Adobe Floor Basics - How to build a dirt cheap floor

Seems crazy, right?  A dirt floor, of all things!  Well, time for a perception shift...

Adobe floors are lusciously beautiful and quite durable.  And best of all, in most regions they can be made from local clay soil.  (Which makes them dirt cheap...sorry, couldn't resist.)

adobe floor in a strawbale cottage in WV

The Concept

An adobe floor, or earthen floor, relies on the sticky binding properties of clay...one of the most versatile building materials I know of.  Clay expands when wet, creating sticky platelet particles, like a bunch of suction cups.  Add to the clay ample course sand, to increase compressive strength and control shrinkage as the clay dries.  Finally add fiber, to increase tensile strength and knit everything together, like reinforcing bar 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.

What I describe below is the method I use to install an adobe floor.  It is by no means the only method!  I recommend reading with the intention to gain an understanding of the concept...the function that each layer serves.  This way you can adjust and experiment while being sure to include the primary functional characteristics (most particularly to keep moisture out of the floor).

The Layers

A typical adobe floor includes a build-up of layers, each with a purpose.  I describe each layer below, including the function it serves and (in some cases) alternatives you can experiment with.  This example demonstrates a floor installed "on grade", similar to a concrete slab.  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.  There are a variety of methods for building up the floor layers, from single pours to multi-layered finishes.  I am sharing a basic version that is conceptually similar to pouring a concrete slab.  I find that this system performs extremely well and is very forgiving to install.

sketch of adobe floor layers

So, starting from the bottom up...

#1: The substrate

You want your floor to rest on solid ground (or a stiff framed floor with little flex).  If you have movement below, it means cracks up above.  So you want structurally strong, compacted earth to build on.  The ground should be free of topsoil or high organic matter, since the organics will continue to decompose and shrink in volume over time, leaving you with voids below your adobe floor.  So dig down until the soil feels solid and compact.  You may even want to go over the ground with a hand tamper to ensure compaction.

#2: The capillary break

Next you install several inches of gravel to provide a capillary break that prevents any potential liquid water in the ground from rising up into the floor.  I use 4-6" of pea gravel or angular 1-inch stone.  If you live somewhere where pumice is commonplace (or any insulating mineral/rock), you can substitute the pumice for the stone & the insulation (#4), since the pumice will provide both qualities.

#3: The vapor barrier

Next you lay a vapor barrier that covers the entire floor area.  This provides your final moisture control for the adobe, blocking any air-borne vapor (from evaporated ground moisture).  I use 6 mil polyethylene sheeting for this, exactly what is used to prepare a concrete slab.  The plastic is obviously not natural, but it's excellent insurance.

#4: The insulation

This is probably the layer that is most often left out, but it can dictate the energy performance & your comfort level.  If you live in an exclusively hot climate, then you can skip the insulation, because a cool floor is beneficial.  But if you heat your building at all...at all...then you want to keep the heat inside.  If you do not 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 will 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.

vapor barrier (black plastic) with insulation above

#5: The base layer

I do a 2-layer adobe floor, mostly because this allows me to pour the thick base layer before the exterior walls are completely closed in.  That extra air-flow speeds up drying time (and eliminates a highly humid interior later in construction).  I use the same proportions I would for cob, which is generally 15% to 25% total clay plus 75% to 85% concrete sand.  The difference is that I add lots more water.  With cob, the wetter your mix, the more your cob wants to splooge on your walls, so it prevents you from building higher until the material dries.  However, with a floor, you want to be able to pour it, like a really thick liquid.  (I use about the consistency of chocolate pudding.)  If I am using clay soil from the site, I sift it pretty large...through a 1/2" screen is fine.  To this mix, you want to add long fiber, and lots of it.  The fiber knits the floor together and helps prevent cracking in the clay.  I use straw, since it is abundant in my region.

The process for laying this layer is pretty simple.

1. make a batch of your mix
2. place the mix at the farthest corner of your floor, where you are going to begin
3. make sure you have height reference points in place (such as a perimeter beam or braced ledger boards)
4. roughly pack the mixture into place, being sure to push into any corners or voids (you can literally throw the adobe into the corners to be sure you get good fill); make sure your material is filled a bit higher than you want your finished layer to end up
5. level the floor with a long, straight 2x4 by holding each end of the board on your ledgers (so you know your height) and then shimmy the board back-and-forth while pulling slowly toward you (the longer your leveling board, the more hands on deck you need to shimmy it); as material builds up in front of your board, it makes it more difficult to move the 2x4, so simply stop, scrape the excess out of your way, and carry on
6. then let the floor dry completely (and don't let it freeze until it is 100% dry)

Note that it is perfectly normal to get some sprouting in your floor as it dries.  Do not be alarmed if this happens!  It's just seeds from your straw confusing your floor for a garden.  As soon as the floor dries, the sprouts will die, leaving behind some additional fiber reinforcing from their root structure.

