by Kaki Hunter and Doni Kiffmeyer – Utah, USA

We live in the heart of the great Southwestern United States, surrounded by examples of one-thousand-year-old ruins left behind by the ancient civilizations of the Anasazi, Hohokam, Pueblo and many others. It was these original natural builders that inspired us to consider building with earth as a way to create beautiful, low-impact, energy-efficient housing that has endured the test of time to this day.

We started by teaching ourselves how to make adobe bricks, the most common earthbuilding technique native to the U.S. Making adobe bricks turned out to be a lengthy process that involved mixing the mud, pouring it into forms, lifting the forms, and then turning the blocks over the next several days to facilitate even curing. The blocks then had to be stacked and protected until ready for use. Manufacturing the adobes required a considerable amount of space for both the pouring process, as well as for storage of the dirt needed to make them, and then the storage of the adobe bricks themselves until they were ready for building. We live right in the heart of a small town, which made this process a little tight.

The dirt for adobe block and most other forms of earthen architecture require a specific ratio of clay to sand, ideally about 25 to 30 percent clay to 75 to 70 percent well-graded sand. In some cases, a stabilizing agent may be added to an earthen soil to increase its compressive strength and make it resistant to the affects of water. Some earth building techniques like cob require copious amounts of straw fiber added to the mix. In most cases, adobe brick also benefits from the addition of straw or some other kind of natural fiber.

Honey House

After our initial foray into homemade adobes, we read about the work of international award-winning architect Nader Khalili. Nader is an Iranian-born architect who abandoned a successful career designing skyscrapers to follow his heart, which led him to create an innovative sandbag/superadobe/earthbag architecture as a means of providing low-tech, enduring affordable housing. Inspired by the ingenious monolithic adobe buildings of his homeland of Iran, Nader conceived the idea of building domed and vaulted structures with…bags of earth. We took a one-day workshop with Nader and we were hooked! We returned home excited to build our first earthbag-wall project, a privacy wall opposite the busy baseball field across from our house. However, our interest quickly zeroed in on the building process itself. We began innovating tools, tricks, and techniques that we felt made the building process more enjoyable and the results cleaner and predictably solid. We coined the acronym FQSS which stands for Fun, Quick, Simple and Solid. The process has to be Fun, which makes the work go Quickly as long as the procedure is kept Simple and the end results are Solid. Hence the FQSS stamp of approval became our dirtbag golden guideline.

Earthbags (as we were soon to discover) had the advantage of being able to use a wider range of soil types than traditional earth building techniques – “Wow, this dirt’s just got five percent clay and it still works!” We have been able to adapt soils for use in earthbags that have ranged from zero clay to 50 percent clay content. No type of fiber was needed within the soil. Since the bag acts as a textile container for the earth, the woven fibers do the job of stabilizing the soil in place so the soil can have a lesser quality binding strength than required for most other types of earthen construction. When necessary, even dry sand can be used as fill, as could be the case in providing emergency relief shelter. The Earthbag System is a contemporary form of earthen construction that uses modern woven polypropylene feedbags (usually misprints) or long tubes as a flexible textile container (or what we call a flexible form) preferably filled with dampened soil. The bags or tubes are filled in place on the wall being built so there is no heavy lifting. After a whole row is laid, the bags are compacted from above with hand tampers. The compacted earth later cures to a cement-like hardness. Two strands of four-point barbed wire are laid in between every row that act as a “Velcro” hook-and-latch mortar, cinching the bags together while providing continuous built-in tensile strength. Tensile strength inhibits the walls from being pulled apart during stressful conditions like earthquakes, floods, hurricanes, and load-bearing and lateral forces. The combined strength of the four-point barbed wire sandwiched in between the woven textile fabric of every row of earthbags adds a significant degree of tensile resilience that is lacking in most traditional forms of earthen architecture.

The soil we selected for our initial earthbag building projects was delivered from our local gravel yard at 80 cents per ton. That was ten years ago. Today we pay about $1.80 per ton. Reject sand or crusher fines are common names for the clay fines that are the byproduct from the manufacture of washed sand and gravel produced at most developed gravel yards. Often, this reject material has sufficient clay-to-sand ratio to produce strong compacted earthen blocks. However, over the years, we have had considerable success with using almost any type of soil available on site by paying particular attention to adjusting the moisture-to-soil ratio that produces the optimal strength block.

