Construction Technology and Materials
Understanding how buildings are put together
You must be able to describe the performance and typical defects of the main construction forms: traditional masonry (cavity walls, wall ties, DPCs), framed structures (steel, concrete, timber frame), roofs (pitched, flat, warm/cold deck), floors (suspended timber, solid, beam-and-block), and claddings (render, brick, rainscreen).
Key concepts:
- Movement and tolerances: expansion joints, movement joints, differential movement between materials.
- Moisture management: DPCs, DPMs, cavity trays, weep holes, ventilation of voids.
- Thermal performance: U-values, cold bridging, airtightness, and the interaction with moisture (interstitial condensation risk).
- Material properties: brick porosity and frost resistance, mortar strength (weaker than the unit), concrete cover and carbonation, timber grading and treatment.
Traps in questions
- A stronger mortar is not better: hard cement mortar on soft brick causes spalling and trapped moisture. The mortar should be weaker and more permeable than the masonry.
- Cold deck vs warm deck flat roofs: insulation position determines where the dew point falls and thus condensation risk.
- Cavity wall insulation in exposed, driving-rain locations can bridge the cavity and cause dampness.
- Confusing U-value (thermal transmittance) with R-value (thermal resistance).
Worked rule of thumb
For flat roofs, apply the dew-point rule: insulation above the deck (warm deck) keeps the structure warm and above dew point; insulation below the deck (cold deck) requires cross-ventilation of the void. If ventilation is blocked, expect condensation on the underside of the deck.
Example: A cold-deck flat roof with blocked vents shows mould on the soffit - the fix is restoring ventilation and, ideally, converting to a warm deck, not simply re-covering.