EARTH CONSTRUCTION
Rammed earth is essentially manmade sedimentary rock. Rather than
being compressed for thousands of years under deep layers of soil, it is
formed in minutes by mechanically compacting properly prepared dirt. The
compaction may be done manually with a hammer-like device,
mechanically with a lever-operated brick-making press, or pneumatically
with an air-driven tamping tool. Dynamic compaction using manual or
power tampers not only compresses the soil, but it also vibrates the
individual dirt particles, shifting them into the most tightly packed
arrangement possible. When finished, rammed earth is about as strong as
concrete.
Houses built of rammed earth have several advantages over wood-frame
construction. The walls are fireproof, rot resistant, and impervious to
termites. The solid, 18-24 in (45.72-60.96 cm) thick walls are nearly
soundproof. The massive walls help maintain a comfortable temperature
within the house, damping temperature swings that normally occur on hot
summer days or cold winter nights. When designed and oriented to take the
best advantage of solar energy, a rammed earth house can be comfortable
with 80% less energy consumption than a wood-frame house. On the other
hand, initial construction is about 5% more expensive than wood-frame
construction because it is very labor intensive.
History
Humankind has used the earth itself to build homes and other structures
for thousands of years. Jericho, the earliest city recorded in history, was
built of earth. Temples, mosques, and churches were built of mud bricks
and rammed earth throughout the ancient Middle East. Egyptian pharaohs
ruled cities constructed of rammed earth. In the Far East, the technique
was used not just for houses, but even for building ancient forerunners of
the Great Wall of China. Romans and Phoenicians brought the technology
to Europe, where it was used for more than 2,000 years.
In the United States, rammed earth construction enjoyed a period of
popularity from 1780 until about 1850, when mass-produced fired bricks
and sawed lumber became readily available. Houses could be built more
quickly and easily with bricks and lumber, which were considered more
modern and elegant materials than dirt. The supply shortages experienced
after World War I and during the Great Depression brought rammed earth
construction back into favor for two decades. Frank Lloyd Wright designed
houses to be made of rammed earth.
When World War II ended, the country faced a large demand for housing,
and wartime factories turned to manufacturing building materials that
could be used for quicker types of construction. Rammed earth was
brushed aside until it was repopularized during the environmentally
conscious 1970s. A modified version of the technique, invented by Michael
Reynolds, uses building blocks of discarded automobile tires rammed full of
earth. These houses not only keep used tires out of landfills, but they can be
built by inexperienced, first-time builders. When a homeowner uses the
unpaid labor of himself, relatives, and friends, and when he can obtain
many of the building materials for free, the construction cost can be held to
less than half that of a wood-frame house.
For thousands of years, rammed earth construction was taught personally
by one generation of builders to the next. In early twentieth-
century America, such a network of experienced builders did not exist. The
U.S. Department of Agriculture developed and published a manual
titled Rammed Earth Walls for Buildings that showed people how to build
their own homes. Research projects designed to improve the methods and
quality of rammed earth construction were reported in academic journals,
and more than 100 articles on rammed earth appeared in trade and popular
magazines from 1926 to 1950.
Several more recent innovations increase the speed and ease of
construction and enhance the structural integrity of the final product. For
example, pneumatic tampers can be used to compact soil much more
quickly than the traditional manual method. Easy-to-assemble forms allow
walls to be built as solid panels rather than building them up as successive
layers a foot or two at a time. Using time-honored manual methods, a four-
person crew can erect 40-50 sq ft (12.19-15.24 sq m) of rammed earth wall
per day; with power tools, the same crew can construct 300 sq ft (91.44 sq
m) per day. David Easton, founder of Rammed Earth Works, has developed
several earthquake-resistant designs to reinforce and structurally integrate
the walls; the choice of design alternative depends on several
considerations, including the distance to the nearest seismic fault.
Raw Materials
As the name implies, the primary material used in rammed earth
construction is the earth itself. There are five basic types of soil (gravel,
sand, silt, clay, and organic), and the dirt in a given location is generally
some combination of all or most of these types. Historically, the longest
lasting rammed earth walls were made of soil that was 70% sand and 30%
clay. The soil from a new building site is tested to determine its suitability.