Building the earthbags around temporary rigid box and arch forms creates door and window openings. After compaction of the keystone bags, the forms are then removed. Wood-strip anchors are installed during the wall-building process, providing an attachment for bolting on doorjambs, cabinetry or wood-frame intersecting walls, electrical outlets and plumbing systems.

Wall plastering options range from thick natural earthen plaster applied directly over the surface of the bags (yes, it sticks!) or, for additional protection, lime plaster can be applied over an earthen plaster. Cement/lime based plasters perform well when the earthbags are filled with a stable, well-draining sandy soil and applied over stucco mesh (chicken wire). Plasters can be applied by hand or sprayed on with a pressurized plaster sprayer for a unique contoured effect that accents the shape of the bags or tubes.

Earthbag Architecture can be designed to suit a wide variety of climates. Since the woven polypropylene bags are virtually rot proof, earthbags are an excellent choice for underground structures: root cellars, storm shelters, bermed homes and greenhouses. In climates where wood is scarce, whole houses can be built exclusively with earthbags including the foundation and roof, as is the case for corbelled earthbag domes. Earthbags also combine well with other natural building materials that can be combined together to create hybrid structures. Straw bales can be interlocked with earthbags to build sturdy arch entryways or to add thermal mass to the interior wall of an attached sunroom. Or we may choose to use earthbags for the sunken first level of a structure and then switch to strawbale, post-andbeam, cob or adobe brick for the rest of the wall above grade to make use of an available resource or add aesthetic variety.

The advantage of combining two alternative natural building mediums: load-bearing earthbag walls provide mega-thermal mass, while an exterior straw-bale wrap provides mega-insulation.

Insulation strategies for earthbag walls offer a variety of options. Narrow tubes provide a sturdy load-bearing wall with plenty of thermal mass, while straw bales secured to the exterior of the wall provide ample insulation. Now, we have mega mass coupled with mega insulation to provide the best use of both of these materials in one building. Another way to add interior mass is to build our interior walls with earthbags and our exterior walls with straw bales alone. Another approach we have experimented with is mixing a percentage of 3/4-inch pumice to a quality rammed earth soil that captures air spaces within the earthbag itself. A 50/50 mix of suitable earth and pumice make the bags one third lighter than their normal all dirt weight yet still makes a nice hard compacted earthbag.

Building codes

The advantage of combining two alternative natural building mediums: load-bearing earthbag walls provide mega-thermal mass, while an exterior straw-bale wrap
provides mega-insulation.

The earthbag building system has been extensively tested by Nader Khalili in conjunction with the ICBO (International Conference of Building Inspectors) and the Hesperia Building Department in Hesperia, California, at the California Institute of Earth Art and Architecture for earthquake resilience, loadbearing, and shear strength stability, all of which were proven to far exceed conventional code standard acceptance. (See Building Standards issue Sandbag/Superadobe/ Superblock Sept-Oct 1998 for a full article on the merits of Earthbag structural nitty-gritty).

Resources

Sources for bags and tubes can be found on the Internet under woven polypropylene feed bags. Our favorite U.S. supplier for both pillow-pack and gusseted misprint bags is www.innpack.com, toll-free 800.622.3695 in Tennessee. Typical prices for 50-lb misprints are approximately $.17 each (USD), and 100-lb bags are $.25 each (USD). Both come in bales of 1,000 bags. Smaller quantities for bags and tubes are available from a Kansas City, Missouri, source www.centralbagcompany.com 816.471.0388. Ask for Chris Klimek for prices and selection. Also try 800.521.1414 www.fultonpacific.com.

For step-by-step nitpicking details about building with earthbags, check out our book Earthbag Building, the Tools Tricks and Techniques by Kaki Hunter and Donald Kiffmeyer, New Society Publishers, 2004. Or call us at 435.259.8378, or visit our web site www.okokok.org.

Donald Kiffmeyer and Kaki Hunter have been involved in alternative construction since 1993, specializing in affordable, low impact and natural building methods. Inspired by the work of visionary architect Nader Khalili, the grandfather of Sandbag/ Superadobe/Earthbag architecture, they wrote a screenplay entitled “Honey’s House,” a film about truth, justice and affordable housing. From these innocent beginnings, they were launched into the alternative building movement where they were encouraged to share their combined innovations to establish the Flexible Form Rammed Earth technique. Together they co-authored the book Earthbag Building, the Tools, Tricks and Techniques by New Society Publishers. They live in Moab, Utah, where they continue to focus on the research and development of fun, quick, simple and solid natural and alternative building techniques that are inspired by this fabulous planet.

