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Adobe
Adobes are sun-dried mud bricks stacked with a mud mortar to
create thick-walled structures. Their use dates back centuries
in traditional earthbuilding areas such as North Africa, the
Middle East, South America and the southwestern United
States, where in all cases this building method is still in
widespread use. Dramatic examples of vaulted and domed
structures built entirely of earth still stand after centuries in
the Middle East, testament to adobe's timeless beauty and
structural integrity.
While in the "South" its use is mostly confined to those too
poor to have access to other building materials, in the
southwest U.S., adobe is often used by the very rich,
illustrating its wide appeal. Adobe is appropriate in areas which
are labor-rich and capital-poor, because it is so labor intensive,
using local materials and simple tools.
Adobe bricks are made with a completely saturated mixture of
clay and sand (and sometimes straw or manure), poured or
pressed into forms, which are then removed. After the bricks
have dried for several days, they are turned on edge for further
drying, then stacked for transport or for use on site. The
adobes are laid on an appropriate foundation (usually stone or
concrete) using typical masonry techniques with thick joints to
take up the difference in size of the adobes. Typically, mud
mortar is used, but a concrete- or lime-based mortar has also
been employed in certain cases. Various stabilizers for the
adobes themselves have been developed, with most, however,
relying on cementitious or asphalt-based compounds.
While adobe is widely appropriate, certain precautions must be
taken to ensure the durability of construction. Wide eaves are
often appropriate to protect the walls from rain, and
foundations must protect the walls from splashing. Mud plaster
is traditionally used to finish adobe structures with minimal or
no overhang of the roof, requiring replenishment every few
years.
Cement stucco has also been used in an effort to provide longer
lasting coatings, but in many cases has proven to be
destructive, as the brittle stucco can crack, letting in water
which dissolves the underlying adobes. Because the stucco can
hide this damage, buildings can collapse before anything is
sensed to be wrong. In addition, indoor moisture cannot
escape, building up on the inside of cement stucco, eroding the
wall. Because of this, there has been a return to favor of
traditional plasters using mud, straw and other natural
materials (see Natural Plasters and Finishes.)
Many building codes place severe restrictions on adobe
construction, essentially limiting it (and other earth-based
building technologies) to the desert southwest.
Bamboo
Bamboo is a species of plant in the grass family. It grows very
quickly, providing renewable material for building, tools, and
utensils as well as edible shoots. Common in the tropics, many
species of bamboo grow in temperate climates as well. Strong
and beautiful, bamboo has seen a recent resurgence in
popularity with builders.
Bamboo as a building material is not commonly known in North
America because of limitations on the import of living plants,
and lack of knowledge of traditional techniques. This ignorance
is beginning to change, however, as timber prices rapidly
escalate, and western builders become aware of innovative
uses of bamboo that have originated in Asia, Central America
and South America.
Utilized for millenia in these countries, bamboo can replace
wood and steel in many applications. It can replace rebar in
concrete, as pins in straw bale construction, to create trusses
and other structural members, as decorative elements, and has
even been used as plumbing.
Its widespread use in tropical areas, as well as unsustainable
harvest for export, can have its dangers: indiscriminate
cropping can decimate stands and endanger interdependent
ecosystems. To address this danger, an effort to create
sustainable growing programs similar to those for tropical
hardwoods is currently in development.
Cob
Cob is an ancient technique of building monolithic walls using
"cobs" of moist earth and straw. It is being rediscovered as a
multifaceted building material applicable to a number of
conditions. Virtually forgotten in North America, cob was
popularly reintroduced by Welsh architect and builder Ianto
Evans, who inspired intense public interest in his self-built cob
home created for under $500. A sculptural technique which
lends itself to curved organic shapes, cob requires minimal
tools and can be built by young and old alike.
The process of building with cob entails mixing local earth with
sand and/or clay (depending on the composition of the base
earth) and straw or other fibrous materials to create a stiff
mud which is formed into small loaves (cobs). These cobs are
then mashed together to form a monolithic wall on top of a
stone or concrete foundation.
