, but I figured reinforcement would help by containing the separation if it did occur, holding all snug together. I made a jig to simplify cutting the reinforcement wire, cut 2×4 grid fence wire in three pieces (this made the best use of the material), positioned it carefully, then tamped it into place. It was tamped to get any high ends down and to embed it into the first layer.

I made my tamper from a short approximately 8 inch piece of cedar tree trunk, a hole drilled in it to accommodate a broom handle.

Step 8 Get Back to the Dirt at Hand

While I tamped, Robin loaded the next cubic foot of dirt into the mixing wheelbarrow and measured out the cement. If she had the energy, she also started sifting dirt for the next wedge. We scheduled two wedges a week, weather permitting. We got behind when wildfires in Oregon caused our air quality to become unsafe for breathing, but after a couple of weeks we were able to return to the task. We got rained out a few times as well.

Step 9 Screed It!

Once we poured the fourth batch into a wedge, it was time to screed. I screeded using a 1×3, just as I would for traditional concrete. The “sawing” motion while advancing the screen board forward used in working concrete didn’t work very well. When screeding the soil cement, I found that it didn’t pull along like a concrete mix does. It tended to suck the “mud” from behind, pulling it away from the forms and scraping the surface off behind. The “mud” also built up in front, making it difficult to move it forward. I worked out a method using a margin trowel to remove the material from in front without taking too much.

Step 10 Finish It!

When I got it fairly close, I used a finishing trowels to begin smoothing it out. When working it you can get the surface to pull along to lower areas. Occasionally, air bubbles would develop, and I just punctured them and filled with a little excess mud. When I had it acceptably flat, I used a concrete finish towel to get it smooth. It can be worked to a very smooth surface, but I didn’t attempt to do that.

For the areas between the formed wedges, I taped plastic sheeting to the wedges on either side to keep the mud from adhering to the finished wedges and to keep them clean.

Step 11 Do Something With the Leftovers!

Four cubic feet was just a little more than we needed, so we used metal flashing and clamps to make forms for stepping stones. We added leaves to give them a fun look. I have no idea how they will hold up in the rain and freeze, but I guess we’ll find out!

Sections are still curing, but we might have a pinwheel effect going on here!

In conclusion, a crew of two old fahts did take a few weeks to accomplish soil cementing a 200 square foot area, but it’s a unique and durable floor. Plus it adds mass to the building to help maintain a comfortable temperature inside. I’m working on my middle piece now and will post an update when it’s done!

If you’re interested in how I built the rest of the yurt, we published a book. Here are the links:

Building a Wood-Framed Panelized Yurt, in color

Building a Wood-Framed Panelized Yurt, black & white

Text and photos on this blog are copyright ©2020 by Marvin Denmark and Robin Koontz. As much as we love to share, please ask permission before re-posting. Thank you!

Scaling Up the Size of the Yurt – the Beams

Let’s consider the beams for the 20 foot yurt. For the book I used a 4.5/12 slope and for practical reasons I would use it for your project. I’ve walked on many different sloped roofs and on some that couldn’t be walked on without ropes. I can say that you can comfortably walk or work on a roof that is a 4.5/12 slope.

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One thing that you may not immediately notice is that I assign the slope to the beam and not the roof plane. Why do that? If you use the roof plane you subsequently have to calculate the “hip” (that joint that joins the adjacent abutting planes) of the roof structure; i.e. directly over where the beams run. Why make things more complicated? Because the beam’s lengths are determined using a slope calculation, it simplifies your calculations. So now to that calculation.

When you solve for the hypotenuse of your 4.5/12 slope (triangle) you have a 4.5” leg perpendicular to a 12” base leg (level). Both squared (20.25+ 144) gives you 164.25 and the square root of that is 12.816 (your hypotenuse). How does all of this relate to the beam? Two things: first for every foot on the level that the beam travels the beam rises by 4 1/2 inches (so for a theoretical beam of 10 feet the “up” end is 45 inches higher) and second, for every foot traveled horizontally (on the level) the beam’s length increases by X 1.068 (so in our 10 feet run the beam grows to (10 X 1.068) or 10.68’ which is just over 10’-8”.

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So to complete the beam length calculations you have to subtract the skylight ring assembly and add the overhang you want (here beams are assumed to go all the way to the center of the yurt). Both of these are accomplished in the same manner as above.

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For the skylight, take the diagonal of the framing of the skylight ring (a level measurement from one corner to the opposite corner) and then halve that. That is then taken from the beam overall length, after calculating the “sloping” length.