Another option for this base layer is to make adobe bricks.  You are using the same exact mixture but placing it in forms and then letting the adobe dry out in the sun.  The benefit here is that you can carry on with construction, without having to wait for a thick, wet floor to dry so you can walk around unimpeded.  Once your bricks are dry, you lay them like patio pavers in a layer of sand, tamping them to level as you go.

leveling the adobe with a long, straight 2x4

this first layer just needs to be level, but it can have pock marks
(this 550 SF floor took 1 day for about 15 people to install)

#6: The leveling layer

There are a few approaches you can take for this layer.  What I do is 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.  The benefit of polishing this layer as your finish floor is that 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 it is to control the texture of a thicker poured floor.  So you can decide which is less stressful to you...  But 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 adobe mixture as the base layer, with two modifications:

1. if using clay soil from the site, I sift it a bit finer...I use 1/4" screening for this layer
2. any straw in this layer should be chopped to 1" or shorter

Why these changes?  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 finish surface and not everyone wants/likes that look.  If you eliminate the straw, then it is absolutely crucial to get the ratio of clay to sand perfect.  To do this, make some test patches of your floor and see how they dry.  If they are crumbly and weak, there is not enough clay.  If they crack, there is not enough sand.

The process for laying this layer is as follows.

1. make a batch of your mix, making sure that it is fairly creamy; wet it just enough so that when you hold a handful flat in your hand it keeps a mounded shape until you vibrate your hand slightly, then it flows like a liquid
2. make sure you have height reference points in place; I use screed boards that are the same thickness as the floor I want to install (for example, I use a flat 2x for a 1-1/2" thick floor)...see photo below
3. start at the farthest corner of your floor (and work your way out toward an exit path), and work in swaths that you can easily reach without over-extending your body
4. roughly pack the mixture into place, being sure to push into any corners or voids (you can literally throw the adobe into corners to be sure you get good fill); make sure your material is filled a bit higher than you want your finished layer to end up
5. use a very straight 2x4 (or 2x6) that is long enough to touch 2 of your screed boards, push down to create firm contact with the screed boards, and then shimmy your board side-to-side to level the floor; as material builds up in front of your board, simply stop, scrape the excess out of your way, and keep going; the floor shape should be pretty level, but you may have some voids here and there
6. I then use a wood float or a magnesium float and fine-tune the surface; add little bits of material if you need to fill any voids, and just keep working the surface until it is level and reasonably smooth (see photos below for examples of what it will look like)
7. your final pass while the floor is wet is with a flexible pool float; this will bring a bit of water (and with it, clay) to the surface and allow you to really make the surface look pretty; but don't go over the surface too much here, or you will bring too much clay to the top and the floor will crack as it dries
8. continue until your floor is completely installed, moving the screed boards as you go
9. Finally you will burnish the surface of the adobe floor as the clay becomes leather hard (firm enough that you definitely couldn't trowel it anymore, but soft enough that you can just barely make a thumb print in the surface); slightly mist a small area with water and make quick, tight circles using a very flexible float made from steel or plastic; the amount of pressure you can apply will depend on the dampness of the floor; you should see a slight sheen on the surface as you burnish and any hairline cracks will be pushed closed; if you wait too long to burnish, the clay will be too dry & hard, and you will need to skip the burnishing
10. then let the floor dry completely (and don't let it freeze until it is 100% dry)

beginning the final 1-1/2" layer

floating the floor level, first with a 2x4, then with a magnesium float to fill all the voids

working from every direction to meet in the middle

a flexible steel float makes the finish super smoooooth

you can see each step...rough placement, wood board leveling, and smooth finish

Watch my trowel primer to understand different types of floats and how best to use them:

#7: The finish

There are several options for finishing your beautiful adobe floor.  (Have you noticed the trend 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:

• apply an 1/8" finish layer, much like a clay plaster, but applied to the floor; when this layer is dry, seal as described below; you can find more info and a recipe HERE
• apply 1 to 4 coats of clay paint, also call "alis"; when this layer is dry, seal as described below; you can find more info and a recipe HERE

If you chose to burnish your floor, then as soon as the clay is completely dry, you are ready to 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 (derived 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 adobe surface.  Common thinning solvents include citrus solvent (d-limonene based, ie 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.  Yup, not a typo...cow urine.  Traditionally used in parts of Africa and apparently makes for a stunning floor.)  You can also add pigment to your sealer if you want to enhance the color of your floor.

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

• 1st coat - pure hardening oil (1 gallon covers approximately 200 SF 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 highly recommend a final coat of beeswax paste buffed into the surface of your sealed floor

Note on smell...the oils harden by oxidizing, a chemical process that offgases an aldehyde compound.  Aldehydes are technically a VOC, though the particular compound offgassed from oils as they oxidize 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.

the finished floor

Additives

There are loads of variations on the mix for adobe floors, including additives to enhance various properties.  Common additives include cow manure, prickly pear cactus juice, hydrated lime, milk protein (casein), wheat paste, pigments...even blood protein.  Look for a future post describing the benefits and characteristics of the most common adobe, cob, and clay plaster additives.  (I'll update with a link here when that post is complete.)

Recommended Reads

This is a recently published, comprehensive book by Earthen Floor guru Sukita Reay Crimmel.