Cob Building

Part of Permanent Shelter

Cob is one of the oldest and most accessible building materials on Earth — a simple mixture of clay, sand, straw, and water that you shape by hand into thick, load-bearing walls. No bricks, no mortar, no forms, no kiln. If you have soil with clay in it, dry grass or straw, water, and your bare feet, you can build a structure that will last centuries. Cob buildings from the 15th century are still standing and occupied in Devon, England.

What Is Cob?

Cob is not the same as adobe (sun-dried bricks) or rammed earth (compressed soil in forms). Cob is a monolithic technique — you build the walls as a continuous mass, adding wet material in layers called “courses” or “lifts,” shaping it by hand as you go. The result is a seamless, curved, sculptable wall.

The Ideal Mix

Component	Proportion	Purpose
Clay subsoil	15–25% of dry mix	Binder — holds everything together
Sharp sand	65–75% of dry mix	Aggregate — provides compressive strength, prevents cracking
Straw	Generous handfuls	Tensile reinforcement — like rebar in concrete
Water	Just enough	Activates clay — mix should be stiff, not sloppy

Too Much Clay = Cracking

The most common beginner mistake is using too much clay. Pure clay shrinks massively as it dries, creating deep cracks. Sand is the majority component — it resists shrinkage and gives the wall its strength. If your test bricks crack, add more sand.

Testing Your Soil

Not all soil contains usable clay. Before you build, test what you have.

The Jar Test

1. Fill a clear jar 1/3 full with soil from below the topsoil layer (dig down 30+ cm to avoid organic matter).
2. Fill with water, cap tightly, and shake vigorously for 2 minutes.
3. Set the jar down and leave it undisturbed for 24 hours.
4. The contents will settle into visible layers: gravel on the bottom, then sand, then silt, then clay on top.
5. Measure the proportions. You want at least 15% clay.

The Ball Test

1. Take a handful of moist subsoil and squeeze it into a ball.
2. If it holds its shape and does not crumble — it contains clay.
3. If you can roll it into a rope 1 cm thick without it breaking — you have good building clay.
4. If the ball crumbles or will not hold shape — the soil is too sandy. Find a different source or add clay.

The Brick Test

1. Mix a small batch at your planned ratio (roughly 1 part clay soil to 3 parts sand, plus straw).
2. Form a brick about 25 × 10 × 5 cm.
3. Let it dry completely (3–5 days in sun).
4. Inspect for cracks. A few hairline cracks are acceptable. Deep cracks mean too much clay — add sand. Crumbling means too little clay — add more clay soil.
5. Drop the dry brick from waist height. It should not shatter. If it does, add more straw.

Mixing Cob

The traditional method is feet.

Step-by-Step Mixing

1. Lay a tarp on flat ground (makes cleanup and transport easier).
2. Spread sand in a layer about 15 cm deep, roughly 1 m × 1 m.
3. Add clay soil on top of the sand in the correct proportion.
4. Wet it down — sprinkle water, not flood. The mix should be the consistency of stiff bread dough.
5. Stomp and fold — walk on the mix, folding the tarp edges inward periodically to keep material together. Stomp for 10–15 minutes until the clay and sand are fully integrated.
6. Add straw — scatter generous handfuls across the mix and stomp them in. Straw should be distributed throughout, not clumped.
7. Test consistency — grab a handful and squeeze. It should hold together firmly, not ooze water between your fingers. If it is too wet, add sand. If too dry, sprinkle water.

Cob Loaves

Form the finished mix into oval “loaves” about the size of a rugby ball. These are your building units — easy to carry, easy to place, easy to integrate into the wall.

Building the Walls

Foundation

Cob walls must sit on a stone or gravel foundation at least 30 cm above ground level. Cob dissolves in standing water — the foundation prevents moisture wicking from the soil into your wall.

Course-by-Course Construction

1. First course: Place cob loaves along the foundation, pressing each firmly onto the stone and into its neighbour. Build up 30–40 cm high in one session.
2. Thumb and knuckle: Press your thumbs and knuckles into the top surface of each course to create a rough, uneven texture. This gives the next course something to grip.
3. Dry time: Let each course dry for 1–3 days depending on climate before adding the next. The surface should be firm to the touch but not fully hardite. In hot, dry weather you can build faster; in cool, humid weather, slower.
4. Subsequent courses: Wet the top of the previous course lightly, then place new cob loaves on top, pressing firmly to bond with the layer below.
5. Trim the sides: Use a flat blade or machete to trim the wall sides to your desired thickness and plumb as you go. Walls should be at least 30 cm thick — 45–60 cm is typical and provides excellent thermal mass.

Windows and Doors

• Frames first: Set wooden door and window frames into the wall as you build up to their height. The cob is built around and over the frames.
• Lintels: Place a strong wooden beam (lintel) across the top of each opening. The lintel must extend at least 20 cm into the wall on each side.
• Arches: Cob excels at arched openings. Build a temporary wooden form in the arch shape, lay cob over it, let it set, then remove the form.

Wall Thickness and Height

As a rule of thumb, the base of a cob wall should be at least 1/10 the wall height in thickness. A 3 m wall needs a 30 cm minimum base — but thicker is always better with cob. Do not build freestanding cob walls taller than 3–3.5 m without buttresses or embedded timber framing.

Moisture Protection

Cob’s greatest enemy is water. Protect it with three defences:

1. Big roof overhang — at least 45–60 cm on all sides. “A cob house needs a good hat.”
2. High foundation — stone or gravel, 30+ cm above grade. “And good boots.”
3. Lime plaster or lime wash — a breathable exterior coating that sheds rain but lets moisture vapour escape. Do not use cement plaster — it traps moisture inside the wall and causes hidden rot.

Lime Plaster Application

1. Dampen the cob surface.
2. Apply a scratch coat of lime-sand plaster (1:3 ratio) about 10 mm thick. Score horizontal grooves for the next coat to grip.
3. Let cure for 7 days, misting with water daily.
4. Apply a finish coat 5–8 mm thick and trowel smooth.

Drying and Curing Timeline

Stage	Duration	Notes
Between courses	1–3 days	Surface firm, not bone dry
Walls complete to roof-ready	2–4 weeks	Depends on wall height and climate
Full wall cure	2–6 months	Deep interior moisture takes time to escape
Interior finishing	After full cure	Premature sealing traps moisture

Advantages of Cob

• Free materials — soil, water, straw are available almost everywhere.
• No tools required — hands and feet are sufficient.
• Thermal mass — thick cob walls absorb heat during the day and release it at night, naturally regulating interior temperature.
• Sculptable — built-in shelves, niches, benches, and curved walls are trivially easy.
• Fireproof — cob does not burn.

Key Takeaways

• Cob is roughly 20% clay, 70% sand, and generous straw, mixed with water to a stiff dough consistency.
• Always test your soil before building — the jar test and brick test take minutes and prevent disasters.
• Build in 30–40 cm courses, letting each one firm up for 1–3 days before adding the next.
• Protect cob with a big roof overhang, a high stone foundation, and breathable lime plaster — never cement.
• Walls should be at least 30 cm thick. Thicker walls give better thermal performance and structural strength.
• Cob is forgiving, sculptable, fireproof, and built from materials that are free almost everywhere on Earth.