WHAT WE DO...

  DESIGN

  PROJECTS

  WORKSHOPS

  CONSULTING

  TEACHING

  ARTICLES

  BOOKS

  BLOG

  CONTACT

like us on facebook to keep tabs on upcoming events or
CLICK HERE to sign up
on our workshop email list


"Be the change
you wish to see
in the world..."
            -M. Gandhi


t a k i n g   g r e e n   t o   t h e   e x t r e m e

 

BUILDING WITH COB

what is cob?
the ingredients
where to use cob
benefits
challenges
soil testing
additional resources
related articles

 

WHAT IS COB?

Clay soil provides 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, resulting in its nickname: “sculptural adobe”.  The ingredients for cob cost almost nothing, especially if clay can be dug from the building site.

Glass bottles or blocks imbed easily in the cob as the wall is constructed, and provide beautiful light glowing through the wall.  Sculpted niches provide functional and aesthetic shelves.  And benches, desks, and shelves are easily incorporated into the sculpted form.

Completed walls are plastered with clay or lime to obtain a fine surface finish.  Tiles or stones can be installed into the finish plaster to create color and aesthetic details.

 

THE INGREDIENTS

SAND
Course concrete sand and small stones serve as the aggregate in cob, similar to aggregate in concrete.  The sand provides strength and shrinkage control.  Maximizing the sand proportion in the cob mixture results in the strongest possible wall.

STRAW
The long pieces of straw also help to knit together each successive addition of wet cob to the wall.  Straw provides tensile strength for any pulling forces inside the cob wall, which provides additional resistance to cracking.

CLAY
Clay is the key ingredient that binds the sand and straw into a monolithic wall system.  Screening the clayey soil through 1/2-inch wire mesh helps eliminate larger rocks and break up clay clumps for better mixing.

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.  Dig below the top soil to explore for clay below and keep the top soil for gardening.

MIXING
Mixing the cob ingredients together can utilize extremely low-tech tools: a tarp and foot stomping power.  Mix dry sand & sifted clay together first, then add water and start stomping.  Use the tarp to roll drier mix on top of the wetter mix, keep stomping, and continue adding water until the cob is sticky but stiff.  It should look uniform and should roll into a "cob burrito" when the tarp is pulled.  At this point begin adding straw.  Use dry straw if the mixture got on the wet side; and use soaked straw if the moisture content remains low.  Add as much straw as the cob will hold, and continue to flip the pile often with the tarp.  Note that the dryer the cob mixture, the higher you can build in one day, since a wet mixture will slump under its own weight.

Machine-based mixing techniques include roto-tiller and tractor cob.  Both methods create larger batches of cob, however the mixtures tend to be less uniform and less consistent.

 

WHERE TO USE COB

High Thermal Mass Walls
Cob walls provide excellent thermal mass, which allow for absorption of heat energy.  This means in the summer time, a shaded cob wall has the capacity to absorb excess heat.  In a well-designed space, this can be used to great advantage, potentially eliminating the need for air conditioning without relinquishing comfort.  In winter time, a cob wall positioned to absorb low-angled sunlight will warm up, and release heat through the cold night-time.  The result is an equalization of temperature between day and night.

The high thermal mass properties of cob also make it ideal for use around a masonry heater or rocket stove.  High temperatures generated by the wood-burning fires are absorbed into the thermal battery and released slowly over time, extending the benefit of a fire long after it has been extinguished.

 

Cob Bake Ovens
Cob performs efficiently as a wood-burning pizza & bread oven, due to the high thermal mass properties.  To construct, build a negative mold of the interior oven space using sand, then sculpt the cob oven mass over the sand in two layers: first a straw-free thermal mass layer followed by a high straw insulating layer.  When the oven has dried for a few days, cut a door into the mass, pull out all of the sand, and plaster or finish as desired.

A fabulous one-stop resource for building a cob oven is Kiko Denzer's book Build Your Own Earth Oven.

 

Cob Hot Tub
Cob can provide heated thermal mass around a wood-fired hot tub.  Any water-holding basin can be used to form the tub container, such as an old claw-foot tub or a water trough.  The basin is raised off the ground and surrounded with cob around the exterior.  The wood fire burns below the tub to heat the water and the heated cob thermal mass keeps hot tub temperatures constant for a long time.

I recommend reading Becky Bee's book You Can Make The Best Hot Tub Ever if you plan to build a cob hot tub.

 

BENEFITS

Completely Non-Toxic
All of the ingredients used to make and finish cob are completely non-toxic.  It wouldn’t taste good, but technically you could eat them.

Local, Indigenous Materials
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
Non-loadbearing cob walls meet current building codes, as long as their substantial weight is fully supported with an appropriate foundation.  However, loadbearing cob built to support the roof structure above, is allowed only sparsely 

Labor Intensive
Building with cob requires little skill but lots of patience.  The process is time consuming and labor intensive, and progress is usually limited to approximately 30-inches high in a day.  (The cob then needs to dry overnight to be strong enough to support additional wall.)  However, the reward becomes great opportunity for sweat equity construction & cost savings.

Little or No Insulating Value
The primary ingredients in cob, clay and sand, have no insulating properties.  That is, they can absorb heat energy but they do not slow the flow of heat energy from one side of a wall to the other.  The straw content is insulating, though it provides little insulating value when encapsulated in the clay wall.  This means that the total energy to heat a cob building in a cold climate will be greater than if the walls had high insulating properties.  In a temperate climate, however, cob's high thermal mass can be a heating & cooling advantage.

 

SOIL TESTING

Worm Test
Making a worm with your soil provides a quick preliminary test to determine if any clay content exists 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 in your hand into the shape of a worm.

  4. If the worm remains intact and provides resistance to pulling apart, the soil contains clay; if it crumbles apart, the soil likely contains little or no 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.

 
 

Test Bricks
Test bricks allow you to determine the strongest 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.

 

ADDITIONAL RESOURCES

WEBSITES

The Cob Cottage Company
www.cobcottage.com
Ianto Evans & Linda Smiley's school for building with cob in Oregon

Cob on Wikipedia
en.wikipedia.org/wiki/Cob_(material)
information & photos of cob structures

Cob Projects
www.cobprojects.com
information & photos of cob structures

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

 

RELATED ARTICLES

Keeping Strawbale
Walls Dry

Tips on Building Permits for Strawbale

 

Down to Earth Design
Sigi Koko, principal
215.540.2694 PA
202.302.3055 DC
admin@buildnaturally.com

©2000 Sigi Koko & Down to Earth