have by no means mastered Earthen floors but have gained enough experience
to have been hired this past building season to install two, adding
to the ten we have worked on along with teaching another dozen or
so here at LanderLand during our workshops.
Our current style is some times called a poured adobe floor. The term
poured seems to come from the world of concrete floors but a better
word might be placing. No matter what term or method, one needs to
properly prepare the sub floor. We prep to within of 1/2 in./13mm
of finished grade and then basically apply the final 1/2 in./13mm
topcoat. Like any earthen application, one needs to know their material
in regard to the clay content of the soil. Here in Kingston, New Mexico,
our soil has close to a 30% clay content so its a dream to use.
We teach two mixes for floors, what we call our sand mix and the other
a straw mix. The sand mix is made up of 1 part sifted clay soil and
4 parts course fill sand with the largest particles as large as 3/16
in./5mm. These large particle sizes mixed with finer aggregates keep
the floor from shrinking, cracking, and add compaction strength, and
are also easier to apply. The clay soil is more like the binder and
filler. You need enough water to mix damp. You can add a small amount
of 1/2 in./13mm chopped straw for the aesthetics but its not
Our straw mix is a variation of our basic earth plaster with added
sand. 2 parts sifted clay soil, 2 1/2 parts course fill sand and 2
parts 1/2 in./13mm chopped straw. Start with 1 part water. This mix
is harder to apply than the sand mix. The wetter the mixes the easier
they are to apply but the moisture may cause shrinking and cracking.
Lets move away from the mixes and talk a bit about prep work.
The heart of a good strong crack-free earth floor is the base that
it is applied on. This is true for almost any floor be it concrete,
tile or wood. Earth floors are more like concrete in that they must
be properly compacted, graded and screeded flat. For the inexperienced
owner/builder, floor prep can be intimidating, again the compacted
sub floor is the key.
Apply your fill materials in lifts of 1 in./25mm to 2
in./50mm and compact damp. A trick, if you have the time, is to flood
these materials with the garden hose. You can rent noisy, smelly plate
and foot compacters; some big floor jobs require this. I still would
buy an 8 in. x 8 in./20cm x 20cm hand compactor. Ive moved away
from making a compactor from a steel pipe and welded plate or a coffee
can filled with concrete. I recommend prepping your floor early in
the building process so it has time to be compacted naturally from
working on it; this also keeps your walls clean, assuming your floor
is last in the construction process.
In addition to compacting, there are also possible moisture issues
and, in some locations, soil gases like radon. These are issues you
or the builder will need to address. Certainly it is difficult to
write about floor prep and build up in a short article.
There are so many variations and situations depending on your particular
site and needs. I always recommend reading about conventional building
materials and techniques and talking with builders in the area.
Im a big fan of radiant heat and almost always add the pipe
to my concrete slabs even if I am not going to heat the slab. Never
know when someone might. Pex pipe is easiest with concrete slabs,
the most efficient being an isolated slab where 1 in./25mm or 2 in./50mm
foam lines the bottom and sides, (a thermal break) steel mesh is laid
down and the pipe attached with zip ties. On small floors, I now use
cattle panel fencing for my wire mesh rather then the traditional
rolled mesh; its more expensive but for me so much easier to
use. So, in my opinion, the best floor would be a 4 in./10cm thick,
3000 psi concrete slab with added fiber and PEX radiant heat pipe
with a final 1/2 in./13mm earth top. Dont forget your fly ash
and control joints, concrete cracks. This of course is a mix of conventional
and natural, not for the purest.
One 300 sf radiant earth floor we did had 9 in./230mm of pumice put
down over the native rock soil as the insulated layer and then we
brought in another 10 in./250mm of crusher fines (road base), sand,
earth, no foam and no steel. This technique had its own challenges,
where again experience and creativity help. What is interesting in
this house is to notice the different feel to the radiant
floors from the earthen side next to the conventional isolated foam
4 in./10cm concrete radiant floor in the adjacent room. They both
work, the house is warm but the concrete feels hotter to the feet.
