Tend & Keep Natural Building · Lesson 5 of 7 · Plasters and floors Course home
Natural Building · Lesson 5 of 7

Earthen plasters and floors

The finish you can touch
About 30 min to read4 partsPlaster calculatorHands-on practice

Plaster is where a natural building comes alive. The rough golden walls disappear under smooth, soft-cornered surfaces that glow in the morning light and feel cool and solid to the touch. It's also the part of the building you'll see and touch every day for the rest of your life.

This lesson covers the three finishes that make natural homes so lovely: clay plasters, lime plasters and earthen floors. All three are made mostly from sand, clay, lime and straw, and all three can be mixed and applied by a family.

By the end of this lesson you will be able to
  1. 1Choose between clay, lime and cement plasters, inside and out.
  2. 2Test, mix and apply a plaster in coats.
  3. 3Estimate how much sand, clay and lime a building needs.
  4. 4Lay out the layers of an earthen floor.
  5. 5Work safely with lime and linseed oil.
Part 1

Choosing a plaster

Inside, or sheltered outside

Clay plaster

Clay, sand and fiber. Soft, warm and easy to repair with a damp sponge. It handles indoor moisture beautifully, but rain can wash it, so outside it needs deep overhangs or a dry climate.

Outside, or anywhere

Lime plaster

Lime and sand. It slowly hardens by drawing carbon dioxide from the air, turning back into a kind of limestone. Tough, weather-resistant and still breathable. The classic choice for the outside of a bale wall.

Use with care

Cement plaster

Strong and familiar to plasterers, but much less breathable and prone to cracking on flexible bale walls. The code allows it, with limits; many natural builders avoid it on straw.

Plaster goes on in coats, each finer than the last.Straw baletrimmed flatdamp-mistedScratch coatpressed intothe strawBrown coatshapes andflattens the wallFinish coatfine sandsmooth or texturedPaintlime washor clay paintTotal thicknessClay plaster: at least 1 inLime plaster: at least ⅞ in, in three coatsCement plaster: no more than 1½ in(from the straw bale code appendix)
Not to scale. Each coat goes on while the one below is still slightly damp, or is misted first, so they bond together.
Remember Lesson 3: plaster goes directly on the straw, with no plastic behind it, and the finish must stay breathable. That's what lets the wall dry out.
Part 2

Mixing and applying

Test first, always

Every clay soil is different. Use your winning mix from Lesson 1 as a starting point and make small test patches on a scrap bale or your test panel from Lesson 4. Try a few ratios, often somewhere from 1 part clay to 2 or 3 parts sand, plus chopped straw for base coats. Let them dry for a few days. The best mix has few cracks, doesn't dust off when rubbed, and holds firm.

  1. Prepare the wall. Trim the bales flat, fill hollows with straw-clay, and mist the straw lightly.
  2. Scratch coat. Push the first coat hard into the straw with gloved hands so it grips. Rough it up with a rake so the next coat holds.
  3. Brown coat. When the scratch coat is firm but still slightly damp, add the thicker brown coat with a trowel or float to shape and flatten the wall.
  4. Finish coat. A thin coat with fine sand, smoothed with a trowel or sponge for the final texture. Shape soft curves around windows now.
  5. Paint. Clay paint or lime wash adds color and a little extra protection. Several thin coats beat one thick one.

Lime needs looking after

Curing
  • Apply between about 40 and 80°F, never when frost is coming
  • Keep it out of hot sun, wind and rain while it cures
  • Mist it gently so it dries slowly, not fast
  • Wait several days between coats
Safety

Lime is caustic: it can burn skin and seriously injure eyes. Adults handle dry lime and mixing. Everyone wears gloves, long sleeves and safety glasses, with clean water on hand to rinse. Keep small children away from wet lime.

Try it · plan your plaster

How much plaster for your walls?

The 512 sq ft default is the 20 × 20 ft building from Lesson 1. Bale walls are wavy, so plaster runs thicker than planned: this adds 25%. A mixed plaster takes up less room than its dry parts, because fine particles fill the gaps between sand grains, so the dry total is about 1.3 times the plaster volume. Planning numbers; test batches tell the truth.

Part 3

Earthen floors

An earthen floor is poured and troweled like a mud pie, then oiled until it's hard, water-resistant and warm underfoot. It's quiet, beautiful and a wonderful place for thermal mass to soak up the winter sun.