The relatively thick walls (they have been known to be up to
six feet thick) proceed in layers or "perches" averaging 18"
high before slumping occurs. After a period of time to let each
layer solidify, work can continue. Irregularities can be shaved
off with a spade or other sharp tool as work progresses.
A particular favorite of natural builders for its ease, sculptural
qualities, strength, and thermal mass, cob is also useful in
combination with other techniques. Windows and other details
are "cobbed" into place, and niches and reliefs are easy to
create. While extremely economical for owner-builders, cob is
very labor intensive and time consuming, with walls taking up
to a year to fully cure. In addition, the curving architecture is
not to all tastes.
Cob has been used mostly in informal or experimental buildings
in the U.S. while code testing procedures are investigated. Cob
construction is undergoing a revival in England as well, where
proponents are rediscovering five hundred year old houses in
perfect condition.
Compressed Earth Blocks
Compressed earth blocks are similar to adobes, with the main
differences being they are not fully saturated with water, are
more dense than adobes, and are usually significantly more
uniform. These blocks are created using a variety of machines.
Some, like the Cinva-Ram invented in South America, use
human labor and are relatively inexpensive. Expensive fuelpowered machines, on the other hand, can produce thousands
of bricks in a day.
Because of their uniformity, compressed earth blocks need
little mortar, and can even be dry-stacked. This uniformity also
speeds up the laying process and results in straighter walls. A
house was built several years ago by CRATerre, a French
earthbuilding education and research group, in a total of 24
hours using compressed earth blocks.
More recently, an inexpensive, innovative machine has been
invented in Auroville, India, which can make a wide variety of
sophisticated block shapes using human power. This machine
was demonstrated at the UN Habitat II conference in Istanbul
in 1996, where a domed prototype house was built in a week
by volunteers and local labor.
Earthbags
Earthbags are soil-filled fabric sacks or tubes used to create
walls and domes. Traditionally used for flood control and by
armies to create bunkers, this method of construction has been
recently turned to a variety of natural construction purposes.
The technique has been used by Gernot Minke of Germany, and
is currently being pioneered in the U.S. by Persian architect
Nader Khalili of the California Institute of Earth Art and
Architecture (Cal Earth).
The use of earthbags is still in its infancy, but holds much
promise as a quick, easy and forgiving technique which uses
minimally processed soil and few tools. To build with this
technique, moistened soil is placed into a bag set in place on
the wall, the bag is lowered into place, then compressed using
a hand tamper.
Heavy earth mixtures can be used with weaker burlap bags as
the compressed soil makes the bags redundant once it sets.
Stronger, structural polypropylene bags are preferable for
sandy soils. Recycled sacks are often available free or at
minimal cost. In earthquake prone areas, a layer of long-point
barbed wire is used as "mortar" between the bags to contain
slipping. Domes using these materials are easily achieved with
a corbelling system utilizing long tubes made of the
polypropylene bag material.
Numerous natural builders have taken to using the bags as
simple foundations for straw bale or cob structures, or for
simple site walls. Recent projects in Mexico and Nova Scotia
have used gravel-filled bags as foundations for straw-bale
walls to minimize rising damp.
Disadvantages with earthbag construction are that resulting
structures may seem excessively "organic" to some and also
require quite of lot of plaster in order to create smooth walls.
Advantages of this technique include the opportunity to build in
wet conditions and in sites prone to flooding. Cheap and easily
transported, the bags could also be used for disaster relief
housing. Successful ICBO-approved testing has been
undertaken by Cal-Earth, and a code-approved public project
was undertaken in Hesperia, California.
Earthen Floors
The use of tamped or poured earth mixtures to create floors is
currently undergoing a renaissance in the southwest U.S.
