Building a Cob House to Code in the United States

Leave the cob in the box to dry completely, a process that will take a couple of days to weeks, depending on ambient temperature and humidity. If you make several samples, you can break one open to determine whether it’s dry all the way through; if you see a darker color in the middle, it isn’t dry yet. Slow drying (in the shade rather than full sun) and using a stiffer (less wet) cob mix may reduce shrinkage. If you’re in a hurry, you can put the cob block in the oven to speed the drying process after it becomes firm enough to remove from the box without deflection.

All you’ll need to demonstrate to meet the code’s requirements is that the cob block shrinks no more than 1 inch out of its 24-inch length. This is a low bar, and any decent cob mix should easily meet it. The test simply ensures that cracking due to shrinkage will not be severe enough to cause structural weakness in the cob wall.

Building a Cob House to Code: Comprehensive Strength Testing

Any to-code cob project will also require a determination of the mix’s compressive strength: a measurement of the material’s capacity to support weight (including the weight of the rest of the cob higher up in the wall and of the roof) without breaking. All cob walls are required by the code to have a minimum compressive strength of 60 psi (pounds per square inch). If the cob wall will serve as a shear panel to resist loads, such as high winds or earthquakes, the compressive strength must be 85 psi or greater.

Note that these requirements are fairly easy to meet; good cob mixes often have compressive strength values over 150 psi. But since the actual compressive strength of the mix, as determined by testing, will be used to ensure the cob walls are capable of supporting loads, such as the weight of the roof, snow, and so on, this testing should be done early in the design process.

To meet the code’s requirement for compressive strength testing, five samples of the cob mixture, 4-by-4-by-8 inches tall, must be made and dried without using an oven, which can artificially increase the strength of the samples. The easiest way to make these blocks is by packing cob into a wooden form. Avoid large pieces of straw, and pack the cob tightly into the form to prevent seams and voids. For best results, build the samples in the same orientation they’re to be tested in, and handle them with extreme care.

When the blocks are dry, coat the two 4-by-4-inch faces of each one with a thin layer of plaster of Paris to make them completely flat and parallel. (Most labs will do this step for you.) These samples can then be submitted to a laboratory for testing. Counting from highest to lowest, the fourth of the five values determines the official compressive strength of the mix. Note that the first version of “Appendix AU” specified that the samples should measure 4-by-4-by-4 inches high. Research conducted at the University of San Francisco by Hana Mori Böttger and the CRI after the code was written concluded that this sample geometry is not ideal. Cube-shaped samples turn out to yield artificially high compressive strength measurements. When the test blocks are prisms twice as high as they are wide, more accurate results are obtained. The language of the code is being amended in the current code cycle to reflect this discovery.

Laboratory testing can be expensive. Depending where in the country it’s located, a laboratory might charge as much as $300 per sample, which would mean a minimum outlay of $1,500 for compressive strength testing of one cob mix. Luckily, the code also allows for on-site testing by the builder, if the building official agrees. Even if your building official insists on laboratory testing, you may still want to pre-test your mix to be certain it will meet the strength requirements. That way, you can adjust your mix if necessary by using a different clay source or different ratios of clay to sand to maximize its strength before submitting samples to the lab.

Perform Your Own Comprehensive Strength Test

Here’s how you can conduct your own DIY compressive strength testing:

1. Because the standard cob block is 4-by-4 inches when viewed from the top, the block has a cross-sectional area of 16 square inches and must support a minimum of 960 pounds. Testing blocks on-site presents three main challenges: applying force to the test block, distributing that force evenly across the surface of the block, and measuring how much force is being applied when the block fails.
2. Stacking 960 pounds of weight on top of a block would be dangerous; when the block breaks, all of that weight would fall. The safer approach is to use a long (and strong) lever to apply the force, a piece of plywood to distribute the load to the top of the specimen, and a round bar to accurately measure the length of the lever and apply force to the center of the cob block.
3. Be sure to secure the stationary end of the lever under something heavy; that end of the lever will push up on the restraint with over 1,000 pounds of force if the cob meets the standard. Some objects that could provide sufficient anchoring weight include the bucket or structural beam of a tractor, the tow hitch of a fully loaded truck, or a portion of cob wall specifically designed to withstand and distribute these upward loads.
4. To measure the force applied to the block, hang a bucket securely from the end of the lever and slowly fill it with sand. Add the sand gradually while another person watches for cracks in the test block. When the block has broken, the bucket will droop or land on the ground.
5. After the block has cracked or compressed, stop adding more sand. Weigh the sand and bucket to determine the force applied to the lever. The mechanical advantage is found by dividing Length A by Length B in the figure. The equation for how much force was applied to the test block can be written: F = weight × A ÷ B. To calculate the pressure on a standard 4-by-4-inch test block, the equation would be PSI = (A ÷ B) × weight ÷ 16. Lengths are assumed to be in inches and weight in pounds.

How to Make Cob for Shear Panels: Testing Modulus of Rupture

One final test is required by the code, but only for cob that will be used in shear panels. These cob mixes need a demonstrated modulus of rupture of 50 psi or greater. Modulus of rupture, also known as “flexural strength,” is a measurement of a material’s capacity to withstand bending forces without cracking. The samples for this test must be 6 inches wide by 12 inches long by 6 inches high. Again, top and bottom surfaces (two of the larger faces of the prism) should be coated with plaster of Paris after the samples are dry.

Five samples must be submitted to a testing laboratory (unless the building official agrees to on-site testing) and the fourth-highest value must exceed 50 psi.

The same testing lever described above can be used to test modulus of rupture with only minor changes to how the test block is supported. Rather than placing the block on a flat, hard surface, support it on two metal pipes or other strong cylinders. Measure the distance between these cylinders for use in the calculation later. Apply the load to a third cylinder on top of the block, placed halfway between the two supporting pipes. The force applied to the test block can be found in the same way as in the compression testing.

To calculate modulus of rupture, the equation is:

MOR = (3 × force × L) ÷ (2 × W × t²), where “t” and “L” are shown on the figure and “W” is the width of the specimen.

For the standard 6-by-6-by-12-inch test block, this reduces to:

MOR = (3 × Force × L) ÷ 432

Again, lengths are assumed to be measured in inches and force measured in pounds. Setting up your testing apparatus may be a challenge, but no more so than many other stages of the building process.

If you want support building a cob house to code, feel free to email the Cob Research Institute at Info@CobCode.org. CRI would love to hear about your experience; sharing your test results, obstacles, and successes will help facilitate the process for other cob builders.

Flickr images courtesy Creative Commons license CC BY-SA 2.0.

Michael G. Smith helped found the Cob Cottage Company and the Natural Building Colloquium. Over the past 30 years, he has taught hundreds of natural building workshops. He’s the co-author of The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage and The Art of Natural Building. He’s also a founding director of the Cob Research Institute. Learn more at Straw Clay Wood.