It is often over the winter months that buildings are most affected by the weather and walls are one of the main building components to receive such weathering.
Although bricks tend to be fairly robust, suffering usually from ‘spalling’, it is mainly the softer pointings between the bricks that weather at a faster rate. Spalling occurs when brickwork becomes wet due to rain and if freezing conditions occur before the bricks dry out, the entrapped moisture expands and forces off the hard face of the brick thus exposing the softer inner face. This mainly tends to occur on northerly and thus colder elevations.
When the pointings become worn they allow rainwater to enter the gaps created and this allows further deterioration to occur as the water freezes, expands, thereby exacerbating the situation and essentially weakening the wall. If left unattended for too long the collapse of the wall could eventually occur.
The walls will reach a point in time when repointing is required. When any repointing is carried out it is essential that the joints are raked out sufficiently, usually to a depth of at least twice the joint width, so that the new pointing is given sufficient ‘key’. Failure to do this can result in it simply cracking and failing out over a relatively short time. The pointing should not be stronger than the bricks to which it is applied or the existing bed joint mortar. Too high a cement content can allow shrinkage cracking and rainwater penetration to occur again.
If the walls are quite old and were built with a lime based mortar it is essential to repoint with a lime mortar rather than a cement-based one. Lime mortars (composed of lime, water and a fine aggregate such as sand) are flexible and able to accommodate minor building movement without cracking. They also absorb and evaporate moisture and so allow an older building to ‘breathe’. Cement-based mortars are substantially harder and impermeable. They inhibit moisture evaporation through the mortar joints causing moisture to evaporate through the bricks or migrate to the internal face of the wall causing dampness on the wall face.
Additionally, brickwork can be irreversibly damaged by repointing with too-hard mortar. So now we are coming out of winter have a look at your walls and if necessary, call Woodward Surveyors for further advice on 01923 920951. Further information can also be obtained from the Society for the Protection of Ancient Buildings www.spab.org.uk
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