Methods range from the African use of fresh cow dung sealed
with ox blood, to sealed earth mixtures.The technique involves
pouring or tamping one or several layers of an earth mixture
over a substrate of gravel, pumice or sand (a sublayer of strawclay has also been used by builder Robert Laporte for
insulation). Hardening agents such as blood, lime, cement or
glue may be added. This mixture is allowed to dry and any
cracks are filled with more mud mixture. Cracking can be
allowed to occur at random, resulting in a flagstone-like
pattern, or can be controlled by incising the floor to create a
tile-like effect. When the floor is entirely dry, it is sealed, most
commonly with successive applications of linseed oil and
turpentine. The floor is often coated with wax to protect the
surface.
Minor disadvantages of this system are its relative
damageability and need for maintenance. It can also be time
consuming to install because of long drying times. Advantages
include its aesthetic values, softness to the feet, and reliance
on cheap, often free local materials.
Earthships
"Earthships" are the name for the independent living structures
utilizing passive solar design and recycled materials developed
by Michael Reynolds of Solar Survival Architecture. While not
exclusively reliant on "natural" materials, earthships replace
some conventional materials with recycled trash which is found
scattered all over the planet.
Earthships are a system rather than a technique: typically,
earthships are dug into south facing hillsides; soil-filled tires
are then stacked like giant bricks to form side and interior
walls providing a source of thermal mass. Old bottles and cans
are used to create a variety of detail features and to fill in gaps
between tires. The building is framed in wood on the south side
and roofed with metal to collect rain water. Other systems
include integrated wastewater treatment, photovoltaic
electrical systems, solar hot water and passive solar heating.
Advantages of the system include self-sufficiency in all aspects,
the use of recycled materials and local soil, and technical and
aesthetic sophistication. Disadvantages include the complexity
of building such a structure and the amount of labor required
which makes it out of reach for many owner-builders.
The earthship concept has become a well-known and popular
building system, especially in northern New Mexico. Several
earthship communities have been built, and the system has
received widespread media attention. Other builders have used
the stacked tire concept to build houses which don't contain all
of the systems of Reynold's designs.
Hybrid Structures
Hybrid building techniques are a fascinating outgrowth of the
efforts of a few visionary natural builders and the sharing of
ideas in the natural building colloquia. The basic concept is that
several techniques can be combined for increased building
efficiency or unique artistic effect.
An example is to combine a thermal mass technique such as
cob or rammed earth on the south side of a house, with an
insulative system such as straw bales or straw-clay on the
north side, taking advantage of the best qualities of each
system. New solutions to common problems have begun to
evolve from such creative combinations.
A disadvantage of this approach is the lack of practitioners with
wide experience in a number of techniques, pointing to the
need for increased communication and sharing of ideas, and
the elimination of dogmatic insistence on the preference of one
system over another.
Light Straw-Clay (Leichtlehm)
Leichtlehm (literally "light-loam") is a German technique of
ramming loose straw coated with a clay slip into forms as an
infill for timber frame structures. This technique has been
popularly reintroduced to North America by Robert Laporte
who teaches natural building courses nationwide. He has been
joined recently by Frank Andresen, a builder with extensive
natural building experience in his native Germany.
The technique consists of surrounding a frame structure with a
thick infill of the straw-clay mixture. The frame is usually fully
expressed on the interior of the building to take advantage of
the beauty of the timber frame joinery. A lighter frame of wood
is built on the eventual outside face of the building as an
anchoring system for the straw-clay walls.
Loose straw and a clay slurry are tossed with pitchforks or
mixed mechanically, then allowed to age for up to several days
in order to allow the straw to absorb the extra moisture and
thus create a stickier and more easily tamped mixture. For
higher insulation values less clay can be used. Slip forms are
set up between the framing members, and the straw clay
mixture is tamped by hand in two-foot layers.
Occasional horizontal members spanning between the exterior
frame are placed in order to "lock in" the straw-clay mass.