Organic material must be removed from the soil and, if necessary, a
different type of soil can be trucked in and mixed with the existing dirt to
create a blend that will work. Cement may be added to the soil to increase
both its strength and its resistance to moisture—usually at about one-fourth
the ratio that would be used to make concrete.
Steel reinforcing bars are placed in the foundations and sometimes in the
walls. Plywood is used to make the removable forms for standard rammed
earth construction. Sheets of three-quarter-inch (1.9 cm) plywood are thick
enough. High-density-overlay (HDO) panels, which have a thin, plastic
coating on one side, work especially well because they release more easily
from the wall after construction. This not only leaves a clean finish on the
just-completed wall, but it leaves the form boards in good condition to be
used on future projects.
Rammed-earth tire construction uses discarded automobile tires,
aluminum cans, and cardboard in addition to compacted soil. About 1,000
tires are used to build the walls of a 2,000 sq ft (609.6 sq m) house.
Design
Rammed earth houses are custom designed to make the most energy-
efficient use of the site. They can be successfully designed for many climate
regions, including humid areas with cold winters. The size and placement of
windows is an important factor in taking advantage of solar heating in the
winter and cooling breezes in the summer. The house can be positioned to
take advantage of hills that offer protection from storms. Shade trees
or trellised vines offer relief from summer heat but admit warm sunlight in
winter.
The Manufacturing
Process
Rammed earth houses can be built in one of three basic ways. Individual,
rammed earth bricks can be formed and used with standard building
techniques; in fact, such bricks may be used to form the floors in a rammed
earth house built with other techniques. Standard rammed earth
construction involves erecting wood forms and compacting
Standard rammed earth construction involves erecting wood forms and
compacting the prepared soil into these molds, which are removed after the
walls are completed.
In the tire method, a row of used automobile tires is simply laid atop the
concrete footing, perhaps centered around steel reinforcing bars that
extend out of the footing. The tires are then filled with soil. About 1,000
tires are used to build the walls of a 2,000 sq ft 1609.6 sq m) house. the prepared soil into these molds, which are removed after the walls are completed. The rammed-earth tire method is a commonly used alternative. The descriptions that follow are overviews of the standard and tire methods.
Preparing the site
1 An inch or two (2.5-5 cm) of topsoil is removed from the building
site and stored so it can be replaced around the completed structure.
Organic matter such as weeds and roots are removed and may be
composted for use in post-construction landscaping. After the site is
cleared, the outline of the house is staked out. The soil is excavated to
a depth that guarantees a level surface; the excavation includes the
floor area of the building as well as a 3 ft (1 m) surrounding buffer
zone. A trench may be dug so that the walls will be anchored into the
ground to a depth below the winter freezing line.
Laying the foundation
2 The foundation, which is made of reinforced concrete, consists of a
footing that may be as narrow as the thickness of the wall or up to
three times that thickness, depending on the strength of the
underlying soil. The footing is extended above ground level to form a
short "stem wall" that will connect the rammed earth walls to the
footing. Depending on the architectural design, a slab floor may also
be poured.
Analyzing the soil
3 A variety of tests are conducted to determine the suitability of the
local soil for construction material. For example, a particle
determination test reveals the relative proportions of sand and silt in
the sample. A compaction test is performed by forming a ball of mud
and dropping it from a height of 3 ft (1 m); the degree to which the
ball disintegrates on impact reveals its usefulness for building. Other,
more precise, tests can be performed at a geotechnical laboratory. If
the native soil is unsuitable or inadequate for building, it can be
blended with or replaced by soil from another source. Soil may be
purchased from a quarry, or it might be available as refuse from a
nearby construction site, in which case it could be delivered free or at
a minimal cost.
Framing the walls
4 Traditionally, wood forms were used to build up walls 2 ft (0.6 m)
at a time. After the mold was filled with fully compacted soil, it would
be removed and reset to form the next section of wall. More efficient
methods now allow forms to be constructed for the entire height of
the wall (even more than one story). Horizontally, the framework
may form the complete length of wall, or it may form shorter panels
[e.g., 8 ft (2.44 m) long] separated by 6 in (15 cm) gaps that can be
filled with reinforced concrete for enhanced structural strength.
