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Tamped Earth Floors
Unstabilized Earthen Floor Using Road Base
by Frank Meyer

[Note: This technique by Frank Meyer is a alternate method for making earthen floors. Tamped earth floors dry much faster than poured adobe and also cracks less.]

A few years ago, when asked to make an earthen floor, I started by collecting all the soil types available in our area. Austin, Texas is situated in a geologically diverse area, so I got samples from several places including the building site, my backyard and all the commercially available dirts, loams and road bases. After weeks of playing in the dirt, mixing in stabilizers and trying to strengthen and harden the earth to make it suitable for a floor, I came to an interesting conclusion. One particular road base would make the hardest, prettiest and quickest floor without any stabilizers at all. Its rich red color and excellent blend of silt, clay, sand and gravel made test bricks that were more impressive than anything else I came up with.

Not all road bases are created equal. The one I chose is what is known here as city base. It has been approved by road building engineers for use in building roads and streets in Austin. (A quest that seems to have no end). It originates in a quarry where the material is taken from the earth and the silt, clay, sand and gravel are separated, then reblended to proportions specified by engineers. It has a plasticity index of 12 which relates to the way it shrinks and swells. It costs $2.48 per ton plus hauling. A 500 sq. ft. floor will require two twelve-ton truckloads costing $200-$300 delivered.

I recently set out to do a floor using site-available clay. We did several tests, both stabilized and unstabilized. Though the owner of the building was adamantly in favor of using the "dirt under his feet," after comparing the color and hardness of our site tests with road base tests, he changed his mind and we are presently working on his floor using road base.

By using this material, and applying basic roadbuilding techniques, we have a big advantage over traditional poured earth floors. The process uses relatively little water, thereby requiring a much shorter drying time. It can typically be walked on a day or two after installing.

Before proceeding with the installation process, it is necessary to consider vapor barriers. I have done earth floors with and without vapor barriers. Unfortunately, for the sake of observing their performance, conditions here have not varied enough for me to form an accurate opinion, as drought conditions have persisted recently. My feeling is, if in doubt, use a vapor barrier. If your location is high and dry and you want your floor to "breathe," consider not using one.

To begin the process of building our floor, the ground should be fairly level, smooth, compacted and at least 6" below the planned finished height. If a vapor barrier is being used, a layer of sand should be spread an inch or two thick below and above the vapor barrier. This prevents gravel from puncturing it. The first layer of road base is now applied about 2" thick and wetted with a hose or watering can. There is no need to soak it. Just get it wet enough so the silt and clay stick to the aggregate. It must then be compacted. A plate compactor, available at tool rental outlets, works well. Be sure to vent the building with fans if using a gas-powered compactor. Hand-tamping is slower but much quieter and without fumes. Hand-tamping the edges works best even if a powered compactor is used. Wooden tampers are easily made or a steel plate welded to a piece of pipe works nicely. Another layer just like the other follows again and again until we are about 1" from our finish height. When estimating the amount of material to use, figure it will lose about one-third of its original volume when compacted.

At this point we need to level the floor. First we screen out the large aggregate from the base. Using 3/8" hardware cloth or similar screening material, we sift the road base. A simple wood frame with the screen attached works well. Having removed the large aggregate, we are left with a mixture of silt, clay, sand and gravel no bigger than 3/8". This is layered on and leveled off using screed boards much like concrete workers use. By dragging our screed across two level boards set at finish height, we knock down the high spots and fill in the low spots. This process can also be applied to each layer of base thereby controlling the level layer by layer.

After leveling, this mix must be wetted again and tamped, making sure that it bonds with the layer below. For the top coat we screen the 3/8" material again, this time using 1/8" hardware cloth. This fine mixture can be applied dry and wetted like all the preceding layers or mixed wet and troweled on. The most important thing at this point is to make sure it is bonding with the layer below. A little extra water on the coat below the top coat seems to help. Also, scratching through with a trowel will help the two layers bond. I have had better success applying the top coat dry. The top coat is not compacted, but hand-troweled and burnished, using enough water to make it bond and be workable. Trial and error will give you a feel for it.

We seal our floor only after it is thoroughly dry. In the summer in Texas thorough drying may take only a few weeks. In cooler, moister climates it may take several months. The floor can be used during this drying period. Cracks from drying, and any chipping or gouging that may occur while drying, can be patched by again screening the road base, this time using window screen. What is left is a fine powder of silt, clay and fine sand. This mixture makes an excellent patching and burnishing material. It can be sprinkled on, wetted and blended in with the floor. The more time and patience you have at this point the better your results will be, as burnishing with a pool trowel brings out the natural beauty and character of the earth.

The best sealer I have found is boiled linseed oil, thinned with turpentine and brushed on in several coats. (The odor dissipates in a week or two.) I also used Palmer 9400 Impregnant (Phone # 800/252-3690, 800/421-2487 or 301/898-7848) on one floor and sodium silicate on another.

This is an all-natural non-toxic floor that has relatively low embodied energy. With time, patience and an affinity for getting dirty, anyone can do it. Enjoy!

Additional Construction Notes:

- total thickness of floor is 5-6

- place in 2 layers (1 after compaction) with each layer containing progressively finer material

- final layer is thick and sifted through window screen

- use only enough moisture to get it to stick

- extra sand and finely screened straw can be added to top coat

- road base has clay in it already, so there is no need for additional clay

- do not make grooves in earth floors because the edges degrade

- a moisture barrier below the floor is recommended when in doubt

- radiant heat tubing can be placed in sand 1-1/2 below surface (be careful that aggregate does not puncture it)

- seal floor after 6-12 months when totally dry (bone dry)

- wax is not necessary the preferred method is to apply extra coats of linseed oil

- heat linseed oil outside as hot as possible in a double boiler

- combine with Citrosol or odorless mineral spirits 4 or 5 to 1 (aggregate mix varies so linseed oil and thinner mix varies)

- the first one or two coats of linseed oil are not thinned, each progressive coat afterwards is gradually thinner

- saturate floor with linseed oil, then wipe up excess

- each coat must dry thoroughly before applying the next; multiple coats can take weeks to dry

- final coats of linseed oil are extra thin for fastest drying

Question: So Frank, how are these floors holding up?
Frank Meyer: The linseed oil finish if done well over a good clay/sand floor holds up very very well.

Question: Any other suggestions?
Frank Meyer: I now mix my own rather than screen road base but I still use the base for the sub layers. Only the top inch or so is it necessary to use finer material. Experiments and experience are the keys.

Frank Meyer of Thangmaker Construction has over 25 years of construction experience. His focus is sustainable and green building. He specializes in straw-bale construction and earth floors and is available for consultation and workshop facilitation. 904 E Monroe, Austin TX 78704; ph 512 282-2341; [email protected]

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