Plastering is the necessary skin that protects your earthbag
walls from the elements. (In much of the world, "plastering"
refers to the interior walls and "rendering" refers to the
Here in the US, it's all called plastering.) Polypropylene
bags - despite being treated with UV inhibitors - will start
deteriorating after just a few months in the sun, exposing
the baked contents to erosion. Burlap bags will get bleached
& weakened by the sun, abraded by wind & sand &
silt, rotted from moisture, and eaten or harvested for
nesting by insects, birds & animals. You need to
Besides being protection for the bare bones of your
structure, plaster is your structure's skin. It's here that
you can play with color, texture, adornment (inlaying stone,
tile, glass, or adding mica to your mix). And it's here that
you can achieve those rounded corners and sweeping curves
that probably attracted you to earthbag construction in the
It's not in our scope to provide a comprehensive tutorial on
plastering. There are good resources out there; see
the book suggestions at the bottom of this page. It isn't
rocket science. People have been plastering their structures
for about 12,000 years, and continue to do so today in many
parts of the world. Like building with earthbags, it's
pretty intuitive, and you'll quickly begin to get a feel for
Basic earth plaster:
Your base plaster can be the same mud as your earthbag fill,
though you may want to tweak it as needed by adding a little
straw, sand, clay and/or manure (see below).
Just as we advise that you experiment with your bag fill to
get it right before
getting started (jar test, sample clumps, etc.) so we advise
that you do the same with your plaster mix well in advance.
Mix small quantities and plaster a couple of cinder blocks
or 2x4s, whatever, then leave them outside & let the
weather have its way with them. All the better if you do
this several months before building commences so that your
samples will have had a chance to get a little beat up by
the sun, rain, UV, snow, wind, etc.
be looking at plastering in at least two different layers
and with two different compositions: a thick base layer and
a thin finish layer.
Your base plaster (the thickest) should be built up in
several relatively thin, incremental layers. Glomming on too
much base plaster at once (a 1", 2", 3" layer of goopy
plaster all at once) may lead to catastrophic failure.
Imagine a clump shifting, swelling, pressing out against
your finish layer & causing it to crack, allowing water
to get in, and then compromising everything. You can and
should make your base plaster much more bomb-proof by
laminating it, grooving each layer with a trowel to give
successive layers "tooth" to stick to, adding sand to your
plaster for grit (which also creates tooth, giving the next
layer to adhere to), and doing what you can to make your
plaster as sticky as possible by adding the likes of clay,
wheat flour or manure.
The simplest earth plaster for the base layer is essentially
earth, sand, and straw, (or manure, or cellulose, for
binder). You'll want your earth to have at least
20% clay. Too much clay = a tendency for your plaster to
swell, contract & crack. Your sand should be plastering
sand, meaning that the granules are angular & lock
together (rather than beach sand, which has rounded
A good place to start if you're making it from scratch
is two parts clay-rich soil, two parts chopped straw
(about 1" long pieces; a weed eater and a steel barrel work
great), one part sand, and then add water to desired
consistency. If you're using native soil, get a handle on
how much sand-to-clay you have and consider bumping up one
or the other. Again, refer to the
Experiment for best results. Factors like the variability of
soils, daytime & nighttime temperatures, humidity, etc.
all make it impossible to develop a one-size-fits-all recipe
for plaster that works for everyone.
a water source close at hand. Some people we've worked with
have done their building in areas without water (having to
drive for miles to fill up jugs from rivers & creeks).
If you're working on a large scale, consider investing in an
IBC tote ("intermediate bulk container"). These come in 275
gallon & 330 gallon sizes, and can be found used for
$150 - $200, or $250-$300 new. We can get them for you if
you can't find a source. After construction, they'll provide
lasting value for drinking water or to store grey water.
If you buy a used IBC, make sure to determine what was in it
before (you don't want one that was previously used with
solvents or chemicals). They're in a metal cage, and fit in
the back of a pickup. Do be aware that water weighs a little
over 8 lbs per gallon, and at over a ton a filled 275 gallon
tote (filled with sloshing water on an unimproved road
leading to your site) can strain your truck's suspension.
that the face of your stacked earthbag construction is
scalloped, having rounded edges alternating with V-shaped
crevasses. You'll first want to fill these in. How well (or
how sloppily) you laid & tamped your bags will have a
lot of bearing on how deep those crevasses are; and how well
you fill these in will define how well your external
plastering will hold up.
Mix your plaster in a wheelbarrow, turning it with a shovel.
A masonry hoe is a good investment - they're heavy duty
& have two big holes in the blade. Or get a paint-mixer
attachment (like a giant eggbeater) for your powerful and
low RPM power drill. If you have a big job, you can always
rent, beg, borrow, or buy a cement mixer. Or if you're
trying to keep costs down, you can throw a party; toss
a tarp on the ground, dump your ingredients, wet it all
down, put on some music, and have everyone mix it &
squish it with their bare feet. (Kids especially love
this.) Then put everyone to work.
Start by packing your base plaster in with your hands,
glomming it on & pushing it in with your fingers. Again,
consider doing it in increments; an inch or so, give it a
few hours or a day to set, add another layer, let it set. If
you have lots of spaces in the crevices that you can't
reach, then pack as much soggy plaster as you can, using
your fingers to get into the crevices then switch to your
hands & your trowel as the face flattens.