Frank Andresen places the ends of these horizontal members at
the upper ends of vertical slots in the frame in order to allow
for shrinkage of the straw-clay as it dries. Once each layer is
complete, the slip form is moved up and the next layer is
tamped until the wall is complete. The walls are allowed to dry
before final plastering occurs. Any shrinkage is taken up by
stuffing more of the mixture into the cracks.
Robert Laporte commonly uses straw-clay stuffed loose
between rafters as insulation, with the clay discouraging pests.
He has also used it as an insulating layer underneath earthen
floors. Frank Anderson has demonstrated a system of strawclay tiles which can be placed between roof rafters as
insulation and as a plastering surface. He's also introduced
straw-clay bricks that can be used like lightweight adobes.
A variant on the straw-clay technique utilizes wood chips
mixed with clay; the resulting mixture is poured into removable
or sacrificial forms. This technique can also be used to create
lightweight bricks.
Highly accurate buildings are possible with the timber frame
and straw-clay technique, allowing a "natural" structure which
compares in looks to conventional stick-frame housing, making
it an appealing option for some homeowners. Some
disadvantages of the technique include its highly labor
intensive nature, and the long drying time. The straight walls
achievable with this method, however, are desired by builders
in southern countries who seek a western look in their houses,
and to whom an "organic" structure is "old-fashioned."
Hemp and other Fibers
Hemp and other fiber-producing plants as kenaf and sawgrass
are currently being investigated as potential building products.
Commonly used for numerous purposes before drug laws made
its cultivation illegal, non-psychoactive hemp is being
rediscovered as a source of fiber, oil, and hurd; these can
replace less ecologically sound wood or petrochemical products
in a variety of building applications.
Examples include pressed-hemp fiberboards, and use of the
inner pith or "hurd" as an additive for a lime-based concretelike material. Similar studies and experiments are being done
with Kenaf and other fibers.
Use of hemp and other fibers have several advantages. Because
hemp provides four times the usable fiber per acre as wood,
grows in degraded soils, and needs little chemical processing,
this annually renewable crop is being considered more and
more seriously. Several European countries, including England,
Hungary and the Ukraine, have joined major hemp producers
such as China to grow this neglected and useful plant for a
variety of uses.
Drawbacks include the reliance on expensive imported fiber
because of misguided laws which restrict the growing of nonpsychoactive strains of the plant in the U.S. Domestic
manufacturers are also hampered by the lack of modern
processing machinery.
Living Roofs
The Archibio architecture group in Quebec has updated the
ancient sod roof of Europe with a concept called the "living
roof." This type of roof has several advantages: it is an
aesthetic feature, helps the house blend into its environment
and provides climatic stabilization. Particularly useful in wet
snowy areas, this style of earthen roof has more limited
applicability in dry climates.
A living roof is built on top of a sufficiently strong frame with
carefully applied waterproofing, as it is very difficult to locate
leaks once the growing medium is in place. The living roof itself
is a compost based system, usually a base of straw left to
decompose within which native or introduced plants can then
take root.
The living roof will need ongoing tending, and could be a fire
hazard in hot dry climates. It is advantageous in that it
protects the waterproofing from damage by ultraviolet
radiation, and precludes the need for tiles or other shingles.
Natural Plasters and Finishes
Before the advent of portland cement, most earthen and
masonry structures were protected by mud- or lime-based
plasters. While still common in other parts of the world, lime
and mud plasters are relatively rare in the U.S. The advantages
of these plasters include "breathability," softness to the touch,
aesthetic qualities, workability and easy reparability, as well as
economy of materials. Less brittle than cement-based plasters,
those based on lime or mud adhere and "move" with the
underlying wall, lessening cracks and often making stucco
netting unnecessary.
Because they can erode unacceptably when exposed to rain,
exterior mud plasters are generally used in drier climates.
Protected from rainfall and other liquid moisture, earthen
plasters are viable in any climate. They have fallen into disuse
in the U.S. because of their disadvantages of slow setting
times, the perceived need to renew them every several years
due to weathering, and discrimination against their use by
building codes.