This is the second of a two-part article on creating a poured adobe or earth floor. See Earth Floor, TLS#52, for the first article describing how to prepare for and install a poured adobe floor.

By Tom Lander – New Mexico, USA

Now, weeks later after your floor is 100 percent dry, it’s time to seal and fill the floor with Linseed oil. Here in the South West our floors can dry in a matter of a few weeks but in humid climates error on the safe side.

Materials:

Linseed oil. We prefer raw linseed oil, less petroleum additives then the common boiled linseed oil but the boiled works if you are not concerned about petroleum out gassing. Even raw linseed oil has carcinogenic warning labels. Ask for an MSDS sheet. Linseed oil is made from flax seed.

Citrus Solvent (thinner) or mineral spirits, again petroleum out gassing

We are still learning how to estimate coverage and quantity so I’m not sure how much material is needed for your size floor. Maybe buy 2 gallons each for starters; you can buy linseed oil in 5-gallon lots.

Equipment:

4” paintbrushes, natural bristle is always best but pricey

Electric hot plate or gas camp stove

Large pot or kettle

Approved vapor mask

Safety glasses or goggles

Fan for air circulation/expelling fumes if you feel this is necessary

Rags,

Gloves

Prep floor:

Sweep or vacuum any loose debris and dust. You might want to do a light mopping or sponging. Give yourself time for the moisture to dry before applying the oil.

Procedure:

Heat the linseed oil to almost boiling (do not boil). We are just trying to heat the oil to aide in soaking, absorbing in. This must be done outside with caution, flammable. Another option is to pour the oil into a large deep baking pan, cover with a piece of glass and let it sit out in the sun. Leave an air gap. With either method start with a small batch to get the hang of heating and applying.

Transfer the oil into a suitable container. You can paint the material on or if you are quick, you can pour some onto the floor and swoosh it around with the brush. The only risk here is that you will not get an even distribution of material. Try it. Be consistent and watch how the floor is absorbing. If more than one person is applying, then you might get varying results but by the time you are done it shouldn’t matter. Use up your first small amount then decide how much more (a large batch) to heat for your next go at it. For reference keep track of how much material you use for each coat and offer this info to others.

The floor will soak up this first coat and there should not be any pooling of the oil on the surface. Plan your route of attack so you end up working yourself out the door, window or hallway. You should be able to go back to the start and do a second full strength coat right a way. Remember your shoes will be picking up dirt and dust from the outside so take steps to minimize this. There are disposable booties one can buy to cover their shoes.

What we are trying to do is seal the floor but think of it more like filling the floor. Filling all the little air voids between the sand and clay particles with oil.

The floor will dictate the timing and how much material. Watch how the material soaks in. You might be able to continue with more heated, thinned coats the same day, unless you are tired or sick from the fumes and not wearing a vapor mask.

Diluting:

The first two coats can be applied full strength. For the third and fourth coat combine 75% oil with 25% thinner, heat and apply. Watch the absorption, watch for pooling or puddling but also give the material some time to soak in; you just don’t want it to dry on the surface. Have a rag and thinner handy to wipe up any excess otherwise the material dries on the floor and becomes sticky. If this happens then it’s quite a job to use thinner and rags to clean the floor. Apply at least two coats of this first diluted mix.

Next is a 50% to 50% heated mix. Hopefully by now you have learned if pouring and brushing works for you (certainly faster) or just brushing or maybe it’s time now to just brush. Isn’t this fun learning as you go? Like all earthen materials, they tell you when and what to do, what’s the word? Experience.

Remember, oily rags and brushes are flammable so hang out to dry and do not leave a pile of rags unless it’s in the middle of a gravel driveway and you want to have some fun.

www.bioshieldpaint.com

Sunny Side http://www.gillroys.com

by Gernot Minke – University of Kassel, Germany

Note: This article is excerpted from Earth Construction Handbook (by Gernot Minke, WIT Press, Southhampton, Boston, 2000) which contains further information about weather protection, physical and mechanical properties of clayey soils, treatments and additives and modern earth construction techniques worldwide.

