Issue 67

By Kyle Holzhueter

Traditional Japanese earthen floors are called Tataki. The pronunciation is similar to the Japanese verb to pound, tataku, as these floors are pounded similar to traditional earthen floors in many other countries throughout the world. However, the word Tataki is written in Chinese characters as 三和土, meaning literally “three and earth” or “three harmony earth”, which reflects the fact that Japanese Tataki floors consist of three ingredients, earth, lime and bittern, a byprod the creation of a traditional earthen floor in Northern Japan as conducted by Keisuke Noda-san of Noda Plaster Works.

The floor is completed in stages. First, a 100-150mm layer of crushed gravel is compacted creating a stable and level base for the earthen floor.

Ink lines are snap to identify layers and the finish height.

The earthen floor is 100mm thick and consists of three layers. The three layers are applied, evened, pounded and compressed in the same day so there are no cold joints between layers. The number of laborers is adjusted to ensure that the entire floor can be completed in one day.

In Japan, the dimensions, techniques and tools used vary by region. In some areas, an 80mm earthen floor is applied in roughly two 40mm layers, or 50mm thick floor in two 25mm layers.

The Mix

As mentioned above, Tataki consists of three ingredients: earth, lime and bittern.

Although the proportions depend on the qualities of the materials, the following proportions are a good place to start:

Sandy soil 100g

Hydrated lime 10g

Bittern 1g

In practice, aggregate may be added to balance clayey soils and the amount of lime depends on the size of aggregate and proportion of clay in the soil. A greater clay content will require more lime.

The following is the recipe developed by Keisuke Noda-san using entirely local materials in Hokkaido:

Clayey soil 1kg

Volcanic sand 3.5kg

Hydrated lime 150g

Calcium Chloride 150g

Red clayey soil 100g (for color)

When mixing, first the volcanic sand, dry powder clayey soil and hydrated lime are mixed dry. Because clayey soil and lime have a tendency to stick to mixers, the volcanic sand is added first. Next the clay is added and after that the lime. Lastly the red clay is added.

Separately, the calcium chloride is mixed with water, approximately 2g of calcium chloride to 1g of water.  The calcium chloride will give off heat when mixed with water and care should be taken. When the calcium chloride is completely dissolved, the mixture is added to the dry ingredients.   Because the finish mix is relatively dry, the dissolved calcium chloride should be added slowly and well dispersed in the mix.

The amount of water is very important for a number of reasons:

1. To prevent cracking: If too much water is added, when the water evaporates, it induces shrinkage which could lead to cracking.
2. To improve workability. If too little water is added, the mix will not compress sufficiently. The mix should form a solid ball when squeezed in one’s hand. However, if too much water is added, the entire mix becomes viscus, acting like a liquid, so that when one area is pounded downward another area rises.

Tools

Noda-san has developed a variety of tools for pounding.

Tataki specialty trowels consisting of thick honyaki steel produced by Yamanishi-san and available from Kiwado – Japanese Plastering. Heavy compression with regular trowels would cause the trowels to bend out of shape.

Simple lumber and hammer: next to walls and other difficult to reach areas, a metal hammer can be used to hit a narrow piece of lumber, which will compact the earth under the lumber.

Application

To construct one layer, roughly 50mm of material is spread evenly on the floor. Screeds and guides can ensure the mix is applied evenly.

The 50mm of material is then pounded and compressed to 30-33mm.

Again, special care must be taken where the floor meets walls or difficult to reach areas to ensure proper compression.

A network of Iguasa rushes is sandwiched between layers. This helps prevent large vertical cracking.

The same process is repeated for the second and third layers.

After the third layer has been completely compacted, the final finish can be decided. There are several options:

1. For a more rustic, traditional appearance, leave the floor with an open pour finish. This finish hides future scratches and wears and tears. Regularly watering and sweeping the floor results in the “cement-like” earthen floors found throughout the world.

2. Immediately after completing the third layer, mist or lightly water the surface of the floor and compress the surface. Additional material can be added in depressions to create a flat surface. The compressed finish results in a harder surface more water and abrasion resistant. This floor is quite comfortable barefoot. Although future scratches and wear and tear will be more prominent, these areas can be re-wet and compressed again.

The area of the wall which meets the floor is cleaned with a wet brush to create clean lines.

After compression, the floor is protected for approximately one month.

In this case, to ensure slow and even drying, the floor is covered with loose straw and then straw mats.

Lastly, a dilution of bittern is sprinkled over the straw mats.

In cases where the floor must be walked on immediately, protective flooring can be used as seen in the photo above.

Many thanks to Keisuke Noda-san of Noda Plaster Works for sharing his experience and knowledge.

Kyle works as a builder, consultant, researcher and educator specializing in natural building materials such as straw bale, light straw clay and natural plasters.  He has a PhD in Bioresource Sciences from Nihon University where he researched the hygrothermal environment of straw bale walls in Japan and building practices to control moisture.   Apart from academia, Kyle has studied natural farming in Japan, permaculture in Australia, and organic and biodynamic farming in the US.   Further details can be found at the following links: http://holzhueter.blogspot.com  http://japaneseplastering.blogspot.com