Framing is a major component of the construction process, in terms
of both importance and time; it usually takes less time to fill and
compact the soil within the forms than it does to set, align, and
remove the framework.
In the case of rammed-earth tire construction, wood forms are not
used. A row of used automobile tires is simply laid atop the concrete
footing, perhaps centered around steel reinforcing bars that extend
out of the footing. After each layer of tires has been filled and
compacted, another layer will be added, offset by half the tire
diameter from the layer below.
Tamping the soil
5 Traditional tampers are made of a heavy wooden block with a
handle extending upward through its center. A more compact version
can be made from a 4 in (10 cm) square steel plate welded to a section
of 1 in (2.5 cm) pipe. A 4-6 in (10-15 cm) layer of moistened soil is
placed inside the form, and a worker drops the tamper from a height
of 12-18 in (30-46 cm). In fact, most of the work is now done quickly
with pneumatic tampers, and manual devices are used only in tight
spaces around electrical boxes or plumbing pipes. After many
repetitions with the tamper over the entire surface of the layer, the
noise made by the impacting tamper changes from a dull thud to a
ringing sound. This happens when the soil has been compacted to
about half of its original volume. At this point, another layer of
prepared soil is added, and the tamping process is repeated. When
the tamping is finished, the wood forms are removed.
The tamping process is different when tires are used as the
framework. In this case, a sheet of cardboard is placed across the
bottom of the hole in the tire, and moistened soil is shoveled into the
tire. The dirt is packed by hand into the interior of the tire, and then
it is compacted by repeated blows with a sledge hammer. Using this
technique, about three wheelbarrow loads (350 lb or 158.9 kg) of soil
can be packed into each tire. Pounding the dirt causes the tire's walls
to bulge, interlocking the tire to the row below. As additional layers
are added and the wall becomes taller, scaffolding must be
constructed so workers have a place to stand while filling and
pounding the tires.
Finishing the walls
6 Interior faces of walls are often finished with plaster. If such a
coating is not applied, the wall should be treated with a clear,
penetrating sealant to prevent dust from sloughing off. Because stone
(even when manmade from rammed earth) is somewhat porous, it
may be necessary to apply sealant to weatherproof the exterior faces
of the walls in certain climate areas.
Rammed earth tire walls are finished by inserting aluminum cans
into gaps between the tires and filling remaining voids with adobe
(straw-reinforced mud). Earth is packed against the exterior face of
the wall, creating a flat surface that completely conceals the tires.
Wall interiors are finished with 2-4 in (5-10 cm) of plaster or stucco.
Byproducts/Waste
Rammed earth structures use natural resources efficiently. Those made of
packed tires even make productive use of some of society's trash. Because
the tires are sealed within 3 ft (0.9 m) thick walls, neither oxygen nor the
sun's ultraviolet rays can react with them. This means they cannot catch fire
and they do not release toxic chemicals. The structures qualify for better
fire ratings than wood-frame buildings, and they do not smell of rubber.
The Future
During the late 1700s, a French builder named Francois Cointeraux
founded a school in Paris to study and publicize rammed earth
construction, which he called pise' de terre (puddled clay of earth). Today,
David Easton has developed a new version of rammed earth construction
he calls PISE (Pneumatically Impacted Stabilized Earth). It involves
spraying the prepared soil under high pressure against a one-sided form.
This technique can produce 1,200 sq ft (365.76 sq m) of 18 in (45.72 cm)
thick wall per day, which is four times faster than a typical, four-person
crew can fill box-like forms and compact earth with power tampers.
Where to Learn More
Books
Easton, David. The Rammed Earth House. Chelsea Green Publishing
Company, 1996.
McHenry, Paul Graham. Adobe and Rammed Earth Buildings: Design and
Construction. University of Arizona Press, 1989.
Periodicals
"Rammed Earth Construction." Countryside & Small Stock
Journal, March/April 1992, pp. 32-33.
Other
"Rammed-Earth Tire Homes" February 9,
1997. http://monticello.avenue.gen.va.us/Community/Environment/Yello
wMtn/men .html (May 22,1997).
