Cob Building

Practical

A cob mix typically contains:

• Subsoil (clay-and-sand) — the structural binder. Subsoil rich in clay (~25–30% clay) is the working range. Topsoil is too organic; pure clay too cracking.
• Sand — added if subsoil is too clay-heavy; typically coarse, sharp-edged sand.
• Straw — long-fiber straw (wheat, rye, barley); the tensile-reinforcement that prevents cracking.
• Water — enough to bring the mix to a stiff but workable consistency.

Construction:

1. Mixing — traditionally by foot-tramping on a tarp; modern practice often uses tractor-loaders, mortar mixers, or skid-steer concrete-mixer attachments.
2. Lifting — wet cob is lifted to the wall (one large clump at a time, typically 12–18 inches across).
3. Building up — clumps placed on the wall, joined by working-and-pressing into the previous course; 8–12 inches of vertical lift per day to allow drying-in-place between courses.
4. Shaping — sculpting curved walls, niches, built-in seating, alcoves; cob’s hand-formed nature allows organic and curved geometry that rectilinear systems can’t.
5. Window and door frames — typically embedded in the wall; rebar, rocks, or timber lintels span openings.
6. Drying — slow, weeks to months; wall must dry sufficiently before plastering.
7. Plastering — interior and exterior earth or [[lime-plaster|lime plaster]]; the plaster is the weather face.

Scientific

Cob walls work through:

• Mass — typical thickness 18–24 inches; substantial thermal mass smooths interior temperature swings.
• Moisture buffering — earth walls absorb and release indoor humidity, reducing condensation and supporting comfortable humidity ranges.
• Vapor permeability — cob breathes; moisture moves through wall in vapor phase rather than condensing inside (provided lime-based plaster is used and synthetic vapor barriers are absent).
• Compressive strength — cob is strong in compression (load-bearing single-and-two-story construction is well-documented), weak in tension.
• Seismic — some seismic-zone application; modern engineered cob with embedded reinforcement is increasingly understood.

The building-physics is well-developed in the natural-building literature. Building-code acceptance is a different question: cob construction is permittable in some U.S. jurisdictions (especially Oregon’s appendix-cob codes), challenging in others. The Cob Research Institute and Cobitatus have done substantial work on engineering data and code-pathway development.

Cultural

The U.S. cob revival traces from the 1990s — Ianto Evans, Linda Smiley, Becky Bee, Michael Smith — through the Cob Cottage Company workshops in Oregon, the spread of cob workshops nationally, the publication of The Hand-Sculpted House (Evans, Smith & Smiley, 2002) and The Cobber’s Companion (Bee, 1998), and the network of builders, owner-builders, and small-cob-school operations that work today.

The British cob tradition is centuries-deep: thatched cob cottages in Devon, Somerset, and Cornwall date to the 16th–18th centuries, many still occupied. Yemeni mud-brick high-rise (Shibam, “[[manhattan|Manhattan]] of the Desert”) demonstrates earthen-construction at multi-story scale. The architectural form is genuinely scalable.

Lenses still to grow

• Specific cob projects in the U.S. (Cob Cottage Company demonstration buildings, Oregon County permitted cob homes)
• The British cob tradition and its conservation
• Cob and seismic-zone engineering; the Cob Research Institute’s work
• Hybrid cob systems (cob-and-straw-bale, cob-and-light-clay-straw)

See also

Auto-generated from this entry’s typed relations: frontmatter, grouped by relation type so the editorial signal isn’t flattened.

• Shares approach with: [[adobe]] · [[hempcrete]]
• Counterpart to: [[rammed-earth]]
• Member of: [[practice]]
• Enabled by: [[clay-plaster]] · [[lime-plaster]]

Sources

• Ianto Evans, Michael Smith, & Linda Smiley, The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002)
• Becky Bee, The Cobber’s Companion: How to Build Your Own Earthen Home (1998)
• Cob Cottage Company (cobcottage.com) educational publications
• Cob Research Institute (cobresearchinstitute.org) engineering data

Rooted in life.