Clay Building Overview

Clay soil provides a versatile building material, used for thousands of years to create  
beautiful and durable structures.  Every region of the world has an indigenous  
method for building walls of clay.  Clay materials can be formed into blocks  
(such as adobe) or built monolithically (such as rammed earth or cob).

Walls made with clay are really a mixture of two ingredients: clay &  
sand.  Coarse sand provides strength, similar to aggregate in concrete.  Clay is the  
sticky ingredient, providing glue to hold everything together.  Many types of clay  
construction contain a third ingredient: straw (or other fiber).  The long  
straw knits the wall together, providing an internal network, similar to reinforcing  
bars in concrete.

The binding nature of clay is activated by adding water.  Wet clay is sticky  
and malleable.  Once dried, the clay holds its shape and remains strong.

Clay is ubiquitous in nearly every region of the world.  Harvesting is as simple as  
digging, since clay is found in soil.  Ideal soil makeup requires a minimum of  
20% clay, with little to no organic humousy soil and less than 10% silt.  The  
techniques used to build clay walls are extremely intuitive and are  
therefore quick and easy to learn.  However, construction is slow and heavy,  
and often requires a drying period before finishing.  Careful construction timing is  

Best Use

Use clay walls to capture and store heat for later dissipation.
Clay walls provide excellent thermal mass.  Thermal mass materials are like  
a rechargeable battery for heat energy: mass absorbs heat when in excess and  
releases heat when the mass is warmer than its surroundings.  This means mass  
materials even out fluctuations in temperatures, always creating an  
average.  Well-positioned mass walls capture free solar heat in winter,  
when the sun peaks low in the sky.  Or can be used to absorb heat from a fire and  
store it for dissipation after the fire goes out.  Shaded clay walls provide free  
air conditioning, absorbing heat and humidity out of the air.
sculpted clay wall creates this winding staircase, with integral handrail
Good thermal mass properties (acting like a rechargeable battery for heat energy storage)
Simple, easy-to-learn construction techniques
Uses low-tech inexpensive tools
Can be used structurally
Inexpensive material that is ubiquitous around the world
Natural and completely biodegradable materials, that create a healthy indoor space
Absorbs excess humidity (acting like a passive dehumidifier)
Aesthetics of a thick-walled building with large widow sills
Can be used additively and subtractively
Promotes healthy indoor air quality


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cob wall that absorbs low winter sun for free heat inside (stays cool in summer)

The Ingredients

Coarse sand and small stones serve as aggregate in any clay wall, similar to  
aggregate in concrete.  Sand provides compressive strength (for pushing forces).  Sand also  
helps control shrinkage, and therefore reduces cracking.  This is because clay absorbs water  
and swells when wet, then shrinks again when dry.  Whereas sand does not change volume  
whether it is wet or dry.  Usually the proportion of sand comprises between 70% and 75% of  
the clay recipe.  Maximizing the amount of sand that still creates a dust-free clay surface  
results in the strongest possible wall.

Long pieces of straw or other plant fibers help knit the wall together, similar to  
reinforcing bars in concrete.  Fiber creates internal tensile strength (to resist pulling forces),  
which provides additional resistance to cracking as the clay dries and shrinks.  Rammed  
earth is the only clay material that never contains fiber.  The best fibers are pliable and  
have good surface texture for the clay to bond to.  The fibers remain resistant to rot once  
the wall is dry, since biodegradation requires persistent moisture.

Clay provides the key ingredient that glues the sand and straw together into a  
cohesive material.  Wetting clay creates a sticky, malleable material.  Once wet clay is  
shaped and allowed to dry, the particles remain stuck together to create a strong monolithic  
material.  Harvesting clay involves digging, since it appears as a component of soils in nearly  
every region of the world.

What's So Magical About Clay?

Clay becomes uniquely sticky when wet, and then strong and cohesive when  
dry.  The reason lies in the clay particles, that absorb water and become fat.  These fat  
particles act like suction cups that stick well to one another.  When wet, those clay particles  
also slide, which explains why wet clay malleable.   

This wet the clay, create a sticky glue, shape as desired, and then dry into a  
strong precisely what makes clay a magical ingredient in natural buildings.  This  
property allows clay to be used for so many applications: pottery, bricks, walls, floors,  
plasters, and more!!
Does not provide insulation, so should only be used where thermal mass is beneficial (often misunderstood with clay walls)
Cannot be used for exterior walls in cold climates or where insulation is required for building codes
Wet clay walls (such as cob) require substantial drying time, often months
Cannot freeze during drying time
Requires breathable finishes, usually natural plasters
Extremely heavy materials (literally tons!)
Extremely labor intensive, so can be expensive if hiring out construction labor and time-consuming if building yourself
Requires testing to determine the proper recipe for your materials (there are many types of clay)


sculpted clay bench, shaped for ergonomics


1. Clay is sticky when wet and strong
when dry.  The stickiness is what makes clay
a superior binder for clay wall materials,
plasters, paints, etc.
2. Clay materials are heavy, averaging
120 to 200 lbs. per cubic foot.  So each linear
foot of wall can weigh over 1,000 lbs.  Clay
walls require solid support, usually directly
down to the ground.
3. Clay acts as thermal mass, NOT
insulation.  So if you will heat your space,
keep clay materials as interior walls only.


All clay walls act as thermal mass,
which means the material performs like
rechargeable batteries for heat
energy.  Clay absorbs excess heat when
cooler than its surroundings, and releases
heat when it is warmer.  Adding a heat
source to the clay (from the sun or a fire)
charges the battery.  The stored heat
dissipates when the mass is warmer than the
surroundings.  Similarly, clay mass walls
absorb excess heat from air in summer,
passively cooling the surrounding air...similar
to a cave.