— Loretta Hall
Read more: http://www.madehow.com/Volume-3/Rammed-Earth-
Construction.html#ixzz39P9ZJTiQ
What is Cob?
Cob is earth used as a building material. Straw is mixed in with it to improve its
strength.
If your soil is too sandy, you add a little clay. If your soil has too much clay in it,
you add a little sand. If you are lucky, your earth may be just right. We call this
‘ready-mix’.
Can I Use the Soil from my Garden?
You don’t use any old soil. You discard the top soil, the stuff that gardeners
love. It is full of decomposed leaves and micro-organisms. You are after the soil
below that, the sub-soil, the inorganic material. Nobody wants that. But it is
exactly what you need. One man’s junk is another man’s treasure.
You need to dig up the earth to lay your foundations. Instead of dumping it,
you will turn that earth into the building that will sit on top of those
foundations. That’s pretty amazing.
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How do I Mix in the Straw?
You have your supply of sub-soil with the right balance of sand and clay. Now it
is time to mix in your straw. You can do this by dancing on top of it, by letting
livestock trample on it, or by mixing it using various basic machines. Using
livestock to trample the cob overnight is a very ancient (and sensible) method.
Animal dung can improve cob’s workability, but be careful of this approach if
you live in a country with dung beetles; I saw one destroy a mud-dung floor in
Oregon. It looked like a tiny mole had crisscrossed over the entire surface
(being cob, though, it was very easily repaired).
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What Happens Next?
You build.
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So Cob is Environmentally-Friendly?
From an environmentally-friendly point of view, it does not get much better.
You source your raw material yourself, right where you want to build. No tree
is cut down, no rock is quarried, no metal is mined, no oil is extracted.
Your raw material does not require any melting or heating at high
temperatures, or the addition of any chemicals or massive quantities of water,
to turn it into a building material.
There is no need for transport from the forest/quarry/mine/rig to the factory;
no transport from the factory to the builders’ providers; no transport from the
builders’ providers to your site.
Cob is not toxic. It will not harm your health if you live a lifetime within earthen
walls.
In generations to come, if your home is no longer occupied, cob will eventually
disintegrate back into the earth. It will leave no trace. That is a pretty special
quality in a building material and often all too rare. When checking the 'green-
ness' of a building material, you need to think about its whole life cycle - from
the cradle-to-the-grave. If you would like to read a bit more about this, please
click here.
Cob goes even one step further. It is perpetually re-usable. This is known as
cradle-to-cradle. When a material has finshed its first tour of duty, it can be
used a second time. In the case of cob, it can be used a third, fourth, fifth time.
By adding enough water to a cob wall, you can actually re-sculpt it. If you
knock a cob wall down (for an extension for example), you can simply add
water and use that original cob to build your new wall.
I visited a 150 year old Irish cob cottage in 2009. The wall had been severely
damaged due to the application of cement render in the 1950s. The owner had
the foresight to gather up and store the crumbling cob. The wall could be
rebuilt using the original material. Isn't that wonderful?
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What Does it Cost?
It is free, except for the labour you put in. You do put in a lot of labour.
Be aware too, that you will need to roof your cob home, have built-in
furniture, etc. It is more time-consuming and trickier to add these elements to
a curved building. If you need to pay trades people to help you with these
aspects, it will be more expensive than if they were fitting out a rectilinear
house. There are tricks that we can teach you to minimise this kind of
expensive input. Check out our courses.
At the end of the day, your cob home should not cost any more than a
standard-built house. It is possible to build it cheaper than a standard house.
We certainly did. At the end though, you will not have a standard house. You
will have created something beautiful, sculptural, personal and enduring.
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Can Anybody Build with Cob?
Anyone can build with cob, small children and grandparents alike. Personally, I
am nervous of power tools. There is no need for them when cobbing. When a
batch of cob is made, traditionally it is rolled into ‘loaves’. You don’t need
strength to carry it from A to B. You size the loaf, or cob, to your ability. Each
loaf gets worked into the monolith of cob below.
If working on a larger scale, you can pitchfork or shovel cob into place and
work it in with simple wooden tools. Whether working with small cobs, or
larger shovelfuls, this is building at a human scale (something which is absent
from so many contemporary buildings, with all of their components craned
into position).
