Earthen floors - introduction

“Heaven is under our feet as well as over our heads.” ― Henry David Thoreau, Walden

What are earthen floors?

[Main image: 50m² Earth floor in a roundhouse yoga studio. Pic: Earth Floors UK.]

An earthen floor (or adobe floor in the US) is a floor made of earth that has been compressed, polished and oiled. It relies on the sticky binding properties of clay – that most versatile of building materials. Clay expands when wet, creating sticky platelet particles, like little suction cups. When coarse sand is added, compressive strength is increased and shrinkage reduced as the clay dries. Fibre is added to increase tensile strength and knit everything together, like reinforcing steel bars in concrete. The finish mix is essentially the same as adobe or cob, and the installation is similar to a concrete slab, only without the environmental impacts or cold nature of cement.

Throughout history, most homes have had earthen floors – usually just rammed earth, often with a layer of straw for warmth, and that could collect dirt and be swept out and replaced. Mint was sometimes strewn on the floor too, and treading on it helped spread a minty smell around the room.

A typical modern earthen floor involves a build-up of layers, each with a purpose. Each layer is described below, including the function it serves and (in some cases) alternatives you can experiment with. The example here demonstrates a floor installed “on grade”, similar to a concrete slab. This is a basic method that performs well and is forgiving to install. You can also install the adobe on a framed floor, but be sure to stiffen the joist framing to minimize flex, otherwise your floor will crack over time.

What are the benefits of earthen floors?

• They have a warm, natural feel, but are also cool in summer.
• Earth is a natural, biodegradable, non-toxic material, with no energy required for processing.
• The earth can be sourced on-site, reducing transport distances and therefore fuel.
• The (main) material is free.
• Lower embodied energy than any other flooring, and hugely less than concrete slab.
• They invite creativity – objects or other natural materials such as stones can be embedded in the floor.
• Thermal mass can reduce heating bills.

Step-by-step guide to the installation of a sealed earthen floor in Oregon.

What can I do?

First, read more. Our advisor Sigi Koko recommends Earthen Floors by Sukita Reay Crimmel. 

Below, Sigi describes one method of installing an earthen floor – but there are many more. Read a bit around the subject to gain an understanding of the function that each layer serves. You can adjust and experiment while being sure to do the most important thing – keep moisture out of the floor.

So, starting from the bottom up…

1. Substrate

The floor has to rest on solid ground (or a stiff framed floor with little flex). If you have movement below, the floor will crack. So you want strong, compacted earth to build on, free of topsoil or organic matter, since it will continue to decompose and shrink in volume over time, leaving you with voids below your floor. Dig down until the soil feels solid and compact. You can even go over the ground with a hand tamper to ensure compaction.

2. Capillary break

Install several inches of gravel to provide a capillary break that prevents any water that may be in the ground from rising up into the floor. 10-15cm of pea gravel or angular 2cm stone.

3. Vapour barrier

Lay a vapour barrier that covers the entire floor area. This provides your final moisture control for the earth, blocking any air-borne vapour (from evaporated ground moisture). 6mm polyethylene sheeting works – exactly what’s used to prepare a concrete slab. The plastic is obviously not natural, but it’s excellent insurance.

4. Insulation

This is the layer that is most often left out, but it can dictate energy performance and comfort. If you live in a hot climate, skip the insulation, because a cool floor is beneficial. But if you heat your building, you want to keep the heat inside. If you don’t have insulation below your floor, then you are, in effect, heating the endless thermal mass of the ground below. I use R-10 insulation for a typical floor and bump up to R-15 if the floor will have radiant heating in it. You want that heat to follow the path of least resistance into your space, not down into the ground. And you need that insulation to be non-biodegradable, otherwise it will compost under your floor and disappear over time, leaving you with a cracked & heaving floor. This is one place where I’ll use rigid foam, since the reduction in energy over time quickly offsets the impact of the foam manufacture. For a natural alternative, you can use a rigid insulating mineral, such as pumice or perlite.

5. Base layer

Two layers mean that I can pour the thick base layer before the exterior walls are completely closed in. That extra air-flow speeds up drying time. I use the same proportions I would for cob, which is generally 15-25% total clay plus 75-85% sharp sand – but I add lots more water. With cob, a wet mix means you can’t build higher until the material dries. But with a floor, you want to be able to pour it, like a really thick liquid (about the consistency of chocolate pudding.) Sifting clay from on-site through a 1cm screen is fine. To this mix, add lots of long fibre (straw, or whatever is abundant in your region). This knits the floor together and helps prevent cracking.

