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NFPA 220 · Construction Types · Materials in Fire · Collapse Indicators
← Study GuidesMemory aid: Type I = best protected → Type V = fully combustible. Each type down the list = less fire resistance, faster failure, higher risk to firefighters.
Steel expands ~9.5" per 100' at 1,000°F. Expanding beams push walls outward. Wall collapse can occur before beam failure. Sagging steel = elongation, not just weakening.
Bar-joist roofs fail rapidly — often within 5–10 minutes of direct flame contact. No warning sagging. Collapse is sudden. Known as "void space" hazard after failure.
Firefighter impact: A steel beam that appears structurally intact may have lost half its load capacity. You cannot see steel failure coming — time in a fire building is the critical variable.
| Feature | Balloon Frame | Platform Frame |
|---|---|---|
| Era | Pre-1940s | 1940s–present |
| Studs | Continuous from sill plate to roof (2–3 stories) | Each floor is a separate platform; studs stop at each floor |
| Fire spread | Continuous void from basement to attic — rapid vertical spread | Floor plates act as firebreaks — slower vertical spread |
| Identification | Older homes, no fire blocking visible in walls | Modern homes, fire blocking between floors |
| Risk | Fire travels unseen — attic involvement before detection | Lower — but lightweight truss still a concern |
Moisture in concrete converts to steam under heat. Steam pressure exceeds tensile strength → explosive spalling. Exposes rebar → rebar loses strength → structural failure. Post-tensioned slabs: failure can be catastrophic and progressive.
Concrete Block and Stucco — dominant South Florida residential type. Non-combustible exterior walls. Interior wood framing still present (roof trusses, floors). Don't assume "concrete block" means non-combustible structure.
| Type | Description | Warning |
|---|---|---|
| 90° (Curtain Fall) | Wall falls straight down, parallel to face. Debris lands at base of wall. | Stays close to building |
| Inward/Outward | Wall falls inward onto firefighters or outward into street. Most dangerous. | Collapse zone = 1.5× wall height |
Collapse zone rule: Stay back 1.5× the height of any compromised masonry wall. This is a minimum — go further when conditions allow.
| Roof Type | Construction | Failure Risk | Warning Signs |
|---|---|---|---|
| Flat (Built-up) | Layers of tar/gravel over deck, often Type II | High — bar-joist failure, no slope drainage | Sagging, soft spots, water ponding |
| Pitched (Truss) | Lightweight wood trusses, OSB sheathing | High — rapid truss failure, void space | Smoke from eaves, fire in attic space |
| Bowstring Truss | Arched wood or steel truss, older commercial | Extreme — collapses as a unit with no warning | Arch visible on exterior; older buildings |
| Heavy Timber | Large dimensional lumber, Type IV buildings | Lower — chars, provides warning | Visible charring, smoke before failure |
Bowstring truss: The most dangerous commercial roof. Look for curved/arched exterior parapet or ceiling profile. Any fire involvement = defensive operations. These roofs collapse without warning as a single unit, taking walls with them.
Champlain Towers South (Surfside, FL — 2021): Progressive concrete failure, long-term rebar corrosion from saltwater intrusion, and post-tensioned slab deterioration led to total pancake collapse. 98 deaths. Reinforces: you cannot see what you cannot access. Construction type + age + environment = risk profile.
High-rise = occupied floor >75 ft above lowest fire department vehicle access. Not just building height. Most are Type I (fire resistive) — but fire dynamics, not collapse, are the primary hazard.
Temperature differential between inside/outside drives vertical air movement. Winter: air rises inside (normal stack). Summer: reverse stack (downward). Moves smoke to uninvolved floors. Elevator shafts and stairwells are primary pathways.
Water table elevation (often <2 ft below grade) makes basements impractical. Homes are slab-on-grade. Utilities run in exterior walls or under slab. Below-grade fire hazards are rare — but slab construction affects access and hose deployment.
Post-Andrew (1992) codes require stronger roof-to-wall connections (hurricane straps), impact glass, and tie-down systems. Paradox: stronger connections mean roofs may stay attached longer — but still fail catastrophically when they do.
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