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Michael Parrett considers issues around damp diagnosis and the impact of retrofits
Damp is a significant issue in listed buildings as well as millions of ordinary traditional buildings. But it can be difficult to identify its true cause. For example, in the case of damp on an internal wall, is it caused by the building's design, by a defect or by occupiers' use? Or by a combination of these?
To help understand these issues, we need some basic definitions:
* penetrating damp describes moisture that has moved from one side of a wall to the other
* rising damp is moisture that has moved vertically upwards from the ground below the building by capillary action or suction.
These definitions are important because:
* rising and penetrating damp and condensation are not themselves causes of damp but mechanisms
* all the causes of rising and penetrating damp create condensation
* condensation caused by use or occupation rarely creates penetrating or rising damp.
Surveyors should consider the failures associated with different building periods, which often provide clues on the risk of damp. For example, Victorian properties with chimneys rarely have a physical damp-proof course (DPC) to fender walls - that is, brick support walls for fire hearths - which means these then act as conduits for upward moisture transfer.
The main causes of damp are:
* high abutting external ground levels
* blocked cavity wall voids at low level
* leaking rainwater goods
* physical blockages underneath suspended floors
* blocked external vents to suspended ground floors
* leaking internal water pipes
* high local water tables
* chimneys and fire hearths
* drainage defects
* general building defects, including those related to the use and occupation of the property, such as cooking, washing, bathing or drying of laundry internally, inadequate or defective ventilation.
Pointing issues
Before the 1965 Building Regulations, many older buildings suffered from poorly conceived repairs, such as the use of inappropriate Portland cement mortar to replace lime mortar pointing to external brickwork. This made the damp worse by preventing the wall from 'breathing', and, as old buildings move slightly, it also cracked because it is brittle. Those cracks then suffered from frost action and widened, causing rainwater penetration and internal damp problems.