Foundations

This article was originally printed in Issue #52, 2005

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 <[email protected]> www.LanderLand.com

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