Natural Construction Materials
Ecological considerations :
sustainability implies using natural
materials with minimum
Health considerations :
In this page I consider the necessary minimum processing for making natural building materials ecologically and safely. Minimum means less resources, energy, pollution, transport, health risk and more useful life for us and our products.
Tradition offer us many solutions and science gives us the tools to research them. The problems are rarely as serious as those associated with the use of manufactured products with which we have, typically, little long-term familiarity.
The major quality of natural materials is their accessibility. Requiring little energy or processing for their use, more people can employ them for their constructions. Direct access makes us more aware of their environmental and health impact, facilitating the management of potential hazards. Accessed locally their transport has little or no impact.
What are they :
The fact that natural materials are traditional does not relieve us of the need to think about how we extract and use them.
Actually they are few and fall logically into three categories :
The mineral world is made up of particles. Large rocks, detached from the Earth by geological and climatic forces are scattered with extreme violence and and broken into smaller fragments. These are being buried, compressed, re-exposed, eroded, dissolved, and sedimented. They are ground by glaciers and they descend by avalanche or are carried by rivers.
~~~~They are distributed
Gravel is the large particles that are the basis of the structure of earth, they are the monsters of the particulate world.
Gravel is the stuff of avalanches and sweep all before them. They pound and crash in the water. They are the mortar and pestle that created the sand and silt. On a slope they are scree - very dynamic, dangerous and sometimes exciting - scree jumping.
They are deposited in valleys and form large flat beds; which gives us the clue as to where to find them and how to use them.
world of sand is a world of tumbled rocks, full of unstable cavities.
Water floats the grains into place to form a compact mass. Dry,
it flows like water and forms large puddles, filling the hollows.
It is grains of silica and quartz, sometimes with pulverised limestone, measuring between 5 mm and 0,06 mm. The grains are visible with the naked eye. Sea sand, being round, is useful for masonry, but is not so good for rendering. River sand is sharp and generates open structures that leave more space for the silt and the clay to contribute to the cohesion. The structure of sand stabilised in this way has improved resistance to compressive forces.
world of silt has been tumbled for millenniums. It is inhabited
by small, round, cuddly particles. Water carries them to fill the
cavities in the sand where they form a network of small groups which consolidate
This is really fine sand which gives earth a loamy texture, composed mainly of silica and quartz particles of between 0,06 mm et 0,002 mm dia. We see it as a layer of particles too fine to be seen separately. Its contribution to the earth's stability is due to it's internal friction by the filling the openings in the sand. Too much makes the earth more friable and crumbly.
micro fine is a slippery world. We can feel it with our fingers.
Water lubricates it and leaving, allows it to repose in electrical equilibrium.
Dry, it shrinks back on itself and becomes hard and stiff.
which bind the clay particles
ensure in large measure the cohesion of the earth.
This is a mixture
approximately in the proportions of ½,
1/3, 1/6, without humus and colloïdes other than clay;
the ideal mixture is a structure
of large grains, with the spaces
between filled by smaller and
smaller grains. The mixture
is held together by a thin layer
of clay and water
surrounding each grain. Gravel will be present according
to the proposed use, medium
gravels being acceptable in a beaten
earth pavement, (often built on a bed of large
gravels left with an open structure).
The reactions between the water and the materials that make up the earth, including the impurities that may be present, explain all the qualities and imperfections that we can detect in our construction.
Stabilisation:Stabilisation is not always necessary but can be useful in difficult circumstances. It can help to improve resistance to erosion, to distortion or to humidity. We can add 20 to 30 kg of fibres to each m3 of earth. Animal manure, ox blood, rye flour, sap and oxidising oils are some of the modifiers used as stabilisers. The use of mineral stabilisers such as lime and cement are only useful in particular cases because they are hydrophilic, they attract water and are more cold to the touch than earth.
The sand that François found on the river bank was sufficiently well dosed with clay to be used as it was for rendering his wall
Daub is crude constructible
earth used in a malleable
state (and the name of the act of manipulating it), mixed by hands or
by feet. It is traditionally mixed with a little straw
or cereal husks and is used to
fill timber frames or to make
The earth is improved
by adding sand or clay
to obtain the optimal proportions (1/2
sand, 1/3 silt, 1/6 clay ).
are good once dried. We
add from 5 to 10%
to the earth to stabilise
the daub. The optimal
length is found to be around the thickness
of the layer in which it is to
is made in a pit or a specialised
mixer.(Those for mixing bread dough work well). It is too
stiff to be mixed in a concrete
mixer. It has good resistance
in compression. It is not
very insulating, like stone
or baked bricks, but it has a
good thermal capacity.
to daub and it becomes mortar
which can be mixed in a concrete mixer
and can be applied with a trowel.
(I am not familiar with English Documentation -help!)
which contains a lot of granules,
shavings or fibres, typically 90%;
covered with a little mud (clay). The woodchips come from woodworker's waste.|
It is available in prefabricated panels, see Akterre, and is used to make lightweight earth concrete.
Terre-paille is a mixture traditionally used in Normandy, made from long straws, covered with mud. It is mixed with a pitchfork or a special mixer.