Plastering's best done on hot days. Keep a bucket of water
nearby and add some to your plaster mix when it starts to
dry out or gets crumbly. A squirt bottle close at hand can
also be helpful. Once your plaster mix has had a chance to
cure (it'll be pretty hard), you can add the second layer of
your base plaster, which will completely cover your bags. You'll
want to groove with a trowel to give some "teeth" for the
finish plaster to key into (this is called the "scratch
You can opt to cover your bags with some sort of mesh after
filling your bags' crevices. Commonly used are chicken wire,
metal lath, & plastic mesh. If you choose to do this,
you'll want to apply a second coat of your base plaster
(referred to as the "brown coat"), finishing it smooth in
preparation for the finish layer.
Besides defining texture & color, your finish layer
provides all-important weatherproofing. A basic finish
plaster will consist of finely-sifted dirt (a 1/8" screen is
good) with the same basic 20-30% clay content, plus perhaps
more sand added than you'd be using for your bag fill &
your base plasters. When sifting, it's pretty critical to
remove all organic material (fine root clusters, seeds) so
you don't have things growing out of it. You can also add a
variety of sealants (wheat flour paste, egg whites, linseed
oil, etc.) which will give it a fair degree of water
repellancy and help decrease the need to re-plaster every
year or two. You can add cement and/or lime to natural
plasters (up to, say, 10%), but it's a trade-off; what you
gain in strength, you lose in elasticity (crack resistance)
Lime is another option, as is stucco or papercrete.
The latter are both cement-based, which is good in humid
climates, but they don't breathe, meaning that any humidity
or water that get inside your walls can soften your
earthbags over time & compromise your structural
integrity. There's a little more discussion about these
further down the page, but - again - this is just an
introduction and you're advised to dig deeper or pick up one
of the excellent books (see the bottom of the page) that are
When choosing what kind of finish layer to use, consider the
- UV rays
- blowing abrasive sand or dust
- seasonal temperature variations
- freezing & thawing
In all cases, you might need to re-apply periodically
(typically every couple of years). This will depend on what
finish plaster you choose, and how much of a beating your
climate dishes out. Like so much else in life, it's all
about the inverse relationship between time & labor vs.
convenience & cost, and where you choose to draw the
- settling (how good is your foundation?)
- vibration from nearby construction, road traffic,
You can add pigments to your finish layer for color (here's
we like). Methods range from slathering it & smoothing
it by hand or trowel, to pump & spray applications with
rental equipment. If you want a professionally-finished
look, you shouldn't have much trouble finding contractors to
handle this phase for you... but you'll want to be clear
with them about what materials they use.
Plastering as you go:
This is a little controversial, and you'll need to use your
best judgement. Ideally, you want your bags to be fully
cured before applying plaster (which otherwise will keep
moisture inside your walls - you don't want that).
It is possible to alternate between filling/stacking bags
and plastering, perhaps switching off every few days. There
are four benefits to doing so:
If conditions dictate (let's say your bags are setting up
nicely in 2-3 days) then you can plaster the sun-exposed
side of your walls while leaving the shady side unplastered;
this will allow the woven polypropylene bags to breathe so
the moisture in your mud can transpire, and your bags will
have a chance to fully dry & set up. Later, you can go
back & apply a 2nd (or "scratch") layer.
- Many people don't appreciate how much work plastering
is. Getting a leg up as you go along will minimize the
potentially-daunting job of plastering a finished
- Shifting gears every few days from filling/stacking
bags to plastering gives your body a break by utilizing
- Slapping or smearing on one layer of base plaster to
the walls which receive the most sun exposure will
provide your bags with protection against UV
degradation. We advise elsewhere on this site to keep
your work covered as you build - tarps, black plastic
sheeting. Even though all the bags we provide are UV
treated, they can start to photodegrade after just a few
weeks of heavy exposure to the sun.
- Getting a late start in the season with your
construction & facing the possibility that you'll be
encountering adverse weather as autumn rolls in &
temperatures start to drop will give your structure a
protective edge (especially if you have to stop work
over the winter & continue the following spring).
So, what to use? Once you start looking, you'll discover
that there's a lot of discussion & even a little
controversy over the pros & cons of various materials.
Here are just a few, with some links:
Basically high clay-content soil, sand, binder, and water. New
Clay has a good line of clay additives (if your soil
isn't up to par),
Rabbit has a very nice page on techniques. Click here
for a YouTube video on natural & food-based additives
for mud plaster finish layering. (A few examples include
milk, wheat flour, and egg whites.)
Besides having a beautiful finish, lime has the virtue of
being flexible after it's cured and offering breathability.
It's essentially lime (binder) mixed with sand (aggregate)
and water. A bit complicated to use (buy hydraulic, not
hydrated). It's caustic and hard on the skin... so, goggles,
gloves, and long sleeves are called for. An excellent
resource is Barbara Jones' article "Working with Lime" inThe
Art of Natural Building.
Limewash is an easier application to mix & use over a
finished plaster. The National Lime Association offers a
great article on this here
A mix of Portland cement, paper & water. Sand can
be added to the final coat for texture. Rumored to be prone
to black mold growth if exposed to moisture (not that much
of a problem here in NM). There's a discussion that touches
on it at Autonotopia,
while Gordon Solberg of Las Cruces offers his book and DVD,
Building with Papercrete
& Paper Adobe, here.
By contemporary definition, stucco contains cement. It may
contain lime (for flexibility), but most stucco used today
is basically cement, sand, and coloring. Widely available
for purchase and for application by contractors.
Also referred to as clay slip. Less of a plaster and more of
a clay-based finish paint for indoor use, which may offer a
greater range of colors & tints to choose from. New
Mexico Clay has a page on aliz here,
and you can also find more on the Web.
For a much more detailed exploration, we suggest: