Cob buildings are an important part of Devon’s architectural heritage. When properly maintained using compatible materials, cob walls can remain strong and healthy for hundreds of years.
However, cob behaves very differently from modern brick or blockwork. Applying the wrong render or paint can prevent the wall from managing moisture naturally, leading to cracking, trapped damp and gradual deterioration.
We are currently working on a property that someone else had previously repaired, and fixing a heritage mistake. They had applied a hard sand-and-cement render over cob, followed by an oil-based paint. Although these materials may appear to provide a strong, weatherproof finish, they are not generally suitable for traditional cob construction.
The work now needs to be carefully corrected using materials that are compatible with the original building.
Why Does Cob Need to Breathe?
Cob is traditionally made using a mixture of earth, clay, aggregate and natural fibres such as straw. Unlike a modern cavity wall, it does not rely on an impermeable outer surface to keep moisture out.
Instead, a cob wall absorbs and releases small amounts of moisture as weather conditions change. Historic England explains that traditional materials such as earth can absorb, store and later release moisture, with permeable lime renders and limewash helping the wall through its natural wetting and drying cycles.
This is what people mean when they describe a traditional building as needing to “breathe”. It does not mean that air passes freely through the wall. It means that moisture must be able to move through the building fabric and evaporate safely.
When a non-breathable coating is applied, that drying process can be severely restricted.
What Happens When Cement Render Is Applied to Cob?
Sand-and-cement render is harder, more rigid and less permeable than a traditional lime render. This can make it incompatible with a softer, naturally moving cob wall.
Cob will respond slightly to changes in temperature and moisture. A suitable lime render can accommodate some of this movement, whereas a rigid cement-based covering is more likely to crack or detach from the wall.
Once cracks form, rainwater can enter behind the render. Because the cement surface restricts evaporation, that moisture may then struggle to escape.
The outer finish may therefore appear strong while moisture is being held against the vulnerable cob behind it.
Over time, this can contribute to:
- Cracking across the rendered surface
- Hollow or detached areas of render
- Damp patches internally or externally
- Loss of adhesion between the render and the cob
- Erosion or softening of the cob beneath
- Damage around the base of the wall
- A distorted or uneven wall profile
Historic England notes that cob can begin to deteriorate when moisture is trapped behind an impermeable surface.
Why Is Oil-Based Paint Also a Problem?
Even where a suitable render has originally been used, an incompatible paint can change the way the whole wall performs.
Oil-based and other impermeable masonry paints create a relatively sealed film over the surface. This may initially look durable and weather-resistant, but it can prevent moisture within the wall and render from evaporating effectively.
Historic England advises that modern alkyd oil-based paints and acrylic emulsions should not be applied over lime plaster or render because they impede moisture transfer. Traditional permeable finishes such as limewash allow moisture to evaporate through the surface instead.
As the building continues to move and moisture pressure develops behind the paint, the finish may begin to crack, blister, peel or flake. Simply repainting the surface will not resolve the underlying compatibility problem.
Why Lime Render and Limewash Are Better Suited to Cob
Lime render is softer and more flexible than cement render. It is also vapour permeable, allowing moisture to move through the wall and evaporate from the surface.
Rather than attempting to seal the cob completely, lime render provides a protective outer layer while continuing to work with the natural behaviour of the building.
A limewashed finish provides additional weather protection without creating an impermeable barrier. Limewash can be maintained by applying further coats as the surface gradually weathers.
Unlike hard cement coatings, which may fail suddenly in cracks or detached sections, lime render is designed to weather more gradually and sacrificially. This means that the render takes the effects of exposure rather than forcing the softer historic material underneath to do so.
Lime materials also follow the natural character and profile of a cob wall more successfully. Traditional cob buildings are rarely perfectly flat or uniform, and trying to force them into the appearance of modern blockwork can create unnecessary thickness, stress and weak areas.
Correcting the Previous Work
Repairing this type of problem is not simply a matter of covering the cracks with another coat of paint.
The condition of the existing render, paint and cob beneath must first be properly assessed. Any sources of excessive moisture, such as leaking gutters, defective drainage, damaged roofs or high external ground levels, should also be investigated before the wall is refinished.
Inappropriate coatings may then need to be removed carefully, avoiding unnecessary damage to the cob underneath. The wall must be allowed to dry naturally, and any damaged cob should be repaired using compatible materials.
Once the substrate is stable, an appropriate lime-render system can be applied in carefully built-up coats. The finished render can then be protected with a breathable limewash or another suitable vapour-permeable finish.
The exact approach should always be based on the condition and construction of the individual property. Historic England recommends obtaining advice from someone with experience of earth walls and matching repair materials as closely as possible to those used in the original building.
Why Specialist Knowledge Matters
Many modern building products are designed to create a hard, sealed and uniform surface. That approach can work well on a modern building designed around those materials, but it can be damaging when transferred directly to a traditional cob property.
A repair may look acceptable when it is first completed, while the consequences only become visible months or years later. By then, the inappropriate material may need to be removed and the work completed again correctly.
Choosing a contractor with experience in cob, lime and traditional construction is therefore crucial. The contractor needs to understand not only how to apply the material, but also:
- How the original wall was constructed
- How moisture moves through traditional building fabric
- Which lime specification is suitable for the exposure and substrate
- How to remove unsuitable coatings safely
- How long the wall and each lime coat must be allowed to dry
- How to retain the natural form and character of the building
- How surrounding details, drainage and maintenance affect the wall
Using the correct materials is only one part of the solution. They must also be specified, mixed, applied and allowed to cure correctly.
Protecting Traditional Buildings for the Future
Cob buildings do not need to be sealed in modern materials to survive. They need appropriate protection, regular maintenance and finishes that allow them to perform as they were originally intended.
When unsuitable materials have already been used, the damage should be assessed and corrected before the problem becomes more extensive.
At Quoin & Oak, we specialise in the repair and conservation of traditional and heritage properties. Our team understands cob construction, lime rendering, lime plastering and breathable finishes, allowing us to develop a repair approach suited to the individual building rather than applying a standard modern solution.
Getting the work right protects more than the appearance of the property. It helps preserve the original building fabric, supports healthier moisture management and reduces the risk of the same repair needing to be completed again.
To discuss repairs to a cob, stone or traditional property, contact Quoin & Oak.
Email: quoinandoak@gmail.com
