Building Pathology and Defects
Diagnosing defects systematically
Building pathology is the study of why buildings fail. A candidate must be able to distinguish symptoms (damp staining, cracking, spalling) from mechanisms (rain penetration, thermal movement, carbonation) and causes (design, workmanship, materials, use, neglect). Always establish a defect's pattern, location, timing and progression before proposing a cause.
Key concept families you must know:
- Moisture: rising damp, penetrating damp, condensation (surface and interstitial), plumbing leaks. Differentiate by pattern and by the presence/absence of hygroscopic salts.
- Cracking and movement: settlement, heave, thermal and moisture movement, sulfate attack, wall-tie corrosion, roof spread. Crack width, taper, and whether it is active or dormant matter.
- Material degradation: carbonation-induced corrosion, chloride attack, frost damage, sulfate attack, timber decay (wet rot vs dry rot), and incompatibility between materials.
Traps in questions
Questions often present a plausible but wrong mechanism. Watch for:
- Attributing all damp to rising damp when the pattern is condensation (cold bridges, poor ventilation).
- Confusing active and dormant movement - a tapered crack that has not changed may be historic.
- Assuming dry rot requires free water; it needs moisture but can travel through masonry.
- Ignoring incompatible materials (e.g., cement render on soft brick trapping moisture).
Worked rule of thumb
For cracking, apply the crack-width and taper test: hairline, uniform cracks often indicate shrinkage or thermal movement; wider, tapered cracks (wide at one end) suggest differential movement or settlement. Always recommend monitoring (tell-tales or gauges) over a defined period before concluding.
Example: A 3 mm crack tapering from the corner of a window to the eaves, stable over 12 months, is most likely thermal/moisture movement, not active subsidence - advise monitoring and repointing, not underpinning.