Mud-woodchip mixtures can be made in an ordinary concrete mixer. They have little structural resistance. Good insulators, they contribute little to the thermal capacity. They are good acoustic insulators and research suggests that the size of the filler and the weight of earth included influences the frequency of the sounds which are reduced.
Rendering with Earth
Rendering with Earth - application
Mortars for rendering:
They are made with dry river sand (sharp), mixed with clay mud to the standard consistency to obtain a mortar which is easily manipulated with a trowel. Each successive coat should be less rich in clay than the preceding one and the maximum size of the grains of sand in the body and finishing coats should be less than half the thickness of the coat in question.
The adhesion of earth rendering to it's support is mostly due to it's rough texture. The support must be stable, not friable and dust free. If this is not possible, we must provide a mesh of split bamboo, willow or fabric firmly fixed to the support. This reinforcement is necessary at the angles and the changes of the nature of the support. The hydrometry of the support influences the suction between the support et the rendering. A light suction is necessary to ensure the continuity of the liaison between the support and the rendering. Too much suction provokes quick drying, with regions starved of clay and the appearance of fissures. Too much water in the support or in the rendering limits the adhesion. Watering the support should be spread over several days. It is given until the support remains wet but not enough to swell the clay in the support and we wait for it to dry. When it takes from 10 to 20 minutes to dry, the support is ready.
Splash coat :
A first coat of mortar is thrown onto the support. This splash coat, quite liquid and rich in clay, materialises the liaison between the rendering and its support. It insures the conformity with the rough support and presents a rough finish. It is not necessary for a support made with daub. It should be from 2 to 4 mm thick. We wait 2 to 8 days for it to dry completely.
Body coat :
Rehumidified, we apply the body coat, thrown or spread over the splash coat. Being the body of the rendering; it is regularised with the edge of the float to a thickness of 8 to 20 mm to equalise the surface of the support. It can be applied in several layers, the dose of clay diminuing, and is dressed with a straight edge to obtain a flat surface. Without fissures, it is scratched or brushed to improve the adhesion of the finishing coat. It must be allowed to dry thoroughly.
Finishing coat :
This is the decorative
layer, it covers the remaining imperfections. It is less
dosed with clay to prevent any fissures and provides
the colour and the texture.
It should not be heavily
trowelled to avoid any risk of crazing. So finally
we rehumidify and apply this thin coat, 2 to 4
mm thick, with minimum trowelling. It is
also the coat which takes the
wear and might need occasional
resurfacing. We can add up to 5% of fixative.
Paint :Inside we use liquid mud (or slip) to which we add fixatives. These fix the dust and equalise the surfaces. This offers another occasion for decoration. Butter milk, whey, egg-white, sap and oxidising oils are modifiers can give good results as fixatives.
Organic Structural Materials:
( in construction )
Using green wood is a long history
that we have forgotten since the arrival of large mechanical saws.
These cut small sections from large
trees, which although dry remain unstable. To understand this we
know that green wood contains sap and it shrinks on drying.
Sawing wood cuts across the fibres and the drying cannot be regular. Correct seasoning is complex and difficult to achieve.
Green wood is well adapted to being squared with an adze or side-axe because the wet wood is softer.
From each length we take a single piece of wood with the heart at its centre.
We respect the natural form of each length to make our work easier and to make the structure more attractive.
Like that it doesn't move.
The wood obeys a simple rule. C=~2r
circumferential shrinkage (C)
radial shrinkage (r)
This difference in shrinkage means
that we must take precautions to limit and direct the eventual cracks.
Using traditional joints (except the halving joint in the case of chestnut), allows the wood to shrink and to expand without weakening the stability of the structure.
is an ancient practice for harvesting
trees known to the Romans. There are coppices
in Europe that have been harvested since the Middle
Ages.Mature trees are felled
and the remaining stump is left
which throws out sticks which
can be cropped; each year
for willows, but every 20
to 35 years
for carpentry timber. The
sticks are selectively
pruned to provide a small number of fine timber
This production cycle has been maintained for at least 800 years without negative effects, neither for the crop, nor for the environment. The soil mineralisation is improved from year to year by the addition of minerals concentrated in the leaves, having been brought up by ever deeper roots.
timbers are pre-sawn on
the bandsaw and if required hewn
with an adze or a side
axe. It produces spars
from 10 to
20 cm square, often curved
in one plane, but straight in
the other. These I use for traditional
timber-framed walls. I have met some colleagues in the Aveyron
who have been doing this for the last 21 years.
référence : LE CHÂTAIGNIER
UN ARBRE UN BOIS - Catherine Bourgeois - IDF
Straw slides, Pampas grass hooks on.
Fibres employed in earth construction are of all kinds. Straw, hay, pampas grass, chaff, linen waste, hemp, nettles and woodchips are all useful. They bring suppleness and resistance to the composite that contains them (see below).
What fibres can teach us
Contrary to popular belief, fibres
rarely contribute their resistance
to stretching, it is the weak links
between the fibres and the matrix
(the earth), which obliges micro
fissures to go off in all directions
when the material is subjected to external
forces . The micro-fissures
energy which would have produced a large
split. The mass doesn't split
( in construction )