Cob is Devon's traditional building material - earth, straw, and water mixed and built in layers. Beautiful, sustainable, but it needs understanding to maintain properly.
What Is Cob?
Cob is essentially mud brick made on-site. Traditional Devon cob contains:
- Local subsoil (clay and aggregate)
- Straw or reed for binding
- Water
- Sometimes horse hair or dung (really)
It's been used in Devon for centuries. Some cob houses are 500+ years old and still going strong.
Why Cob Is Different
It Breathes
Cob walls absorb and release moisture constantly. This is a feature, not a bug. It regulates humidity naturally.
Critical rule: Never seal cob walls with cement, modern paints, or waterproof membranes. They trap moisture and cause rot.
It's Soft
Compared to brick or stone, cob is relatively soft. You can damage it with heavy impacts or by drilling too aggressively.
It Needs Protection
Cob's enemies are:
- Water (especially penetrating damp)
- Ground moisture rising up
- Frost damage when wet
- Poor repairs trapping moisture
Common Problems
Cracked Render
What you'll see: Cracks in the external lime render, sometimes with exposed cob behind.
Why it happens: Settlement, impact, or failed render that's lost its key.
The fix:
- Small cracks: Lime mortar pointing
- Large areas: Re-render with lime (never cement)
- Exposed cob: Patch and re-render urgently
Damp Patches
What you'll see: Dark patches on internal walls, musty smell, peeling paint.
Common causes:
- Blocked drains causing splashback
- Ground level raised above damp course
- Cement render trapping moisture
- Leaking gutters
The fix: Address the cause first. Then the cob usually dries out naturally.
Crumbling at Base
What you'll see: Cob deteriorating at ground level, especially on exposed corners.
Why it happens: Ground moisture rising, splashback, or lack of stone plinth protection.
The fix: Specialist repair - often involves cutting back and rebuilding with new cob on a proper plinth.
Render Bulging
What you'll see: Render pulling away from the wall, sometimes hollow-sounding when tapped.
Why it happens: The key between render and cob has failed.
The fix: Remove loose render, prepare the cob surface, re-render with lime.
Maintenance Essentials
Annual Checks
Every autumn, check:
- Gutters and downpipes - Clear and functioning?
- Render condition - Any new cracks or damage?
- Ground levels - Soil heaped against walls?
- Pointing - Around windows, doors, at base?
- Thatch or roof edge - Water throwing clear of walls?
Keep Water Away
The single most important thing for cob:
- Maintain gutters religiously
- Keep drains clear
- Ensure ground slopes away from walls
- Clear vegetation from wall bases
- Don't pile soil, mulch, or materials against walls
Use the Right Materials
| Right ✅ | Wrong ❌ |
|---|---|
| Lime render | Cement render |
| Lime wash | Masonry paint |
| Breathable paints | Vinyl emulsions |
| Lime mortar | Portland cement mortar |
What You Can DIY
Limewashing
Traditional finish for cob. Needs redoing every few years.
Basic process:
- Dampen the wall lightly
- Apply thinned limewash in several coats
- Each coat should be thin (not thick like paint)
- Allow to cure slowly (mist with water if very sunny)
Small Crack Repairs
Hairline cracks in render:
- Brush out loose material
- Dampen the crack
- Fill with lime mortar
- Smooth and blend
Pointing Around Windows
Lime mortar pointing is achievable DIY:
- Mix lime mortar (NHL 3.5 or 5 with sharp sand)
- Pack into joints firmly
- Finish to match existing
What Needs Specialists
Some cob work needs people who really know what they're doing:
Structural Repairs
If the cob itself (not just the render) is damaged or crumbling, you need a cob specialist or conservation builder.
Major Re-rendering
Large areas of render replacement should match original specifications. A conservation architect may be needed for listed properties.
Underpinning or Foundation Work
Old cob buildings often have minimal foundations. Any work near the base needs expertise.
Damp Investigation
Before treating damp, understand the cause. A buildings surveyor experienced with traditional buildings is worth the fee.
Living With Cob
Heating
Cob has good thermal mass - slow to heat, slow to cool. Works well with:
- Constant low heat rather than blast-on-demand
- Woodburners
- Underfloor heating
Hanging Things
For lightweight items:
- Picture hooks are usually fine
- Use longer screws into deeper cob
- Spread the load
For heavy items:
- Anchor into timber lintels where possible
- Use cob-specific techniques
- Consider freestanding solutions
Alterations
Planning permission aside, any alterations to cob buildings should:
- Maintain breathability
- Not add point loads without spreading
- Preserve the building's thermal behaviour
- Use sympathetic materials
Finding Help in Exeter
For serious cob work, look for:
- Conservation builders with cob experience
- SPAB (Society for Protection of Ancient Buildings) trained craftsmen
- Devon Historic Buildings Trust recommendations
For general maintenance that respects the material:
- Handymen who understand traditional buildings (that's me)
- Local builders with old-property experience
Resources
- Devon Rural Skills Trust - Runs cob building courses
- SPAB - Technical guidance on traditional materials
- Historic England - Free guidance documents
- Listed Building consent - Check before any work if your property is listed
Got a cob property needing maintenance? I work with traditional buildings across Exeter and understand what cob needs. For general maintenance and minor repairs, call 01392 964094 or get a quote. For major cob repairs, I'll recommend specialists.
Sam Hembury
Sam is the founder of Hembury Contracting, providing professional handyman services across Exeter and Devon. With years of experience in property maintenance, he shares practical tips to help homeowners tackle common tasks.