More details on installing the base layer.

Underfloor heating pipework can be laid in the base layer.

Explanations of different kinds of plastering trowel.

6. Leveling layer

There are a few approaches you can take for this layer. I float this layer smoooooth and then polish it as it hardens. Some people float this layer reasonably smoothly, and then apply a super-thin layer for the desired texture. By polishing this layer as your finish floor, you avoid the extra step of additional layers. The benefit of applying a thin final layer is that it’s easier to control the texture of a thin coat than a thicker poured floor. You decide which is less stressful to you. Either way, the goal of this layer is to end up with a floor that is level (and if it’s your finish, then level & smooth). In the first layer, small imperfections are not a concern, but this layer is what you will see for years to come. This second layer uses the same mixture as the base layer, with two modifications:

1. if using clay soil from the site, I sift it a bit finer – 5mm screening
2. any straw in this layer should be chopped to 2cm or shorter

This is because smaller particles allow you to float the floor to a finer finish. I don’t always use straw in this layer of the floor, since it will show in the finished surface and not everyone likes that look. If you eliminate the straw, then it’s crucial to get the ratio of clay to sand perfect. To do this, make some test patches on your floor and see how they dry. If they’re crumbly and weak, there’s not enough clay. If they crack, not enough sand.

More details on installing the leveling layer.

7. Finish

There are several options for finishing your beautiful adobe floor. (Have you noticed that there’s never just one way with natural building?)
If you chose to apply a thin final layer of clay, you have 2 options:

1. apply 3mm finish layer, much like a clay plaster, but applied to the floor; when this layer is dry, seal as described below (more info and recipe).
2. apply 1 to 4 coats of clay paint, also call “alis”; when this layer is dry, seal as described below (more info and recipe).

If you choose to burnish your floor, then as soon as the clay is completely dry, you can apply a sealer. The sealer densifies the top layer of clay (making it more durable & scratch-resistant), prevents dusting (so you don’t get clay on your butt when you sit on the floor), and reduces absorption (so makes it more stain-proof & easy to clean). The most common sealers are hardening oils, which react with oxygen in the air to chemically change into a hard, transparent, water-resistant, but breathable resin. Hardening oils include linseed oil (from flax seeds), hemp oil, tung oil, walnut oil, etc. The oil is applied in multiple layers, and each subsequent layer is thinned with a solvent to promote deep absorption into your floor’s surface. Common thinning solvents include citrus solvent (d-limonene based, i.e. orange peel) or mineral spirits (petroleum based). There are alternatives to oil & wax sealers, so feel free to do additional research / experimentation on that. (The most intriguing of all sealers to me is cow urine. Traditionally used in parts of Africa, it apparently makes for a stunning floor.) You can also add pigment to your sealer if you want to enhance the colour of your floor.

Here are the layers I most commonly use to seal a floor:

• 1st coat – pure hardening oil (1 litre covers approximately 4 square metres when applied full strength)
• 2nd coat – 80% hardening oil with 20% thinning solvent
• 3rd coat – 60% hardening oil with 40% thinning solvent
• optional 4th coat for high traffic or damp areas – 40% hardening oil with 60% thinning solvent
• optional beeswax paste to finish – if you want a really luscious surface that feels like leather, then I recommend a final coat of beeswax paste buffed into the surface of your sealed floor

Note on smells: the oils harden by oxidising, a chemical process that offgasses an aldehyde compound. Aldehydes are technically volatile organic compounds (VOCs), though the particular compound offgassed from oils has extremely low toxicity (unlike their cousin, formaldehyde, which is carcinogenic). However, there is definitely a smell that can linger (especially from linseed oil) and some people with respiratory issues may experience discomfort.

And what not to do…

1. walk on them in stillettos (and probably, a ‘shoes off’ regime is best).
2. install them in rooms where they’re likely to get wet – like bathrooms.
3. drag heavy furniture around on them.

Thanks to Sigi Koko of Build Naturally for information, and Sigi and Earth Floors UK for images.

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Sigi Koko is the principal designer at Down to Earth Design, which she founded in 1998 to help her clients manifest their dreams of living in a natural, healthy home. She also teaches natural building workshops that empower her clients to contribute creatively during the construction of their own home. You can find out more about her work on her website and blog.

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The views expressed here are those of the author and not necessarily lowimpact.org's

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