Cob
Subsoil, straw and water trodden into a stiff mix and raised in lifts without formwork. Thick, monolithic, sculptural walls with high thermal mass.
Carbon and evidence
How This Is Made
An Environmental Product Declaration is issued by a manufacturer against a declared unit. This record has neither, so it states sourced facts about process and provenance instead of a carbon figure.
Devon and the South West; a large stock of listed cob buildings remains in use
Process
Feedstock
Earth
Site-won subsoil with a straw binder; never topsoil
Process heat
No heat
Binder
Clay
Fabrication
Site-formed
Sourcing: site-won
Who Builds It
Specialist trade
Rare in the UK
Slow: each lift must dry before the next. The programme, not the cost, is usually what kills it
End Of Life
Compostable
Unstabilised cob returns to subsoil; nothing to separate or landfill
Maintenance: Repaired with fresh cob or earth mortar; must be finished vapour-open
Specification Risk
What a specifier has to resolve before this can be built.
- Certification
- No Agrément; specified by test and precedent, structural engineer sign-off needed
- Fire
- Non-combustible mass, but the straw content means no Euroclass classification is held
- Warranty
- Workmanship only
- Insurance
- Warranty providers may not accept it — check NHBC or an alternative provider early
- Supply
- Very few UK contractors. Secure the trade before committing the design.
Read This Honestly
- •Not rammed earth. No formwork, no compaction, no cement — different material, different figures.
- •A cement render or an impermeable paint will trap moisture and cause structural failure.
Design & Specification Notes
Subsoil, straw and water trodden into a stiff mix and raised in lifts without formwork.
Material Note
Unfired subsoil raised by hand, so the material carries almost no process energy — the impact is in the labour and the roof and plinth that keep it dry. It is not rammed earth: no formwork, no compaction, and no stabiliser.
Typical Uses
- •Primary frame
- •Long-span elements
- •Load-bearing supports
Key Performance
High load-bearing capacity for primary frame.
Specification Actions
- •Confirm the subsoil is site-won and its clay content tested — cob is mixed to the soil, not to a recipe.
- •Detail a generous roof overhang and a masonry plinth; cob fails from water, not from load.
- •Specify a lime render or limewash finish. A cement render will trap moisture and destroy the wall.
Indicative Health & Indoor Air
Unfired subsoil and straw. Vapour-open; must stay vapour-open to remain sound.
Products in this category
6 products from 4 manufacturers · 354 in this category. Ordered by the evidence each record holds, never by commercial standing.
Specification
- Finish
- Monolithic cob, lime-rendered
- Service life
- 100 years