Building Pathology, Defects and Repair
Diagnosing defects before specifying repair
Building pathology is the study of why buildings fail. A surveyor must separate symptom from cause: cracking may indicate settlement, thermal movement, roof spread, wall-tie failure or lintel deflection; dampness may be rising, penetrating, plumbing-related or condensation. Common domestic pathologies include moisture ingress through roof coverings, flashings, gutters, parapets and failed damp-proof courses; condensation and mould linked to ventilation and heating; timber decay (wet rot and dry rot); insect attack; corrosion of wall ties and lintels; concrete deterioration (carbonation, chloride attack, spalling, alkali–silica reaction, high-alumina cement); render and plaster failure; and drainage defects.
Distinguish wet rot from dry rot. Wet rot needs high moisture and causes localised softening; dry rot (Serpula lacrymans) can spread through drier masonry, often with a fruiting body and cubical cracking. Neither is cured by fungicide alone - the moisture source must be found and corrected. For cracking, know the BRE Digest 251 categories: 0 hairline, 1 fine, 2 slight, 3 moderate, 4 severe, 5 very severe. Categories 0–1 are often decorative; 2–3 need monitoring and investigation; 4–5 need structural engineer involvement. Movement should be monitored over time, with tell-tales or crack gauges, before a cause is asserted.
Traps: calling all damp 'rising damp'; diagnosing subsidence from a single crack; recommending a repair without identifying the cause; confusing a lintel crack with foundation movement; ignoring ventilation after new windows are fitted.
Worked rule of thumb: diagnose cause, assess significance, recommend action, advise monitoring. A repair that hides a symptom without removing the cause will fail again.