Clay walls do NOT act as insulation!!
(They do not prevent the flow of heat from
one side to the other.)


Cob and adobe use the same recipe: 
25% to 30% pure clay with 70% to 75% sand.  
In regions around the world that have
indigenous clay construction methods, you
also find soils that have exactly these ratios
in them.  The clay is the sticky binder and
the sand supplies strong aggregate.  Long
straw is added for tensile (pulling) strength.

The difference between cob & adobe is that
cob is sculpted in place, whereas adobe
is formed into bricks (and dried in the
sun).  They otherwise contain the same


Rammed earth walls provide the densest
type of clay wall, and thus the highest
thermal mass per thickness.  Rammed
earth walls do not contain
fiber/straw.  Instead, the mixture includes
only sand (~70%) and clay (~30%).  The
mixture of clay and sand is compacted
(rammed) into forms.  The ramming
turns the loose mixture into a dense, strong,
sedimentary stone-like material.


clay wall separating entry & kitchen
sculpted clay sink with oil sealer to waterproof
cob mass wall warmed by a masonry heater
sculpted cob niche & embedded glass block


Clay materials are inherently non-
flammable. You know how fire fighters dig
a trench around forest fires to contain them? 
They are clearing out flammable brush and
exposing non-flammable earth.  Similarly,
walls built of that same earth are also fire


One of the coolest properties of clay is the
ability to regulate humidity.  Clay is
hygroscopic, meaning it acts as a
desiccant, absorbing humidity when
surrounding air exceeds 50%.  When
humidity drops, the clay releases moisture
back into the air.

This constant absorption and release of
humidity cannot be seen.  The clay walls
never feel wet or sticky.  They just absorb
and then release humidity.

As a result, spaces with ample clay do
not experience peaks in humidity.  My
favorite example of this is taking a shower. 
When you have ample clay inside your
bathroom, you won't see fog on your mirror
after taking a hot steamy shower.  The clay
absorbs the steam before it can condense on
your mirror.  The humidity then releases
slowly the rest of the day.


Material costs for clay walls are dirt
cheap.  (Sorry, couldn't resist.)  Clay,
sand, and straw cost significantly less
than materials used in conventional
construction.  (Material costs make up
about 1/2 of conventional construction costs,
compared to only 1/4 of a natural home.) 

The main difference from conventional
construction: clay walls require a lot of
heavy labor to build.  If you are building
yourself, you save significant costs by using
clay materials.  However, labor costs can be
significant, if you hire out the construction
force (instead of building yourself).

Techniques for mixing and building are
extremely easy to learn.  Clay is highly
intuitive to work with.  The feasibility of
building high quality with little experience is
positively within reach.


Clay walls meet current U.S. building
codes...provided they are not used
where insulating walls are required.

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R-0.125 per inch of thickness
meets all fire safety codes
meets hurricane wind-load codes
inexpensive (almost free) material
high labor to build
can be monolithic or formed into bricks

use to capture and store heat energy
(such as solar heat or from a fire)

Read the glossary description here.
Made with sticky clay soils, coarse sand for strength, and, usually, agricultural fibers (such as straw) to create strong, heavy, high-thermal mass walls.  Clay walls are typically finished with vapor-permeable plasters, made from clay or lime.

Techniques are quick to learn and intuitive.  Almost no tools are needed, and most are simple and can be hand-made.

Clay walls are NOT insulating.
rammed earth wall with layers of different colored clays
pressing wet clay, sand, & straw mix into adobe molds
building with cob is intuitive, tactile, hands-on sculpting

How to test soil for clay content

Determining if your soil has clay is shockingly simple: just add some water. Really!  Remember that clay particles  
become fat and sticky when they absorb water and swell.  This means you can take a handful of clay, add water, knead it in  
your hands for a bit and then observe if the mixture becomes sticky.  Wet sand will not become sticky.  Wet organic gardening  
soil will not become sticky.  Only clayey soils become sticky by adding water.  It's that simple.  So there you have it, a  
low-tech, 30 second test to determine if your soil contains sticky clay.

If you want to get more scientific and determine what else your soil contains (and in what proportions), another simple test  
separates the soil components into layers.  This test is commonly called the "shake test" because it involves adding a soil sample  
to a straight-sided jar, adding water, and shaking.  The agitation washes any sand aggregates clean of the smaller clay particles.   
The heavy sand then sinks to the bottom of the jar, while the clay remains suspended in the water for a longer period of time.   
Because the settling times are different for each component of the soil, they settle in layers...allowing you to measure the  
proportion of each in your soil.

Recommended Reads

Here are my favorite books about clay construction (click the cover to purchase)

If you are looking for one book on cob wall construction, this is my favorite. Detailed. Step-by-step photos. And lots of construction tips.

Buy this book

Great book on rammed earth construction, including info on how to test materials, create a strong recipe, and build formwork.

Buy this book

This book explores an intuitive connection with cob construction, including design considerations, construction tips, and many sketches.

Buy this book

A great way to test out clay construction is to tackle a small a clay oven! And this how-to book explains all you need to know.

Buy this book
If you are looking for one book on cob wall construction, this is my favorite.  Detailed.  Step-by-step photos.  And lots of construction tips.
Great book on rammed earth construction, including info on how to test materials, create a strong recipe, and build formwork.
This book explores an intuitive connection with cob construction, including design considerations, construction tips, and many sketches.
A great way to test out clay construction is to tackle a small a clay oven!  And this how-to book explains all  you need to know.

Start-to-finish guide to preparing, installing, and finishing a beautiful floor made from clay

Buy this book
Start-to-finish guide to preparing, installing, and finishing a beautiful floor made from clay...the finishing touch for your strawbale home.
Watch a demonstration of testing clay soil here:
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