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No. 65, Fall 2014

Getting Grounded on an Adobe Floor

By Sigi KokoOctober 20, 2014May 9th, 2022No Comments
By Sigi Koko

This is the first article in a two-part series on adobe floors.  In the next issue Sigi will walk us through the installation process of an adobe floor.

Why Would You Want A “Dirt” Floor?

The truth is, you probably wouldn’t want to live on a dirt floor. A dirt floor implies the lack of any finished floor…where the unrefined ground acts as your floor. Plain dirt is difficult, if not impossible to clean, and persistent dampness is likely (due to moisture rising from the ground below). Chronically damp surfaces can pose a host of health issues, including toxic forms of mold. A dirt floor has a connotation of poverty or is used purely for utilitarian (not living) spaces.

Adobe floor in a strawbale cottage in West Virginia

Adobe floor in a strawbale cottage in West Virginia

But this article is not about dirt floors

What we’re talking about here, is an adobe floor system…made from earth, but with specific material ratios and with controlled installation processes. And the difference is not nuanced. An adobe floor, also called an earthen floor, is lusciously beautiful, completely non-toxic, and quite durable.  The floor is easily maintained, cleanable, and promotes a healthy indoor space. And best of all, in many regions adobe floors can be made using local clay soil. (Which makes them dirt cheap…sorry, I couldn’t resist!)

What exactly is an adobe floor?

An adobe floor uses the same materials as its namesake “adobe”: clay, sand, and fiber (most commonly, straw). The integrity of the floor relies on the sticky binding properties of clay…one of the most versatile building materials I know of.  Clay expands when wet, creating fat, sticky platelet particles, that act like suction cups.  As the clay dries, those suction cups stay connected, creating a strong bonded material. But, if you use pure clay for a surface as big as a floor, the clay shrinks significantly as it dries, which means it cracks everywhere.

To control the shrinkage and cracking as the clay dries we add lots of sand to the mix. The sand does not expand when wet or shrink when dry, so it reduces the total amount the adobe will change in volume, wet versus dry. The sand also increases compressive strength, which means the floor is stronger.  The sand should be angular in shape (not smooth) and should include a variety of sizes, up to ¼-inch across and down to fine particles. This way all of the sand interlocks, with the clay in between to bind it all together.

Finally, we add some fiber, such as chopped straw. The fiber knits everything together…similar to reinforcing bars in concrete.  Alternatives to straw include dried grasses, horse or cow manure, and even recycled paper pulp. Anything that will give your floor additional tensile strength…which is the ability to withstand movement and internal pulling forces without cracking.

The finish mix is essentially the same as adobe or cob

The difference between adobe bricks (or cob) and an earthen floor, becomes clear when it comes to processing and installation. Cob involves using the adobe mixture to sculpt thick walls. Adobe bricks are prepared by pouring the mixture into oversized brick forms, baking the blocks in the sun, and then building a masonry wall using the bricks and adobe mortar. For a floor, we use essentially the same adobe mixture, but pouring and floating it to form a thick, monolithic slab.

The installation of an adobe floor applies the same skillset used to pour a concrete slab. The wet adobe is poured in place and leveled. As it begins to dry, you “float” (smooth) the surface, creating a continuous floor slab. The “curing” process is just evaporation of the moisture. As the floor dries, you can burnish the surface as smooth as you like. Once the floor is completely dry, you seal the adobe to densify the finish surface and ensure that it’s easy to clean.

The finished floor

The finished floor

Since there is no cement in the adobe floor, you avoid the environmental impacts and cold nature of cement. Instead, you have a warm, grounding floor, with a super low eco-footprint…that begs you to walk barefoot.

What it looks like…

What the actual finished adobe floor feels like depends on how much you trowel and burnish the surface as it dries. Burnishing tightens the particles together, reducing the micro-pores in the surface. The tighter the pores, the smoother the finish feels. You can make your floor feel smooth like leather with modest skill and a bit of patience. (I recommend practicing on smaller test panels to get your technique down, before committing to an entire floor.)

The color possibilities are as expansive as your imagination. A basic adobe mixture takes it’s pigmentation from clay soil, which means color ranges from red to brown, green to yellow, purple to gray. You can enhance or change the color by adding pigments to the mix or staining the surface when your floor is dry. The sky’s the limit for color possibilities.

You can also embed stone or tile into the surface to create additional patterns. Just be sure to finish the surfaces flush, so you don’t stub any toes.

Where (and where not) to install an adobe floor

An adobe floor can suit nearly any use…and yes, that includes kitchens and bathrooms. Contrary to intuition, adobe floors can get wet…they just can’t stay wet for prolonged periods of time. Wet materials deteriorate faster, harbor pests, and promote mold growth. Areas prone to surface spills are no problem, because a spill is easy to wipe up. What you want to avoid is consistent moisture over time. This means a floor poured directly on the ground needs to include a capillary break and a vapor barrier, to keep out rising ground moisture. (More on this in the next issue…)

Don’t install an adobe floor if you are in a hurry. The thicker the floor, the longer it takes to dry…which can be anywhere from 2 to 8 weeks. Additionally, the sub-surface under an adobe floor needs to have very little flex. Any movement can cause the finish floor to crack. If you plan to install an adobe floor over framing, I recommend stiffening the floor joists to minimize flex.

Do install an adobe floor if you are looking for a non-toxic floor option that is do-it-yourself friendly, breathtakingly beautiful, and sensuous to walk on. And stay tuned…in the next issue we’ll cover a step-by-step “how to” for installing your own adobe floor.

Sigi Koko founded Down to Earth Design, an architectural design firm that specializes in durable strawbale buildings for wet climates.  Her projects use natural materials that ensure healthy indoor spaces, super energy efficiency, and minimal environmental impact.  She teaches a wide variety of natural building techniques that empower her clients to contribute meaningfully during the construction of their home.

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