Cob Construction

Cob is earth building without the mold. Where adobe forms bricks and rammed earth compresses layers within formwork, cob is mixed wet — clay, sand, straw, and water — and stacked freehand, one course at a time, shaped by nothing more precise than the hand and the eye. The result is a monolithic wall with no joints, no units, and no planes that were imposed by anything other than the material's own willingness to hold its shape.

The mixture is similar to adobe in composition but different in water content — cob is wetter, plastic enough to be kneaded and placed without molds, firm enough to support the course above it without slumping. The proportions vary with the local soil: typically 15 to 25 percent clay, the balance in coarse sand, with long straw mixed throughout at roughly 3 to 5 percent by volume. The straw is critical. In adobe, it prevents cracking during drying. In cob, it does that and more — it provides tensile continuity across the wall, knitting each course to the one below it so that the finished wall behaves as a single mass rather than a stack of layers. The straw does not decompose within the wall — the alkalinity of the clay inhibits biological activity, and as the cob dries, moisture content drops well below the threshold for fungal growth.

A cob wall is built in lifts — courses approximately 300 to 450 millimeters high, each allowed to stiffen before the next is placed. The stiffening period depends on weather: a day or two in warm, dry conditions; several days in cool or humid air. During this interval, the surface of the lift firms enough to support additional weight without deforming, while the interior remains soft enough to bond chemically with the fresh cob placed above. The timing is critical. Place the next lift too soon and the wall bulges under its own weight. Wait too long and the bond between courses is weak, creating a horizontal plane of separation that compromises the wall's monolithic character.

Thermal Mass Without Insulation

Cob's thermal behavior follows the same principles as adobe and rammed earth — high thermal mass, moderate thermal conductivity, negligible insulation value. A 450-millimeter cob wall has a thermal lag of approximately eight to ten hours, shifting the exterior temperature cycle so that the heat of the afternoon arrives at the interior surface in the late evening. In climates with significant diurnal temperature swings, this produces comfortable interior conditions without mechanical heating or cooling.

In climates that are continuously cold, cob walls lose heat steadily. The mass stores heat, but the conductivity — approximately 0.7 to 1.0 watts per meter-kelvin — allows it to escape faster than a well-insulated wall would. Cob construction in cold climates is not impractical, but it requires either supplemental heating, additional exterior insulation, or both. The material does not pretend to be something it is not. It provides mass and modulation. Resistance must come from elsewhere.

The Monolithic Advantage

The absence of joints in a cob wall is structurally significant. Adobe masonry has mortar joints — planes of potential weakness where settlement, moisture movement, or lateral force can cause separation. Rammed earth has lift lines — thin boundaries between successive layers where the bond is less complete than within the layers themselves. Cob, when properly built with overlapping straw fibers and correct timing between lifts, has neither. The wall is continuous, and loads are distributed through the full section without interruption.

This continuity also means that cob walls can follow curves, incorporate niches and shelves directly in the wall thickness, and taper from thick at the base to thin at the top without any change in construction method. The wall is shaped as it is built, and the shape is limited only by what the material will support. Straight walls require guide strings. Curved walls require nothing — the builder simply follows the intended line, and the cob holds whatever form it is given, provided the base is wide enough to carry the height.

Foundations and Protection

Cob's relationship with water is the same as all unbaked earth construction — it must be kept from sustained wetting, or it will revert to the mud from which it was made. The foundation serves as the primary moisture barrier: a plinth of stone, concrete, or rubble, typically 300 to 500 millimeters above finished grade, that prevents ground moisture from wicking into the earth wall above. The plinth must extend below the frost line to prevent heaving, and its top surface should be level to within a few millimeters to ensure even load distribution across the base of the wall.

The roof is the second line of defense. Generous overhangs — 450 millimeters or more — protect the wall face from direct rainfall. In exposed locations, rendered surfaces shed water more effectively than bare cob, and lime render is preferred for its vapor permeability and durability. Earth plaster is appropriate for sheltered locations and interior surfaces, where its softness and warmth of color are assets rather than vulnerabilities.

What Persists

The oldest surviving cob structures in Devon, England, date to the fifteenth century — five hundred years of continuous occupation and maintenance. These buildings have been re-thatched many times, their plaster renewed on cycles measured in decades, their foundations repaired as settlement required. The cob itself — the monolithic earth wall — remains substantially as it was built, sound and load-bearing after half a millennium.

This durability is conditional. The buildings that survive are the ones that were maintained — roofs kept intact, plaster renewed before erosion reached the structural cob, drainage maintained around the foundation. A cob building that loses its roof will erode to a mound within decades. The material makes no claims of indestructibility. It asks only for a roof over its head, a dry foundation under its base, and periodic attention to its surface. In return, it offers walls that are massive, quiet, thermally stable, and built from whatever earth was nearest at hand. The exchange has been sustained for centuries, and the terms have not changed.