However, as the limitations of cement-based plasters become
increasingly evident (see Adobe section), many natural
builders are returning to ancient plastering techniques from
around the world. An extensive collection of traditional plaster
and paint recipes and application methods can be found in The
Straw Bale House and The Earthbuilders' Encyclopedia (see
Bibliography).
Paper Blocks
Printer Eric Patterson of New Mexico has discovered a use for
his (and anyone's) waste paper. He pulps this paper and mixing
it with cement, makes lightweight, strong and easily-worked
paper blocks. This material can also be poured and used as
mortar or plaster. Only a few examples of this system with
varying degrees of success currently exist.
While most recipes call for as much cement as is used in a solid
concrete wall, some practitioners are experimenting with clay
as a binder. Concerns include flammability with some examples
having burned, and the effects of freeze-thaw.
Rammed Earth
Rammed earth is an ancient earthbuilding technique currently
undergoing a renaissance in the U.S. and abroad. It has been
revived in France by CRATerre, in Australia by Giles Hohnen
and others, while its main proponent is the U.S. is David
Easton, author of The Rammed Earth House (see Bibliography).
These pioneers have updated the technique with improved
engineering, sophisticated forms, and innovative design to
make rammed earth competitive with conventional
construction, even in earthquake-prone California. While
rammed earth is in limited use in the U.S., builders in western
Australia have captured up to 20% of the housing market in
many areas.
Rammed earth has the advantages of excellent thermal mass
(which in many climates may be a disadvantage unless
insulated), strength, comfort and beauty. Rammed earth can be
built with simple forms and tools with less handling than other
earthbuilding techniques, as the material cures in the wall. It
can be built in a variety of climates. Walls do not need to be
plastered and will last for hundreds, even thousands of years
(the great wall of China is partially built of rammed earth). It
has been used to build structures of up to thirteen stories in
Yemen.
First proceeding by setting up forms on top of an appropriate
foundation (usually stone or concrete), a soil mixture with a
clay content of 20% and a moisture content of 10% is then
rammed in layers or "lifts" of 6-8 inches using mechanical or
hand tampers. Different soil types can be used to create
decorative effects and the whole is topped by a concrete bond
beam which then holds the roof. Procedures are discussed in
detail in The Rammed Earth House.
To build efficiently for the North American market, David
Easton has developed sophisticated forms and tools which
decrease time and labor costs. It is, however, still extremely
labor intensive, and codes demand intensive reinforcing in
earthquake-prone areas.
In an effort to make rammed earth even more competitive,
Easton has developed a system called Pneumatically Impacted
Stabilized Earth (PISE). In this system moist earth is sprayed
against a single form allowing thick walls to be built extremely
quickly.
Recycled Building Materials
In an effort to reduce waste, many builders are seeking to
reuse materials which would otherwise end up as trash. Many
structures, especially in urban areas, can be effectively
rehabilitated, saving immense amounts of new construction
costs, maintaining important cultural links, as well as avoiding
massive amounts of landfill waste. Other structures can be
carefully dismantled, saving lumber, bricks and other
recyclables for new projects. High quality old-growth timber
has been salvaged from old barns which is then used for new
homes.
Waste wood can be reused to create a number of board
products. Many of these, unfortunately, depend on toxic
binders, but efforts to make healthy alternatives continue.
Additionally, sawdust and recycled plastic have been used to
create long-lasting lumber substitutes. Efforts in these areas
have been spearheaded by the Wood Reduction Clearinghouse.
Windows, doors and other fixtures can be refurbished and
reused, saving valuable architectural heritage and creating
unique resources for owner-builders. Others use "junk" in
innovative ways (see Earthships.)
The advantage of this approach is the obvious environmental
benefits and aesthetic possibilities. Disadvantages include the
time, labor and cost to build with old treasures because of
refurbishment, special detailing necessary, and the time spent
to find, dismantle and transport these resources.