Figure 3.1 Testing samples of earthen plasters.

1) General. Earth plasters mainly consist of sand and silt with only as much clay as is necessary (usually between 5% to 12%) for developing their adhesive and binding forces. It is difficult to state what the proportions of an ideal earth plaster should be, because not only does the proportion of clay, silt and sand influence the properties, but also the grain size distribution of the sand fraction itself, the water content, the type of clay, the method of preparation and the additives. In order to test the appropriateness of earth plasters, samples with varied compositions should be tested. Earth plasters stick very well not only on earth surfaces, but also on brick, concrete and stone surfaces, if the surface is rough enough.

2) Preparation of substrate. As earth plaster does not chemically react with the substrate, the surface has to be sufficiently rough in order to develop a good physical bond. A good method of getting a strong bond is to wet it sufficiently until the surface is soft, and than scratch diagonally patterned grooves with a small rake or a nail trowel. In order to ensure that the plaster adheres better, it is also possible to use latching in the form of galvanised wire mesh, plastic mesh, reed mats, and such on the substrate before plastering.

3) Composition of earth plaster.

3.1 General. In order to get earth plaster free of shrinkage cracks, the following points must be kept in mind:

• The earth should have enough coarse sand.
• Animal or human hair, coconut or sisal fibres, cut straw or hay should be added (however, too much of these additives reduce the ability of the plaster to adhere to the substrate).
• For interior plastering, sawdust, cellulose fibres, chaff of cereal grains or similar particles can also be used as additives.
• In order to develop enough binding force, the adhesive forces of the clay minerals should be sufficiently activated by adequate water and movement.
• When the plaster sticks to a sliding metal trowel held vertically, yet is easily flicked away, the correct consistency has been achieved.

In order to test the characteristics of an earth plaster, a simple adhesion test can be carried out. The plaster to be tested is applied 2cm(3/4-inch) thick to the flat surface of an upright burnt brick. The plaster has to stick to the brick until it is totally dry, which might take two to four days.

If it falls off in one piece by itself, as seen in the left sample of fig. 3-1, it contains too much clay and should be thinned with coarse sand. If it falls off in portions after the sample is hammered on the floor like the second sample in fig. 3-1, then it has insufficient binding force and should be enriched with clay. If the plaster sticks to the brick but shows shrinkage cracks, like the third sample in fig. 3-1, it is too clayey and should be slightly thinned with coarse sand. However, it can be used without thinning as the first layer of a two-layer plaster. If the surface shows no cracks and the plaster does not come off when hammered, as in the fourth sample in fig. 3-1, then the sample might be adequate. In this case, it is advisable to make a larger test about 1×2m(40×80-inches) high on the actual wall. If shrinkage cracks now occur, this mixture needs either to be thinned with coarse sand or mixed with fibres.

3.2 Exposed exterior earth plasters. Exposed exterior plasters have to be seasonably weather resistant or must be given perfect weatherproof coating. It is important in cold climates that the plasters together with their coating have a low vapour diffusion resistance, so that water condensed in the wall can be easily transported to the exterior. The exterior plaster must be more elastic than its ground in order to meet thermic and hygric influences without cracking. In general, for cold climates, an external earth plaster is not recommended unless sufficient roof overhang, plinth protection and good surface coating can be assured.

Since plastered wall edges are very easily damaged, they should either be rounded or lipped with a rigid element. In extreme climates when the elasticity of large expanses of flat plaster is insufficient to cope with the influences of weather, vertical and horizontal grooves filled with elastic sealants are recommended.

3.3 Interior earth plasters. Interior plasters are less problematic. Usually they create no problem if they have fine shrinkage cracks because they can be covered with a coat of paint. Dry earth plaster surfaces can be easily smoothed by wetting and being worked upon with a brush or felt trowel.

If the surface of the walls demands a plaster thicker than 15mm(5/ 8-inch), it should be applied in two layers, with the ground layer containing more clay and coarse aggregates than the second layer. If the ground layer gets shrinkage cracks, it is not problematic, but could actually help by providing a better bond to the final layer of plaster.

