Failure Mechanisms & Failure Modes
P02 covered stressors and P03 covered loads. Now we connect the dots: How do buildings actually fail? We’ll build a repeatable way to diagnose and prevent failures without jumping ahead into the later “control layer” and system-design sessions.
Potential agenda
- Intro (2 min): Failures are rarely random—most are predictable chains.
- Desired outcome (6–8 min): A shared “failure chain” language for diagnosis and prevention.
- Common failures (6–8 min): The most common modes we see and what drives them.
- Core concepts (6–8 min): A simple diagnostic toolkit
- Source → Path → Accumulation → Damage
- Time turns small problems into expensive problems
- Edges and interfaces are where failures start
- Redundancy beats perfection
- Takeaways (2–3 min): A “walk the failure chain” checklist you can use tomorrow.
- Discussion (30–45 min): Diagnose real examples from the group.
Desired outcome: shared language for diagnosing failures
By the end of this session, you should be able to look at a symptom (stain, smell, crack, callback) and ask the right sequence of questions—without immediately blaming a single product or trade.
The “failure chain” (simple version)
- Source: Where is the stress coming from? (rain, snow melt, sun, interior moisture, movement)
- Path: How does it get to the vulnerable place? (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 failure, structural degradation)
Potential discussion prompts
- Fast test: Think of a recent failure—can you describe it as source → path → accumulation → damage?
- Weakest link: Which step is hardest to identify on your projects?
Common failures (mechanisms and modes)
We’re staying “big picture” here: the recurring mechanisms that show up across different assemblies and products. Later sessions get specific about moisture control, air control, and system integration.
Common failure mechanisms Patterns that repeat across many homes
- Repeated wetting: small leaks that happen often (edges, penetrations) beat dramatic one-time events
- Trapped moisture: wetting + low drying = hidden damage over time
- Thermal cycling: movement at joints/interfaces opens paths and degrades sealants
- Material sensitivity: some materials fail quickly when wet, hot, or UV-exposed
Examples (for discussion)
Example 1 — “The stain that keeps coming back”
Rain event → small path at a penetration → wetting repeats → cosmetic repair fails because the source remains.
Example 2 — “Rot where nobody can see”
Minor wetting behind finishes → slow drying → rot/corrosion shows up late and expensively.
Example 3 — “Cracks, gaps, and callbacks”
Temperature swings → movement → sealant/joint failure → new paths for water/air → comfort and durability issues.
Potential discussion prompts
- Hidden failures: What failures were invisible until finishes came off?
- Repeat offenders: What detail or interface causes the most repeat callbacks?
Core concepts
Source → Path → Accumulation → Damage
Don’t jump to “damage.” Find and fix the source and path—or the problem returns.
Time is the multiplier
A small defect that wets 50 times beats a big defect that wets once.
Interfaces are where failures start
Penetrations, transitions, edges, and changes-in-material are the highest-risk zones.
Redundancy beats perfection
Assume something will be imperfect—design layers and details so one miss doesn’t become a disaster.
Takeaways
- A shared “failure chain” language: source → path → accumulation → damage.
- A habit of focusing on interfaces first (that’s where failures start).
- A reminder that repetition + time is what turns small misses into big costs.
- A preference for robust, redundant details over “perfect installation” assumptions.
Further reading & references
These lean toward durable mental models. Later sessions will translate this into specific control strategies.
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BSD-003: A House Is a System
Helpful for diagnosing failures without blaming one component in isolation.
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BSD-014: The Four Control Layers
A compact framework for “what must be controlled” (we’ll apply it later in more detail).