Straw Bale Construction
The use of baled straw to create superinsulated walls has
become an extremely popular method of construction in recent
years. Most common in North America, bale buildings have
been built around the world. Originally used by the pioneers of
the Nebraska sandhills, straw bales are cheap to buy and easy
to build with, lending themselves to "barn-raising" parties
where structures and community are created at the same time.
Straw is an annually renewable crop, available wherever grain
crops are grown. It is indeed a waste product, much of which is
currently burned in the field. The thick walls offer superior
insulation value when appropriately built. Bales are easy to
work with, lightweight and require a minimum of tools. With a
natural plaster, straw bale walls "breathe," and together with
the sound absorbing qualities, provide a quiet, healthful
interior environment. Straw bales can also be combined to
great effect with other natural building systems.
Straw bales are used either as infill in a post-and-beam
structure, or as a load-bearing system where the bales
themselves support the weight of the roof. The bale walls are
commonly wrapped with stucco netting and plastered with
mud, lime-sand or cement plaster. In many cases, the netting
has been found to be unnecessary, and plaster is applied
directly to the bales.
Structural, fire, and moisture tests have been done on the
system with great success, leading to easier code approvals.
Several insurance companies have insured bale buildings, and
bank financing is becoming available. The system is also
gaining acceptance with HUD and Fannie Mae, as well as with
large home building organizations as Habitat for Humanity.
Straw bale construction has a few drawbacks; the walls tend to
be "organic" and if not carefully built, can use excessive
amounts of plaster. Appropriate bales can sometimes be
difficult to locate, and if inadequately stored can be susceptible
to damage by rain. Current code restrictions demand
"overbuilt" systems which can increase costs, and a contractorbuilt straw bale house can be slightly more expensive than its
stick frame counterpart. Straw-bale buildings have been built
with code approval in every climate from New Mexico to
Minnesota.
Because straw bale construction is still so new, innovations are
continually being developed. An external pinning system shows
great promise, as do the possibilities of vaulted and domed
roofs entirely of straw bales. Straw is also being used to create
compressed panels to replace plywood and as interior partition
walls. Ongoing innovations are recorded in The Last Straw, an
essential journal for the field.
Thatch
The use of reeds, grasses or palm fronds as a roofing material
is still common in Europe and many southern countries. This
natural roofing material is of increasing interest to builders
seeking an alternative to industrial roofing methods. Thatched
roofs, if well built, can last up to sixty years or more, and
provide a pleasing counterpoint to many of the wall systems
mentioned here. Thatch breathes, can use local materials, is
highly insulating, and is extremely beautiful.
Thatch however, is a highly skilled and time-consuming craft,
with only a few practitioners left. This can lead to excessive
costs. If a thatched roof is not well built, it will need to be
replaced within a few years, and is prone to leaks. Thatch can
provide a home for undesirable pests, and fire danger is also a
distinct disadvantage for improperly thatched roofs. In many
countries in Africa, thatching grass is increasingly rare and
expensive, and the desire for "modern" roofs has led to its
decline.
Wattle and Daub
The technique of weaving branches (wattle) as a support for
mud plaster (daub) is perhaps the oldest of earthbuilding
techniques and is still used for traditional architecture in many
parts of the world. Uncommon in the U.S., it can be used in mild
climates to create thin earthen walls, but lacks the thermal
mass or insulation desirable in other climates. An intriguing
use of wattle and daub is to create interior partition walls.
Wood
Wood is an ideal building material: strong, easily worked and
beautiful. Its major disadvantage is that current rates of use
are unsustainable, leading to widespread deforestation. Natural
builders are seeking alternatives to conventional stick-frame
construction where wood is used indiscriminately, and have
begun to use wood in new ways.
Building with exposed timber-frames surrounded by materials
such as straw-clay or straw bales can take advantage of the
beauty and structure of wood while eliminating unnecessary
use. Innovations in bentwood truss construction in Nova Scotia
shows ways to create strong members using smaller diameter
trees. Other builders are finding uses for driftwood, and
irregularly shaped trees which would otherwise go to waste.