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Can I Form Organic Shapes with Cob?
Cob is sculptural. You can curve it. You can carve into it. You can add on to it. It
can be reworked at any stage in its life. Cob works really well in curves. A
curved cob wall is actually stronger than a straight one, as it becomes self-
buttressing; it supports itself. This opens up so many possibilities for a
completely individual building, full of personality and free from the 1.2m x 2.4
(4ft. x 8ft.) module dictated by so many modern building materials.
Curved or non-uniform rooms feel good. There is a theory that as humankind
evolved in nature, we can only be truly comfortable when we are in ‘natural’
environments. The box-shaped rooms that most people live in nowadays are,
in fact, alien to us. We are not meant to spend so much time around so many
straight lines. Modular, straight building components are the result of
mechanisation for mass production. They are driven by profit margins and
convenience, not by any regard for the health or well-being of the future
occupants of these buildings. A curved or randomly shaped room feels like it is
embracing you. There are no dead corners. There is a flow. It is a pleasure to
spend time there.
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A Feast for All the Senses?
Architecture should be about all the senses. Too often, architecture is purely
visual. Cob buildings are extremly tactile. The first thing that people do when
they visit our house is carress it. They touch the walls, they follow the line of
the curves with their hands. Visitors are surprised at how much they want to
do it, but everbody does it, spontaneously and without exception.
The mass of earth and non-uniform shapes of cob also allow for wonderful,
gentle acoustics - that's the sense of hearing covered. Cob buildings do not
really smell - perhaps the slightest hint of earth - an outdoors, natural smell.
There is a definite absence of chemical smells. Cob buildings look good enough
to eat. Geophagy, or 'eating clay/dirt/earth', happens worldwide for cultural,
dietry and/or medicinal purposes. I wouldn't recommend it, but maybe cob
can really claim to satisfy all of the senses.
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I Want to Get Back to a Sense of Craftmanship
Cob is labour intensive, but it is extremely satisfying. As there is a slow food
movement, so cob belongs to the slow build movement. Cob-builders are not
getting a consumer product instantly off the shelf. They are spending time,
crafting their building, taking great pride in their work. The building process
may take longer than usual, but the legacy will last for generations.
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Are There Any Special Considerations Working with Cob?
Cob buildings are built as monoliths – huge, thick, solid walls. Traditionally they
were built approximately 600mm (2 ft.) wide, up to a storey-and-a-half or
more.
They need ‘a good hat and good boots’, large overhangs and stone or block
plinths to minimise the amount of rain reaching the cob walls. They need to be
finished with compatible materials, which will protect the cob but also allow it
to breathe.
If you maintain your cob building well, such as re-newing your external
limewash every few years, it will last for generations.
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What About Insulation?
Cob does not have a "good" u-value. The current building regulations in Ireland
require that the maximum u-value for a wall should be 0.21 W/m²K. A 600mm
wide cob wall will only achieve values from approxiamtely 0.4 to 0.65 W/m²K,
pretty far off the mark. This implies that cob walls need to be heavily insulated
to achieve current standards of thermal comfort, but our experience begs to
differ.
The building regulations do not take into account cob’s excellent thermal mass
properties (the walls act like slow release storage heaters), its monolithic
nature (which has a effect on how heat passes through the wall), its thermal
inertia properties and the fact that the mass of earth in the walls is a huge
reservoir for water vapour (reducing humidity levels in a room, which allows
you to feel more comfortable at lower temperatures). These qualities need to
be investigated and measured further if cob is to be accepted as a viable
modern building material.
In our house, we compensated for cob’s alleged inadequate insulation
capability by super-insulating everything else – the floor, the roof and the two
coldest walls of our home (north and east – these are timber-frame, with
straw-bales for insulation and cobbed up the inside with 100mm (4”) of cob).
In the winter, our house is toasty with only a 10kW turf-and-wood-stove to
heat 130m² of floor area, as well as our hot water. We have no back-up heating
system (we have solar water panels for our summer hot water). I cannot feel
any difference between the rooms which are entirely surrounded by cob
compared with the rooms which are partially or totally super-insulated.
The problem of straight-forward compliance with the building regulations will
be an issue for anyone wanting to take on a cob building. If you want to read
more about this, please click here.
