Down to Earth Design

Sigi Koko. Principal admin @ buildnaturally.com

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www.buildnaturally.com

Down to Earth Design

COB

WWW.BUILDNATURALLY.COM

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WHAT IS COB? Clay soil offers a versatile building material used for thousands of years to create beautiful and durable structures. Clay can be formed into building blocks (such as adobe) or monolithic walls (such as rammed earth and cob). Cob walls combine the same ingredients as adobe sun-baked bricks – clay, sand, and straw – but cob is sculpted in place when wet, thus is familiarly referred to as “sculptural adobe”. Completed walls are plastered with clay or lime to obtain a fine surface finish. The ingredients for cob cost almost nothing, especially if clay can be dug from the building site.

THE INGREDIENTS 1. sand: aggregate, provides strength and shrinkage

control; use course sand, such as “concrete sand” 2. straw: provides tensile strength and helps minimize

cracking; straw should not be chopped 3. clay: binder that holds all of the ingredients

together

Do not use top soil! Organic top soil adds inert fill into the cob with no benefit. Top soil shrinks over time, potentially creating voids in the finished wall. Remove top soil to stockpile for gardening & look for clay below.

BENEFITS

Completely non-toxic. All of the ingredients used to make and finish cob are completely non-toxic. It would not taste good, but technically you could eat it.

Local, Indigenous. Often the soil dug from foundation excavation contains sandy clay and can be used to build with. If you do not find clay soil locally, dry bagged clay works as well, however it should hydrate for several days to make the clay sticky enough.

High Thermal Mass. Cob walls have high thermal mass that can store the sun's heat energy in passive solar design and provide thermal storage for a masonry heater. Fully shaded cob provides free cooling in summer by storing cool energy and absorbing humidity.

Low-Tech & Easy to Learn. Techniques for mixing

and building with cob are extremely easy to learn and fun. Tools needed are few and inexpensive: shovels, tarp, and buckets.

CHALLENGES • building code compliance (if using cob structurally)

• extremely time & labor intensive

• only appropriate where high thermal mass, non-insulating wall provides the best energy efficiency (difficult to heat otherwise)

TESTING SOILS FOR CLAY

WORM TEST Making a worm with your soil provides a quick preliminary test to determine any clay content in your soil. It works because clay is uniquely sticky when wet.

1. Take a small handful of soil to test and remove any visible rocks.

2. Mix in a small amount of water, just enough to make the soil malleable.

3. Roll the soil into the shape of a worm. 4. If the worm remains intact and

provides resistance to pulling apart, the soil contains clay

SHAKE TEST A simple shake test determines relative percentages of clay

and sand contained in the soil. It works because clay remains suspended in water, whereas sand and silt sink in water.

1. Fill approximately ¼ of a cylindrical-shaped glass jar with crumbled soil (free of visible stones).

2. Fill to the top with water, close the lid, and shake well, until all of the clay is dispersed.

3. Set the jar down on a level surface and watch for 10 seconds. All of the sandy solids will settle to the bottom. Draw a line on the jar at the top of the sand. The water remains cloudy with clay.

4. When the water becomes completely clear, draw another line at the top of the settled clay. The ratio between the height of the sand and the height of the clay represents the ratio of sand to clay in the soil. Note: it is difficult to differentiate silt in this test, as silt is similar to sand, only smaller and spherical.

MAKE TEST BRICKS Test bricks allow you to determine the best proportions of clay and sand for your soil. The strongest cob contains the maximum amount of sand, while still having enough clay to provide excellent binding. Not all clay is equally “sticky”, so different clays allow more or less sand. Begin with a brick made with 100% clayey soil, then make bricks with increasing amounts of sand until it is clear that there is not enough clay in the mixture (you can no longer keep the cob intact). Write the proportions directly on each brick. Do not add straw to the test bricks. Once the bricks are completely dry, drop them from shoulder height, starting with the sandiest brick. The first brick that does not break is your ideal proportion of soil to sand.

RESOURCES WEBSITES www.cobcottage.com www.cobprojects.info en.wikipedia.org/wiki/Cob_(material) www.buildnaturally.com/EDucate/Articles/Cob.htm

BOOKS The Hand Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans and Linda Smiley

The Cobber’s Companion: How to Build Your Own Earthen Home by Michael G. Smith

The Cob Builders Handbook: You Can Hand-Sculpt Your Own Home by Becky Bee

Building with Cob: a step-by-step guide by Adam Weismann and Katy Bryce

Building with Earth: A Handbook by John Norton

Build Your Own Earth Oven by Kiko Denzer

You Can Make The Best Hot Tub Ever by Becky Bee

HANDS-ON WORKSHOPS

Down to Earth Design teaches workshops on

building with cob, where we build walls, surround masonry heaters, or construct a cob oven or hot tub.

WORKSHOPS INCLUDE:

• overview of cob construction, including ingredients & properties of cob

• demonstrations of how to test soils for clay content and mixing techniques

• guided hands-on experience mixing & building with cob, including how to trim walls, prep for plaster, create niches, and more

Check our website www.buildnaturally.com for additional information.