The overhang is a bit different. To make it simple just consider the overhang equal to whatever you extend the beam tail (whatever is hanging past the walls). So, if you wanted a two foot overhang, just extend the beam out two feet (using the slope calculation, of course). This will be close enough for a two foot overhang.
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Pic2Otherwise, if you want a more precise dimension for your overhang you will need to do some more calculations. This requires a bit of trigonometry. It goes like this: Say you want a two foot overhang. This is a two foot projection that is horizontal (level) from the wall and parallel to the wall. Since we know the number of sides we have, we get the number of divisions in the 360 degree circle. That gives us the peak angle for our triangle. We will use 1/2 of that angle (see drawing below). Our long side of the triangle is 2’ (our overhang). So solving for the hypotenuse: recalling our trig formulas: CAH — cosine of the angle = adjacent side divided by the hypotenuse. Solving for H we divided through by A to get H=C/A. Now do the “rise” calculation from above to get the actual beam overhang length. This sounds like a lot of work, but honestly, it’s easier than reading this entire article!

Be sure to get a copy of the book which explains the rest of building a wood-framed panelized yurt. It’s available on Amazon in color paperback, color ebook, and b&w paperback. And hey, if you build a yurt after reading our book, please send photos to share, thanks!

Here are the links to purchase the book:

Building a Wood-Framed Panelized Yurt, in color

Building a Wood-Framed Panelized Yurt, black & white

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Scaling Up the Size of the Yurt

I want to talk about scaling up the size of the yurt. We had considered adding this in the book but decided against it. So I will take you through the process now. The size or diameter of the yurt in the book was 16 feet. Considering the next reasonable size up one might think of a 20-foot diameter. Let’s follow the steps to creating it. The parts that get affected are the foundation, floor panels, wall panels (though not height), beam length, roof panels, and possibly the skylight (size). If you follow this process you can create a yurt of any size (within reason, as loads on the beams become a factor). We begin with the floor plan, which in turn informs us of what the foundation will be.

With a diameter of 20 feet we are looking for the circumference. That is found with the formula C = d X pi (circumference = diameter X 3.1416). C = 62.83185. With this information we can find the number and width of the wall panels. First, for the number of walls, we divide the circumference by 4. Four feet is the “ideal” working dimension for our walls. When we divide we get 15.7. An uneven number or decimal number doesn’t work well for wall layout. We want an even number of walls, so we round up to 16. Now we divide the circumference by 16 to get walls of approximately 3.927 feet or 3 ft – 11 1/8 in. That’s a good workable number.

chordlengthLet’s step back a second. When we divide the circumference of a circle by assigning a number of sides to it not only do we get our floor panel angles at the peak and the base (more on that shortly) but we also get a dimension that makes for an approximate length for the base of the triangle, which will be our wall length. But this dimension is not the true base of the triangle. To get that you would have to use the chord length formula. There are variations on that formula depending on the conditions known. For us it is the simple one: 2 r sin (angle)/2. We have a radius of 10 feet and an angle of 22.5 degrees. 2 (10) sin 11.25 degrees = 3.9018′, or 3 feet – 10-13/16 inches. This is our most correct length for the base of our triangle and the width of our walls.

Print

The next step is to find the angles involved in making the various panels. We now have 16 sides around this yurt. In recalling our math classes we know there are 360 degrees in a circle. Dividing 360 by 16 will give us 22 1/2 degrees for the peak angle. The sum of the interior angles of a triangle is 180 degrees so that leaves us 157.5 degrees for the base angles. Since the sides are equal, than the angles are equal at 78.75 degrees. Remember, the peak angle for our yurt in the book had 12 sides and thus (360 divided by 12) 30 degrees with base angles of 75 degrees each. Some people may not like dealing with partial angles, seeking more simplicity in their work. So let’s look at this yurt with more sides, but same diameter, say 18. That would give us angles at the peak of 20 degrees and at the base 80 degrees. Using the chord formula above the wall length will be around 3.49 feet long. That will waste about 6 inches of siding plywood for each wall but is still a workable width.

With the information above we can layout the foundation, and we could finalize the math to build the floor panels. We can build the walls also. If we follow the process in the book you will see that not much changes except for the length of floor joists and later the roof framing members. Walls will stay the same height so just a width adjustment is necessary. I should say at this point that you could make the walls taller to any height chosen, which will only effect a later calculation for the placement for the skylight ring and it’s tower height. So feel free to experiment!

Following this information and combining it with what is in the book you should be able to design your own yurt of practically any size. Just remember the larger you go the more need for paying attention to the loads placed on various members.

Next time I will talk about the beams, tower and skylight, and roof panels for a larger yurt.

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Here are the links to purchase the book:

Building a Wood-Framed Panelized Yurt, in color

Building a Wood-Framed Panelized Yurt, black & white

Building a Wood-Framed Panelized Yurt: the movie

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Robin put together a movie called “How to Build a Yurt in About Five Minutes.” You can view it by clicking here!

If you’re intrigued yet need more information, you can also buy the book in either color or black and white. It’s 176 pages of step-by-step instructions with lots of photos and drawings, and also includes a materials list for a 16-foot yurt:

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Here’s the link to the color version of the book.

Here’s the link to the black and white version of the book.