COB CONSTRUCTION
A building needs a strong foundation to rest upon. This is the first detail to consider when building any structure.
It should be a unified and stable base for your building to sit upon, and must also support the load of the building.
It was our goal during Week One of Aprovecho’s Sustainable Shelter Workshop Series (www.aprovecho.net) to
construct a stone foundation for the building we would be working on for the next 7 weeks.
Alan Ash, a master stone mason (thestonemason.com), was brought in to instruct and guide the foundation
building process. Alan has been working with stone for 30 years and had a huge wealth of knowledge and
experience to pass along to us throughout the week. Alan is a real character, and a joy to work with! He knows
how to get’er done! And as he’ll say, “Damn, I’m good!” He really IS damn good!
The foundation was done with mostly dry stacked stone (local Oregon basalt), and mortared on each pier with
lime mortar. A clay/lime mortar was used between the other stones that were above grade. With dry stone
masonry, gravity, friction, and the skill of the worker are what holds your stone work together.
I thought, rather than writing out big long paragraphs of what we did, here is a basic step-by-step process of what
we did to build our foundation. If you have questions about any steps in the process, please leave a comment
below.
1.SitePreparation
Clear a level pad for your building. Make it at least 3 feet bigger than the size of your building on all
sides so that you have room to maneuver about the site.
Set up batter boards. These allow you to run string lines for the outside and inside of your foundation
trench. Always use a line level to level these strings.
Make sure that your foundation is “square” by using the Pythagorean theorem.
Dig your foundation trench down to the frost line (depends on where you are). Make the sides of your
trench vertical, square on the sides, and tamp down the bottom.
Determine your drainage point, and make the bottom of your trench slope down to where the water will
drain out. Standard good practice is to have it slope down ¼ inch per foot (1 inch per 4 feet). This is
pretty slight. Use either a transit or a water level to do this part. The Egyptians used water levels to level
the pyramids so don’t be turned off by this ancient tool. It works just as well as a transit for this job. Just
make sure that there are no bubbles in it.
2.RubbleTrenchandDrainagePipe
There are a couple different ways to create a rubble trench for your foundation. I have a more detailed guide in
my eBook“Build a Cob House: A Step-By-Step Guide.”
Here are the basics though:
Lay down a few inches of gravel on the bottom of your trench and tamp it down. Use drain-grade gravel.
Use landscaping fabric to cover the bottom and the sides of the rest of the trench surfaces. Leave
enough on top so that you can wrap it back over the top when you’re done.
You typically want to start your stone foundation or stem wall about 6” below grade. So measure down
from the top of your trench about 6”. This will be where your rubble trench ends.
Fill about 1/3 of your trench with drainage gravel again. Now filling over the top of your landscaping
fabric.
Lay in your perforated drainage pipe on top of this layer of gravel. Make sure that it is leading out to your
drainage point.
Fill in the rest of your trench with gravel and cover the drainage pipe, but leave an extra 2” free for now.
(Remember to keep 6” of space below grade to start your stem wall foundation too.) So leave about 8”
free of gravel at this point.
Cover the landscaping fabric over the top of the gravel, making a “burrito” of sorts. Cover this with
another 2” of drainage gravel all around.
Tamp your gravel inside the trench.
Begin your stone foundation or stem wall.
3.StoneLaying
Grade your stone – sort out the stones by size. The larger rocks will go on the bottom of your
foundation. Also pick out corner stones with a nice 90 degree angle, and mark these with a big X.
Set up your next set of tapered batter boards. You need to build these according to your structure. The
top surface of rock foundation walls should be NO LESS than 16 inches wide. The minimum height of a
stem wall above grade should also be 18 inches. Our batters moved in 2” per each foot upward in
height.
Lay your corner stones for the first layer and start from there.
Lay the exterior layer first. You will probably have one interior and exterior layer of stones for your
foundation.
Always lay your stones lengthwise into the wall! This is important in order for your foundation to last a
long time.
Lay your stones so that the “face” that you want is showing on the outside of your foundation wall. Try to
use faces that have a slight upward slope to them. This will help shape your wall nicely.
You want all of your stones to have contact. They all need to touch!
Insert “hearting” in between your large stones. This is a critical step! Hearting is just smaller stones or
gravel to help support the large stones and fill in the gaps. Use your “waste” rock for this. Try to insert
one bigger rock and then some smaller ones. When adding hearting, always lay it in from the inside of
the stone wall, and don’t put it in from the exterior of the wall. This prevents them from wedging out over
time. Also, don’t force the hearting in. You don’t want to disturb your large stones’ positions.
Lay the interior layer of stones now. Use the stones without a good shape or face for your inside layers
since these won’t be seen. As Alan Ash would tell us, “Just get the shittiest big rock!” The words of a
Master…(Note: This is not always his general rule of thumb though. He has reason for what he says.)
Insert more hearting to fill in all the gaps too.
For “static” stone laying, you will lay one layer of stones at a time. For each layer, lay a string line by
attaching it to two opposite tapered batter boards. This will help guide you up at the right slope as well.
Follow the 1 over 2 rule. Lay one stone to cover the break between the two stones below it. This is
called “breaking the joint.”
4.Mortaring
If you’re mortaring your stones together, lift up the stones and place them aside. Keep them in the same
direction so that you know how they should go back in place.
Lay your mortar down where your stone will sit.
Place your stones back in place on top of the mortar.
Re-insert your hearting, and add new hearting inside of the mortar if needed.
We used Natural Hydraulic Lime mortar on this structure. You can also use cement mortar if you
choose. NHL is best for mortaring stones underground as it sets in water. It is harder to come by than
cement mortar and costs a lot more though. If you’re on the East coast, you can get NHL and lime putty
through Virginia Lime Works.
This is a potentially good recipe for lime mortar: Use 3 parts sand to 1 part NHL. The amount of water
can vary, but it will be about ½ part water. We tested the sand that we were using for voids as well as
gradation and that’s how we came up with the 3-1 formula. Often it can be 2.5,1, or even less though.
A good recipe for a clay/lime mortar is: 6 parts sand, 1 & ½ parts sifted clay, ½ parts lime putty, and
about 1 part water. (Add the water and lime first.) This is mortar to only be used on the above grade part
of the wall.
That basically sums up what we did and how to build a stone foundation for your cob house or any other manner
of building you might be envisioning.
ADOBE CONSTRUCTION
In southwestern United States and Mexico (as well as other parts of the world), where there
are not many trees, people often build houses out of mud bricks called adobe. Adobe houses
are warm in the evening and cool in the daytime.
If a mud brick is warmed by the sun, how long will it continue to give off warmth once the
sun goes down?
Round huts made of bricks
Materials for making mud bricks:
Soil
Water
Bowl
Large mixing spoon
Straw, dry grass or pine needles
2 thermometers
One-pint milk carton
Clock
A sunny window
Pencil and paper
How to make mud bricks:
Gather some straw. If you do not have straw you can use dry grass, or dry pine needles.
Put the straw, soil from your yard, and water into a bowl and mix it well.
Open the top of the empty one-pint milk carton. Pour the mud mixture from the bowl into the
milk carton.
Make a hole in the mud by pushing a pencil halfway down in the middle of the opening.
Loosen the mud around the pencil by moving the pencil in a small circle, and then leave it in
the carton.
Place the milk carton in a sunny window and leave it there for several days to dry.
When the brick is firm and dry, take the pencil out of it and peel off the carton.
Leave your brick in a sunny window for one more hour. Then, put the brick on a table out of
the sunlight.
Put a thermometer into the hole of the brick. This will measure the temperature inside the
brick.
Lay another thermometer nearby on the table to measure the temperature of the air outside
the brick.
Wait a few minutes, and then read and write down the temperatures showing on the
thermometer inside and outside of the brick. (How long will it take before the thermometer
inside the brick is the same temperature as the one outside of it?)
Many people around the world use different materials to build their houses. What are some of
the advantages to using adobe bricks to build a house? What could be added to the mud mix
to make stronger bricks?
Adobe bricks are not used for building in places where there is a lot of rain, or where it is
cold. What would happen if adobe bricks froze and thawed a lot? What happens to adobe
bricks if they keep getting wet?