So now you have prepped your floor rock solid (like concrete, huh?)
to within 1/2 in./13mm of finish height. You have also gone around
all the walls and drawn a line at your finished height. A day or two
in advance, you might need to go around and fill any holes, voids
or low spots with a damp clay-sand mix, maybe even tamping a bit with
your nice tamper. When dry you should be able to sweep up any loose
Its a good idea to mix up your material a day in advance. Now,
do your math. Calculate your square footage then your cubic footage
and add about 30%. If your room is 10 ft x 12 ft, then 10 ft x 12
ft equals 120 sf. Multiply this by 0.0416 to get cubic feet. (0.0416
is 1/24th of 12 in.) 120 x 0.0416 = 4.99 cubic feet.
[ For metric calculation: 3m x 3.6m = 10.8m2. 10.8m2 x 13mm = 0.14m3]
Add 30% more material. 4.99 x 0.3 = 1.49 for a total of 6 1/2 cubic
feet. We add 30 % due to the fact that we will be measuring our materials
dry so there is air space. Once wetted and applied, the material gets
compacted by the toweling process and we lose volume. You will need
a container to store all this material. A simple tub can be made out
of a frame of straw bales set on the ground and lined with plastic
or a tarp. You can also buy large kids swimming pools. The color
of your floor will be the color of your dried clay. You can add concrete
liquid or powdered colorants. It is always a good idea to do a few
3 ft x 3ft.90cm x 90cm samples to test for shrink, cracking and color,
also a good way to practice your applying techniques.
How to apply
Again one of those concepts that is best shown during a workshop training
session than through trying to write about it, but here it goes. Ahead
of time make up a few 1/2 in. x 1/2 in./13mm x 13mm screed sticks.
These are also the thickness guides, four per person. Vary the lengths,
12 in. to 36 in./30cm to 90cm. Also make some wooden pool trowels
out of the 1/2 in./13mm thick concrete wood floats from your building
center; they cost around $3.00 USD each. Keep one square for corners.
Plan your route of attack so you will be able to work your way out
of the room. Begin by setting down some pre-wetted wood sticks
trowel lengths apart, shovel down some material and start working
in the material between the sticks. The trick is to make sure the
material is compacted well, no voids. Do a few square feet leaving
the sticks in place to run your trowel over thus establishing the
thickness. Dont spend a lot of time making it look good right
now. Slide out the sticks, you now have a square groove that needs
to be filled. First, take your trowel and press the sharp sides and
ends down to form sort of a vee, now add small amounts of material
in the vee and trowel it flat. Any voids or air pockets will leave
a spot for cracking so compress well. The tendency is to put too much
material in at one time; instead use a small amount frequently rather
than large amounts all at once. Keep your guide sticks clean, wash
frequently so as not to add buildup creating a thicker and uneven
floor. As you progress along placing material and removing sticks,
go back over the previous areas with your trowel to smooth and even
out your floor as far as you can reach back over what you did. Sounds
easy? Hopefully you worked this all out in your 3 ft x 3 ft (90cm
x 90cm) test samples.
Sure looks good doesnt it? Youre not done yet. More steps
involved as the floor begins to dry. A word of caution about drying,
its important to get even drying. If the sun shines in a window
or door, these must be covered up. Air circulation helps to remove
the moisture and speed up drying but again you need even flow.
Now its all about timing. On hot days/in hot climates, we find
it best to apply the floor early in the morning so that hopefully
by late afternoon or early evening we will be able to get back on
the floor with kneeboards and steel pool trowels, or apply late in
the day and hopefully you are back on it first thing in the morning.
Miss this window of opportunity and your floor will be too hard to
steel trowel. If you were so good applying the material with the wood
floats and you are happy with the results, then one can skip hard
troweling so your floor will be a little more course.
So your floor is drying, time to hard trowel on kneeboards
3/4 in./20mm plywood, 18 in. to 24 in./45-60cm square or 2 in./50mm
foam blue board works well. Make sure to wet your kneeboards, otherwise
they stick and pull up your material. Almost like hard troweling a
concrete slab. Steel troweling tightens up and flattens the surface.
We use pool trowels and basically just go over the whole floor again,
pushing hard with two hands in big sweeping motions.
Once your floor has completely dried, its time to seal and fill
the floor with Linseed oil.