Innovative use of scraps and sawdust make the best use of
wood for panels, hybrid products, etc. In addition, increased
popularity of non-wood construction systems can lead to
reduced wood use and ecosystem rehabilitation.
Appropriate timber use is closely tied to sustainable forestry
practices. Selective cutting can provide ongoing sources of
material while saving delicate ecosystems. Using smaller
diameter or unmilled lumber can save large old-growth trees,
innovative uses of non-traditional species or young, second
growth trees can also be an effective strategy.
Conclusion
Natural building has a bright future. As techniques evolve and
more builders, architects and developers employ them,
structures which meet human needs while assisting in the
healing of the planet will become more common. While many
challenges lay ahead, it is still a hopeful and exciting time to be
part of this quest to create a sustainable human culture.
Natural Building Techniques
Straw-Clay
Straw-clay (used interchangably with "light clay") is a building material with un
properties. It is a very low-impact material, site-made of unprocessed earth and
agricultural fiber. In varous forms, straw-&-earth mixtures have a long history o
successful use in buildings of many cultures. It is hard, durable, non-toxic, and s
structural (self-supporting within reasonable limits). It is both a reasonably good
insulator and a thermal storage medium. It accepts natural admixtures such as li
oil, casein and prickly pear extract for greater water-resistance and durability. It
handles water vapor well in a cold climate, and it is resistant to rot, vermin and
Although there are many variations and techniques, the process show here descr
light wall construction of about 12 to 20 pounds per cubic foot (325 to 817 kilos
meter) with an estimated R-value (a measure of insulating value) of about R-19
.0526 BTU/hr/sq.ft./degree F) for a finished, dry 12" (30 cm) thick wall.
Visit our section on the Affordable Natural House for more on our research into
straw-clay
(left) Clay-rich soil, dry, 'shredded' and ready to mix in the mortar mixer.
The process of constructing a straw-clay wall consists of:
1.
2.
3.
4.
5.
6.
constructing the formwork
mixing the clay slip
coating the straw with slip
placing and compacting the straw-clay
removing the forms
finishing the interior and exterior surfaces
Mixing the clay slip...by
hand
Adding slip to the
straw
Making straw-clay
Clay is mixed with water to form slip. Slip can vary in consistency between thin (like
cream) and thick (like a milk shake ~~ at least, such comparisons are meaningful in the
Dairy State!). We're using a medium thickness here, mixed with an electric mortar mixer
with rubber paddle blades. The clay is not a pure potters' clay, but rather simply earth
with a high clay content.
A bale of wheat straw (preferred for it's strong, waxy stalk) is opened on a plywood
mixing floor. Then clay slip is poured over it and , like dressing on a salad, the straw is
tossed until it has just a light coating of clay slip. The mixture's readiness is tested by
grasping a bunch of straw in the hand. If it sticks together in a clump, the batch is ready
to be compacted into the form. Plywood forms can be washed off and reused for floor or
roof sheathing.
wall formwork ready
for straw-clay
placing the strawclay in the forms
legwork -- compacting the
straw-clay
The slip-coated straw is then placed into the forms and compacted. Feet are best; for tight
spots we use tampers made from short lengths of 2 x 4. Forms are removed immediately
after being filled to the top, to avoid surface mildew growth and to allow the new wall to
begin drying immedieately. Depending on the season and the climate (wind and relative
humidity), full drying of a 12" thick wall may take a month to several months. In the first
week or so wheat grass usually sprouts out of the wall until the wall surface dries.
removing the form as
the wall goes up
wheatgrass growing on
the new wall
The room is usable
before plastering.
In this sequence of photos, the work is proceeding by hand. However, farm equipment
can be used to mix and deliver the straw clay mixture to the forms. These and other
means of simple mechanization can dramatically speed up the labor-intensive process.