External wall issues

Earth walls have been used in domestic construction from the earliest times. Usually made up of local earth coated in limewash or lime render, such walls are built up on a stone plinth. Moisture control is critical to these walls:

• too much moisture causes the material to slump;
• too little moisture has been said (but apparently not proved) to make it crumble.

The outer skin is therefore the most vital component of this type of construction. Earth construction is notoriously unpredictable structurally, and great care is needed to identify such walls and adapt and repair them with all due consideration. The walls must not become saturated for long periods, as they then revert to their plastic state. Garden walls are often found unrendered, with a deep straw coping to prevent saturation. The saturation of the soil around a primitive form of building can also lead to rising damp.

One reason why there are no lower-order vernacular houses pre-1400s is because many were made of earth. While the grand houses of the day were built of finer materials, the reality for the majority of the population was that they were living in very poor quality constructions. This is not to say that earth buildings are necessarily inferior - in fact earth is just as good as many other building materials. Perhaps it will make a comeback as part of the eco-friendly movement.

Examples of common earth houses can be found dating to between the 1500s and 1900s, and take the form of simple cottages to country villas. Many agricultural buildings were made with earth walls, and these may now have been converted to domestic houses. Most are covered with a render, making correct identification difficult, but an assessment of the wall thickness on ground and upper floors should reveal thicker walls at the base. As window openings in earth walls are formed by cutting away the dried earth, it is reasonably difficult to alter the size of the windows at a later time - the walls tend to fail if openings are extended, and so those houses that have survived today have largely retained their original window sizes.

The 4 main types of earth construction

• Cob: A mixture of clay/chalk/grit/straw, mixed to a paste and formed freehand into walls. Mainly a West Country formula.
• Shuttered earth: A similar mix to cob, cast within shuttering on top of a masonry plinth.
• Clay lump: A clay, water and straw mix is cast into blocks and left to dry, prior to construction on a plinth wall. Mainly found in Essex, Norfolk and Suffolk.
• Rammed earth (pisé): A dry subsoil with no additives is rammed in thin layers into shuttering, making a monolithic wall. Mainly found in Scotland.

Threats to cob or earth walls

Cement renders seal in the walls and do not allow them to breathe, or may cause them to fill up with water. Whenever an earth wall is thought to be sealed with cement renders, this combination needs very careful consideration as the remedial work to strip the render is very expensive. The greatest danger is when cement renders crack and allow water into the wall. This usually migrates to the base of the wall and stops at the plinth. The walls are then likely to fail at the base. A similar effect can occur to those buildings clad in a brick skin, because bricks are porous.

Roof spread

Earth walls have limited resistance to lateral loads, and if roof timbers are cut or become decayed, the thrust from the roof will cause the walls to bulge and the ridgeline to drop.

Driving rain

Earth houses must dissipate rain very quickly, and so large eaves overhangs and soft landscaping is critical. If eaves are shortened and hard standings laid up against the foot of the walls, then the balance of evaporation is tilted and wetter walls become inevitable. Flooding is disastrous to earth walls, and can cause them to collapse completely.

Defective plinths

Most remaining earth-walled houses are built on stone plinths. If these shift, this can set up vertical stresses in the walls, which can cause them to split.

Double glazing

Double glazing is not in itself a problem, but it does reduce the natural air movement through the house and consequently natural evaporation may be too low. This may cause difficulties and exacerbate otherwise tolerable amounts of moisture in the walls. Introducing trickle vents and monitoring moisture levels is recommended, to assess the natural ventilation rate in such houses.

Internal alterations

The removal of internal walls can severely affect the lateral stability of earth-walled houses. The weight of roof materials can help tension the walls, to prevent bowing and keep the structure stable. Any structural alteration to these types of properties needs very careful assessment and consultation for listed consent.