Failure Mechanisms & Failure Modes
Separate underlying mechanisms (cause) from visible modes (symptom) to enable prevention and better QA.
Builds on: P3 (loads) · Leads to: P5-P10 (system interactions through assemblies) · Cross-series: A4 Infrastructure & Site
P/A bridge: A drainage standard miss or grading error creates both envelope failure risk and infrastructure cost dispute.
Core Concepts
- Failure modes are symptoms; mechanisms are causes. Trace visible failures back to mechanism chains.
- Most failures are multi-step chains. The weakest link is where prevention has highest leverage.
- Mechanism understanding improves QA. Focus inspection on transitions and known failure pathways.
The Failure Chain
Look at a symptom (stain, smell, crack, callback) and ask the right sequence of questions - without immediately blaming a single product or trade.
- Source - Where is the stress? (rain, snow melt, sun, interior moisture, movement)
- Path - How does it reach the vulnerable spot? (openings, cracks, capillary routes, pressure)
- Accumulation - Why does it stay long enough to matter? (storage, low drying potential, repeated events)
- Damage - What fails? (rot, corrosion, odor, comfort complaint, finish/structural degradation)
PF: Mechanical (E1), Thermal/Moisture (E2), Chemical (E3), Biological (E4)
Worked Chain Examples
Template: Symptom -> Failure mode -> Mechanism -> Load/condition -> Control gap -> Field check
Chain 1: Bulk water at a transition
- Symptom: recurring stain on interior wall near window head
- Mode: water intrusion at window-to-wall transition (F3)
- Mechanism: capillary/gravity path through missing kick-out or reversed lap (E2c)
- Load: wind-driven rain on exposed facade (C3a, B2b)
- Gap: flashing sequence not detailed or inspected (G4)
- Check: inspect flashing lap direction and sealant continuity before cladding closes
Chain 2: Condensation from air leakage + thermal bridge
- Symptom: mold on interior surface at rim joist / band area
- Mode: concealed condensation and biological growth (F3, F5)
- Mechanism: warm moist air reaches cold sheathing via air path; surface temp below dew point (E2a, E2c)
- Load: interior moisture + cold exterior temp (C2, C3)
- Gap: air barrier discontinuity at floor line + thermal bridge at rim (G4, G5)
- Check: blower door test for leakage location; thermal scan for cold spots at rim
Common Failure Patterns
1. "The stain that keeps coming back"
Rain event -> small path at a penetration -> wetting repeats -> cosmetic repair fails because the source remains.
- Chain: source + path not resolved
- Check: trace water path from source to penetration; verify flashing and sealant before cladding closes
2. "Rot where nobody can see"
Minor wetting behind finishes -> slow drying -> rot/corrosion shows up late and expensively.
- Chain: accumulation + time
- Check: confirm drainage and drying path exists behind finishes; probe moisture at high-risk locations
3. "Cracks, gaps, and callbacks"
Temperature swings -> movement -> sealant/joint failure -> new paths for water/air -> comfort and durability issues.
- Chain: cycling -> path creation
- Check: inspect sealant joints and flashing at movement-prone interfaces; verify expansion allowance