Adding rye flour improves the surface against dry and moist abrasion. The author has proved by testing that this resistance can also be built up by adding casein glue made of one part hydraulic lime and four to six parts fat-free quark, borax, urea, sodium gluconate and shredded newspaper (which provides cellulose fibre and glue). The mixes in the accompanying chart worked well.

Lime reacts with the casein within the fat-free quark forming a chemical waterproofing agent. A similar reaction is obtained with lime and borax (which is contained in the shredded newspaper). Sodium gluconate acts as a plasticizer so that less water needs to be mixed for preparation (thereby reducing the shrinkage). Urea raises the compressive and the tensile bending strength, especially with silty soils.

Waste paper shreds lead to better workability and reduce shrinkage. The mixes B, C and E showed best workability. When using mixes A and E, it is preferable to first mix the casein glue and the shredded newspaper together with the water, and then, after an hour, add earth and sand.

With all mixes, it was found that the final smoothing of the surface, which was done by a felt trowel, was best done after several hours or even a day.

4) Guidelines for plastering earth walls. As pure earth plaster does not react chemically with the substrate, it might be necessary to treat the substrate suitably so that sufficient bonding occurs. The following guidelines should be kept in mind:

1. The surface to be plastered has to be dry, so no more shrinkage occurs.

2. All loose material should be scraped off the surface.

3. The surface should be sufficiently rough and, if necessary, moistened and grooved or the mortar joint chamfered, as described in section 2.

4. Before plastering, the substrate should be sufficiently moistened so that the surface softens and swells and the plaster permeates the soft layer.

5. The plaster should be thrown with heavy impact (slapped on) so that it permeates the outer layers of the ground and also achieves a higher binding force due to the impact.

6. If the plaster has to be more than 10-15mm(3/8-5/8-inch) thick, it should be applied in two or even three layers in order to avoid shrinkage cracks.

7. To reduce shrinkage cracks while drying, the mortar should have sufficient amount of coarse sand, as well as fibres or hair.

8. To improve the surface hardness, cow dung, lime, casein or other additives should be added to the top layer.

9. In order to provide surface hardness and resistance against wet abrasion, the surface should be finished with a coat of paint[Editor’s Note: breathable paint].

10. While using plasters, the change of physical properties caused by additives and coatings should be kept in mind especially with respect to vapour diffusion resistance.

5) Sprayed plaster. A sprayable lightweight earth plaster with high thermal insulation containing shredded newspaper was successfully developed by the author in 1984. This plaster can be applied in a single layer up to 30mm(1-1/4-inch) thick using an ordinary mortar pump. In order to get a shorter curing period, some high-hydraulic lime and gypsum was added to the mixture.

6) Thrown plaster. Fig 6-1 shows how a traditional African technique, which consists of throwing earth balls on a wall, has been adapted. Here, this technique is used on a wood-wool board for a winter garden wall. In order to increase the adhesion, bamboo dowels were hammered halfway into the board.

7) Wet formed plaster. As loam plaster retains its plastic state for a long time and is not corrosive to the hands like lime or cement plasters, it is an ideal material for moulding with the hands. Fig. 7-1 shows an example of an exterior loam wall stabilised by a lime-casein finish.

Professor Dr.-Ing. Gernot Minke is a professor at Kassel University and a consultant structural engineer since 1967. He has a keen interest in earthen structures and low-cost, low-impact housing. He numerous publications include the Earth Construction Handbook (WIT Press, Southhampton, Boston, 2000). Contact: <feb@architektur.unikassel.de>

This article was submitted by Friedemann Mahlke, a student of Dr. Minke and a straw-bale builder and researcher. Contact <mahlke@architektur.unikassel.de>

by Tom Lander – New Mexico, USA

We 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 it’s 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 it’s not necessary.

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.

Prep Work

Let’s 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. I’ve 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.

I’m 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; it’s 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. Don’t 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 it’s 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.

Installation

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 debris.

It’s 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. Don’t 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 doesn’t it? You’re not done yet. More steps involved as the floor begins to dry. A word of caution about drying, it’s 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.

Hard Troweling

Now it’s 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, it’s time to seal and fill the floor with Linseed oil. We will address sealing in our next article.

Tom and Satomi Lander have been involved with natural building since 1993 and began teaching straw-bale building and earth plaster in 2003.

They can be reached at <tom@landerland.com> www.LanderLand.com

Visit www.LanderLand.com for color images and earthen floor workshop information.