Structural repairs and typical defects
Damp and water ingress can occur due to structural defects in the fabric of a building. They are the source and need to be addressed before cosmetic renovation can take place.
The importance of the diagnosis of damp in buildings cannot be underestimated. If a problem is diagnosed as rising damp and remediation work is carried out on that basis, problems are likely to persist if the source of damp is due to water ingress from some structural defect.
Cracking and movement in external walls is fairly common particularly in older buildings and can be caused by a number of factors.
Rusting wall ties and lintels.
When metal objects such as wall ties and lintels corrode they can expand up to several times their original width and exert tremendous forces within the masonry structure. In modern construction metallic items are protected from corrosion by a galvanised coating but in older buildings where the metal is exposed the wall structure is more vulnerable to cracking. The corrosion is more pronounced where the wall has been subject to water ingress, excess damp or the mortar is composed of black ash which accelerates corrosion due to its acidity.
The pattern of cracking for corroded wall ties tends to be in horizontal lines along the axis of where the ties are placed when the building was originally constructed.
The ends of metal lintels can also be vulnerable especially where a cavity tray has not been installed or has deteriorated. Water ingress onto the ends of the lintel is more pronounced where the cavity tray ends. Cracking tends to be at an upwards forty five degree angle from the end of the lintel in a stepped pattern up the vertical mortar joints and along the bed joints.
Foundation settlement and heave (lift)
If the foundations all settled at the same rate there would be no problems. Differential settlement is when cracking occurs to tp one part of a foundation settling at a different rare than another.
This can occur in several ways.
Inadequate ground bearing. Could be due to variations in strength of subsoil, weakening due to leaking drains or other forms of water ingress.
Inadequate foundation strength. In older buildings the foundations may only be composed of corbelled brickwork or loose stone rubble held together with lime mortar.
Excessive loading. Large masses of heavy masonry such as chimneys may exert larger forces on the foundation and soil structure causing settlement in excess of the surrounding foundations causing cracking in masonry.
Frost heave. In relatively shallow foundations, frost can penetrate to a deep level causing expansion of the soil structure and lifting of foundations. Building regulations have sought to address this by increasing the depth of foundations over the years.
Tree roots. Certain tree types such as Poplar are renowned for removing large amounts of moisture from the soil causing shrinkage and therefore differential settlement.
Timber lintels or joists in decay or undersized
In older buildings it is common for lintels to be made of timber. When they are exposed to the elements they decay and are prone to insect attack causing weakening of the timber and movement of the brickwork above.
Many years ago there was little science behind the correct sizing of timbers for specific loadings. Today things are different and we can refer to British Standards and joist sizing tables to establish the cross sectional dimensions to suit a particular span and required loading.
Inadequate joist sizing can lead to excessive deflection when under load.
Defective or missing cavity trays
The typical scenario is where water and damp can be seen on the underside of the top of a window or door opening. This is actually the effect you are seeing and not the cause. The cavity tray is installed to direct water ingress outside where it can do no harm. Though the problem with the cavity tray should be addressed, the source of water ingress should be identified and repaired.
Lateral spread of roof timbers
The construction of a simple roof structure is designed to transfer its weight loadings vertically onto the wall structure below usually via a timber wall plate. Errors in design , poor alterations or the development of defects through the life of the building such as decay of timber joists or fixings can lead to some loadings being pushed outwards, causing a deflection in the wall below. This can be seen in the wall bowing out or leaning completely.
Mortar used in the construction of walls in older buildings was often made with lime. Over time the lime mortar deteriorates (See the lime cycle) and requires to be repointed to prevent damp and water ingress into the inside and your living environment. Where there is no cavity present and the wall is of solid construction this can lead to serious damp and mould issues. Where repointing has been carried out to a poor standard for example the joint has been raked out to an insufficient depth or an incorrect mix of sand and cement has been used, the mortar can be subject to frost damage, crack, disintegrate and fall out of the joint.
Frost damaged brickwork and masonry
Certain types of bricks are more susceptible to frost attack than others. This could be due to factors such as porosity, position in relation to prevailing weather and the composition of material from which the brick is made. The type of pointing could be an issue, for example recessed pointing leaves the top edge exposed and vulnerable to water sitting on top which could saturate the brick. Frost then acts on moisture within the brick, expands and separates the face of the brick away in a process known as spalling.