Dampness in Earth Walling

Excessive build up of moisture can cause the earth walling to reach a putty-like state, and, if not checked, eventually to collapse.  Sometimes, collapses and crumbling surfaces are perceived as being due to the walling becoming too dry.  However, it is questionable as to whether this is really the case.  After all, many examples of apparently very dry cob chimneys survive in sound condition in parts of South West England, often where much of the rest of the cob structure has been lost or seriously impaired.

A friable surface of the earth walling that gives the appearance of having become too dry is more likely to be due to excessive moisture and temperature variation throughout the seasons, particularly when the surface coating is a hard ungiving cement render.  The tensions created in these situations, coupled with entrapped moisture behind the render, can cause the surface of the earth walling to break up and become powdery, sometimes giving the appearance of having become too dry.

Subject to suitable ground conditions being maintained as indicated at Figure 3, coupled with a suitable underpin course, it is unlikely that rising damp will be the primary cause of excessive moisture in the base of an earth wall.  More likely, excessive moisture is due to water finding its way through cracks in an otherwise impervious render coating and becoming entrapped, building up and eventually saturating the wall base.

As the moisture level increases at the base, the loadbearing capacity of the wall is reduced at the very position where the loading is greatest.  This can cause the wall to consolidate and spread, and the render at the base to crack and spall.  Failure to attend to the problem is likely to result in the wall eventually collapsing.  This can be very dramatic and sudden, particularly where very hard and thick cement rich renders have been applied and, for a while, act as a sort of shuttering to the saturated earth walling.  This type of failure is illustrated at Figure 6.

Approaches to Repairs

In deciding how to tackle repairs, it is essential to recognise the qualities of the particular building in question, and what has caused the defects.  This must be coupled with a clear understanding of the wailing material - its composition and construction technique.  Most earth walling systems serve both loadbearing and weather resisting functions, and clearly any schedule of repairs needs to be prepared with an understanding of the full range of functions performed by the walling.

The following content deals firstly with repairs to wall surfaces in order to improve weather resistance, and secondly with repairs which endeavour also to reinstate the structural integrity of the wall.  A typical example of the first variety is where the

walling has suffered surface erosion through rain splashback, cattle licking or such like, Examples of dealing with these types of hollowing are shown at Figure 7. A careful analysis needs to be carried out to ensure that the surviving original material does in fact continue to provide adequate structural support.  Hard and fast rules cannot be laid down in a document such as this.  In mass loadbearing work such as cob, if the problem is confined to a few hollows no more than about 75mm deep, the chances are that surface applied in-situ repairs using matching materials can be successful.  However, one needs to be aware of the possibility that there are other problems to overcome at the same time, for instance deep cracks or rat holes, dictating the need for a more structural repair.