An earthen floor, layer by layer.Finish coat½–¾ in · fine clay, sand and fiber, troweled smooth, then oiledEarthen baseabout 2–4 in · clay soil, sand, gravel and strawInsulation (optional)rigid insulation or pumice, in cold climatesGravel4–6 in · drains water and stops wickingCompacted subsoillevel and firm
A sample build-up. In very wet ground or cold climates, builders often add a vapor barrier under the insulation; follow your local practice and code.
  1. Prepare the base. Level and compact the subsoil, then lay and compact the gravel.
  2. Lay the base coat. Spread the earthen base between screed boards and level it. It isn't troweled smooth.
  3. Let it dry completely. This can take weeks. Good airflow and warm weather help.
  4. Trowel the finish coat thin and smooth with a steel trowel. This is the surface you'll see.
  5. Oil it. Once it's bone dry, brush on four or more coats of linseed oil, thinned at first, until it stops soaking in. Many builders finish with a wax.
Oily rags can catch fire on their own. Rags soaked in linseed oil heat up as they dry and can burst into flame hours later. Spread used rags flat in a single layer outdoors on concrete to dry, or keep them in a sealed metal can of water, and never leave them in a pile.
Part 4

Color, texture and care

Make it yours
  • Natural pigments (iron oxides, ochres) for warm earth colors
  • Colored clays from your own land for a finish no one else has
  • Sculpted niches, shelves and rounded corners
  • Mica or fine straw in the finish coat for sparkle and texture
  • Pressed leaves, shells or tile set into the wet plaster
Caring for it
  • Clay plaster: patch scuffs with a damp sponge and a little fresh mix
  • Lime plaster: refresh with lime wash every few years outside
  • Earthen floors: sweep, use a damp mop, and re-oil or wax high-traffic spots
  • Felt pads under furniture, and a doormat at every door
The design opportunity

No two hand-plastered walls are alike. The soft shadows of a trowel, the warm color of your own clay and a child's handprint pressed into a hidden corner make a home that couldn't exist anywhere else.

Hands-on practice · two afternoons

Mix and apply an earthen plaster sample

Plaster your test panel from Lesson 4, or a scrap bale, with your own clay. Test a few mixes, apply real coats, and finish with clay paint.

You'll need

  • Clay soil (your Lesson 1 winner), sifted through a screen
  • Sand, both coarse and fine
  • Chopped straw, scissors or a garden shredder
  • Buckets, a tarp, a spray bottle and a trowel or old spatula
  • A test panel from Lesson 4, a scrap bale or a cardboard box stuffed with straw
  • Optional: natural pigment and a little flour paste for clay paint

Do it together

  1. Mix three tests on a board: 1 clay to 2 sand, 1 to 2.5 and 1 to 3, with a handful of chopped straw. Smear each into a patch and let them dry for two or three days.
  2. Pick the winner: the least cracking, the least dusting when rubbed.
  3. Scratch coat. Mist the straw and push the winning mix in by hand. Rough it up.
  4. Brown coat the next day, about ½ to ¾ inch, shaped with a trowel.
  5. Finish coat once it's firm, with fine sand and no long straw, smoothed with a damp sponge.
  6. Paint it with a simple clay paint of fine clay, water, pigment and a little flour paste.
AGES 5–8

Little plasterers

  • Mix the mud with their hands and feet
  • Push the scratch coat into the straw
  • Press a handprint or a leaf into the finish
AGES 9–14

Older plasterers

  • Record each test mix and rank the results
  • Run the plaster calculator for your Lesson 2 home
  • Mix a clay paint and record the recipe that works

Photograph your finished sample for your progress record. It's a small piece of your future home.

Closing

Walls you can touch

Plastering is slow, social, muddy and deeply satisfying work. Families who plaster their own walls say they never see them the same way again: every curve remembers who shaped it. It's the finishing touch that turns a building into a home.

Next, we take everything you've learned to the building department, and walk through how natural methods get approved.

Worksheet

Your Lesson 5 worksheet

Work through these together. Your answers save on this device as you type, and print cleanly for your records.

For your homeschool records

Progress record

What this lesson practices:

  • ScienceChemical change (lime hardening by absorbing carbon dioxide) and properties of mixtures (NGSS 5-PS1-4, MS-PS1-2).
  • EngineeringTesting materials and choosing the best solution (NGSS 3-5-ETS1-3).
  • MathRatios and volume (Common Core 6.RP.A.3, 5.MD.C.5).
  • ArtColor, texture and craftsmanship in a finished surface.

Standards mapping is a draft. If you use charter or ESA funds, confirm it with your education specialist.

Sources for this lesson
← Lesson 4: Straw bale and straw-clay wallsAll lessons in this courseMy library