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BUILDING WITH 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
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.
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.
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 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
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 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.
techniques include roto-tiller and tractor cob. Both methods create
larger batches of cob, however the mixtures tend to be less uniform and
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.
All of the ingredients used to make and finish cob are completely
non-toxic. It wouldn’t taste good, but technically you could eat
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.
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
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.
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.
Take a small handful of soil to test and remove any
Mix in a small amount of water, just enough to make
the soil malleable.
Roll the soil in your hand into the shape of a
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.
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
Fill approximately ¼ of a cylindrical-shaped glass
jar with crumbled soil (free of visible stones).
Fill to the top with water, close the lid, and
shake well, until all of the clay is dispersed.
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.
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 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.
The Cob Cottage Company
Ianto Evans & Linda Smiley's school for building with cob in Oregon
Cob on Wikipedia
information & photos of cob structures
information & photos of cob structures
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
Tips on Building Permits for Strawbale
Down to Earth Design
Sigi Koko, principal
©2000 Sigi Koko & Down to Earth