Earthen Floors:
Higher Ground

The subfloor is just as important to get right as the top layer.

Earthen floors have been used in buildings for thousands of years. As well as an effective way to passively heat and cool your home, they’re aesthetically beautiful, kind to the environment and delightful to walk on.

It’s estimated between ten and 30 percent of the world’s population still live in earthen structures. Earthen floors range from bare and dusty earth all the way through to perfectly levelled modern versions with a gloss finish.

Earthen floors connect you to the natural environment and can be used in most construction situations that would otherwise call for a concrete slab. There are three main reasons you’d consider one in your home; an earthen floor’s response to passive solar design, their positive environmental impact and their numerous health benefits for the home’s inhabitants.

Passive Design

One aspect of designing your home using passive solar principles is to include mass and insulative materials in the correct locations to reduce heating and cooling bills, therefore reducing your environmental footprint. An earthen floor is considered a mass material, dense in nature with very few air pockets. Large and dense areas act as a thermal ‘battery’ when used effectively, areas which catch and store the surrounding temperature for later use when you’d otherwise be switching on artificial heating or cooling options.

With careful design focusing on orientation, the placement and size of windows and the size of the structure’s eaves, a building can allow winter sun to land on the floor during the middle of the day, which then stores and releases the heat when the sun goes down and the temperature drops. Likewise, with the same design considerations preventing hot sunlight from hitting the floor in the middle of summer, the floor stays cool and plays a large role in regulating the indoor temperature.

Cob is created using clay, sand and fibres.

Materials

After water, concrete is the second most used resource in the world and its production accounts for between four and eight percent of the world’s greenhouse gas emissions. The energy associated with its production, as well as its transport and use in construction, makes concrete a particularly unsustainable building material. In comparison, earthen floors have a seriously low carbon footprint, and that becomes significantly smaller again if you can source materials directly from the site you are working on. 

In most residential buildings in Australia, wherever an interior concrete slab would be laid, an earthen floor could take its place. Exceptions would be wet areas like a bathroom, laundry and directly beneath the kitchen sink, or the areas of your home that receive the highest amount of foot traffic. Because the floor is primarily made from natural materials, most of which you can dig straight out of the ground you’re building upon, these floors are great for human health. Clay produces negative ions which release into the air, and these floors ‘breathe’ allowing them to absorb and redistribute the air’s moisture. When designed and built correctly, these floors will create rooms free of dampness.

The Techniques

There are so many recipes and techniques to build an earthen floor, but the technique outlined here is similar to the teachings of Sukita Reay Crimmel who co-wrote the 2014 book Earthen Floors, a Modern Approach to an Ancient Practice.

Earthen floors are generally installed in new homes and on ground level, however some people have successfully installed them on second floors, and have even retrofitted a thin earthen floor over the top of an existing concrete slab. Both of these options are possible but it’s important to note they do require a different method to what’s outlined here in order to be successful.

The most sustainable clay will be dug from the site.

The Ingredients

The four basic ingredients of an earthen floor are clay, sand, straw and gravel. To make your earthen floor as sustainable, healthy and as rewarding as possible, try sourcing the clay from your property’s subsoil.

To see if your subsoil is rich enough in clay content to be able to collect and use on your floor, dig below your topsoil to reveal what’s beneath. Then take a shovelful of subsoil and place into a bucket in order to carry out some tests. There are many different ways to test clay content, but start by adding a small amount of water and break down any hard lumps by squishing the soil between your fingers. Once malleable, first roll it into a ball, then roll it out into sausage shape before forming it into a doughnut. If you are able to create the ring without any cracks appearing, it’s likely you have a clay content that’s appropriate to use in your mix to create an earthen floor. If you cannot find clay-rich subsoil on your property, then you may need to purchase bagged builder’s clay, which also works well.

Plan Your Construction

More complex than simply a layer of smooth clay, an earthen floor is made up of three or four important layers. Foundations are hugely important in any build and beneath every successful earthen floor is a stable and compact subfloor. The easiest way to plan your layers is to decide the height you want your finished floor to be and then work out your layers backwards from there. It’s a good idea to mark the heights of the layers around the perimeter of the room so you know exactly what height you need to work each layer to.

Take time to carry out tests to get the perfect ratio;

Test Your Mix

The top layer of your earthen floor when wet is called cob and, because it’s such an important part of the process, conducting tests of what will be your final mix is crucial to achieving your desired outcome.

As well as the clay, which binds everything together, a cob requires sand and fibres for strength. Chopped straw is a good option for the fibre component, but similar fibres like sugarcane mulch will work, too. Whatever you decide on, it’s critical to get the ratios spot on, which is why you must carry out tests.

A good ratio to start testing is one part clay soil to three parts sand, adding an additional 10 percent of fibres and a little water as you go – how much you need will depend on your materials. Using different ratios to determine which mix will be strongest, form a series of balls. If a ball is dusty and crumbly, you’ve got too much sand, if it’s hard and cracking, there’s too much clay.

Once you’re confident your ratio is close, frame up a square to the same depth as your final floor layer. Lay the mix down and leave it to dry naturally while you prepare the subfloor. Any cracks or significant shrinkage will determine whether you need to alter your mix.

Working quickly will achieve a consistent result.

Preparing The Subfloor

Start with a 150-millimetre layer of tamped gravel for drainage, followed by a 20-millimetre layer of tamped sand. Next comes an optional vapour barrier layer which is recommended to protect your floor from rising damp. It’s a layer of builder’s plastic which is sealed at the joins and sandwiched between two layers of geofabric. If you wish to avoid using of plastic and are confident there’s no risk of damp, you can omit this step.

Next you’ll need a 75-millimetre base layer. Road base, made up of various-sized aggregate and clay soil, when dampened and compacted becomes a really strong foundation for your floor.

The Top Layer

By now you’ve worked out the optimum ratio for the materials so, using a 20-litre bucket as your ‘part’ measure, combine the ingredients on a large tarp, add the water and stomp the mix together with your bare feet. This is then piled on to the prepared area in batches, and smoothed out using a concreter float. You can lay timbers cut to the same thickness to use as screed rails which will ensure a level surface, but be aware the timbers will absorb moisture, so pull them out and back fill once you’re happy with the level. How much you work the surface will depend on how smooth you want your finished floor.

A well-finished earthen floor can feel ‘soft’ underfoot.

Finishing Seal

The final step is the oiling and waxing process. When the floor is oiled it hardens and becomes more water resistant, creating a floor that should outlive its occupants. There are many options for sealing your floor, but one part mineral solvent mixed with three parts linseed oil, and applied in three coats, is what was used here. It’s a good idea to undergo thorough research before deciding on your oil component, as some boiled linseed oils contain synthetic drying agents. Depending on how smooth you worked the surface, once the oil has dried, you can polish your floor using a mix of beeswax and linseed oil.

A well-finished earthen floor will be resistant to general wear and tear, but will be susceptible to scratches much like a timber floor. It is a good idea to keep a small bucket of your cob in the freezer for any touch ups that may be required. And reapply the oil every year or two to keep it looking its best.

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