Chapter 11: Roofs - Fire Protection and Building Construction

Comprehensive practice flashcards covering Chapter 11 (Roofs), including roof styles, framing components, decks, coverings, pre-engineered trusses, rain roofs, energy-efficient designs, and tactical firefighting safety.

Why must firefighters be familiar with all aspects of roof construction?

Roofs play an important role in structural firefighting; firefighters work on them to ventilate heat and products of combustion; roofs may serve as alternative access points for tactics involving neighboring structures; and firefighters work beneath roofs whenever inside a building.

How does an intact roof influence fire dynamics within a compartment?

An intact roof prevents heat and smoke from exhausting into the atmosphere, threatening trapped occupants below; alternatively, it can reduce fire growth due to less available oxygen.

How does the structural strength of a roof compare to that of a floor?

Roofs are usually not as strong as floors because they are typically designed to support lighter loads.

What hazard is created by concealed spaces between the ceiling and roof deck, such as attic spaces and knee walls?

They make it difficult to determine the extent to which a fire has developed overhead.

What types of roof security measures might firefighters encounter that may not be approved by the fire code?

Barbed wire, interior roof access blocked with steel plates or doors, and other hardened security measures.

What are the three architectural styles into which building roofs are categorized from a firefighting standpoint?

Flat, pitched, and curved roofs.

Why are flat roofs typically constructed with a slight slope rather than completely horizontal?

To facilitate water drainage, preventing pools of water from forming that cause early deterioration.

What collapse risk does the application of master streams pose to flat roofs?

Flat roofs may handle heavy rainfall but cannot support the cumulative weight of master streams, which may overload the structure and cause collapse.

How are the slopes of low-pitched and medium-to-high pitched roofs classified numerically?

Low-pitched roofs have slopes up to $3/12$ (rising $3\text{ units}$ per $12\text{ units}$ of horizontal run). Medium- to high-pitched roofs have slopes of $4/12$ to $12/12$ (where $12/12$ equates to a $45^\circ$ angle).

What equipment is recommended for working on roofs with slopes of $18/12$ or greater?

Aerial apparatus or ground ladders, because slopes of $18/12$ or greater are too steep to work from a roof ladder.

Which common pitched roof styles are depicted in this architectural reference?

Gable, Hip, Gambrel, Mansard, Butterfly, Monitor, Sawtooth, Shed, Arched, and Flat roofs.

What is a butterfly roof?

A roof style that slopes in two directions, consisting of two shed roofs that meet at their low eaves to form a valley.

What is the structural distinction between a gable roof and a gambrel roof?

A gable roof has two inclined surfaces meeting at a high ridge. A gambrel roof slopes in two directions and features a distinct break in the slope on each side to create attic or living space.

What is a hip roof?

A roof style that slopes in four directions and has a degree of slope similar to that of a gable roof.

What was the purpose of monitor and sawtooth roofs in industrial buildings, and which way should sawtooth vertical glass face?

They provided natural light and ventilation through openable clerestories. The vertical glass of sawtooth roofs should ideally face north to provide constant daylight without solar glare.

What hazards are posed by false mansard fronts added to flat-roofed buildings?

They contain unstopped concealed void spaces allowing rapid horizontal and vertical fire spread, and they impose an eccentric load on the wall that can pull down walls or parapets if fasteners fail.

What structural systems most frequently support curved roofs?

Arches and bowstring trusses.

How do the structural forces in a dome roof operate compared to a standard arch?

A dome operates like an arch rotated $360^\circ$ on its keystone; its forces are exerted around a complete circle rather than in just one plane.

What is a geodesic dome and how is it constructed?

A curved roof formed using spherical triangulation, where triangles are arranged in three dimensions to create a nearly spherical surface.

What are knee walls, and what fireground problem do they cause around dormers?

Knee walls are void spaces formed around and between dormers when an upper attic floor is finished; they permit uninhibited fire spread and offer limited access for suppression.

What hazard must firefighters verify before entering through a dormer window?

They must ensure a floor is present, because false dormers are purely decorative and lack interior flooring, creating a severe fall hazard.

What are common structural supports used for flat roofs?

Open web steel joists, steel beams, ordinary wood joists supported by load-bearing walls, solid or laminated beams, walls, and columns.

Why are wood I-joists and open web truss joists susceptible to early failure in fire conditions?

Plywood I-joists have thin web portions that burn through quickly, truss joists use slender components, and open web designs allow fire to spread rapidly perpendicular to the joists.

Why might a modern flat roof vibrate or deflect noticeably under firefighters without signaling imminent collapse?

Roofs are designed for lighter live loads than floors and are required to support only a minimum maintenance live load; deflection or vibration often simply reflects lightweight roof construction.

What conventional roof framing members and structural thrust forces are illustrated in this diagram?

Rafters, ridge boards, floor joists, and purlins; raftered roofs exert an outward thrust against exterior walls that must be resisted by ceiling/attic floor joists or a structural ridge beam.

What components make up conventional roof framing?

Rafters, collar ties, ridge boards, ceiling joists, and rafter ties.

What is the typical on-center spacing of wood rafters in conventional roof framing?

$12\text{ to }24\text{ inches}\,(300\text{ to }600\text{ mm})$, depending on span and design load.

What does a spongy feel on partially supported roof decking signify to firefighters?

It indicates that one or two underlying supports have failed and that the remainder of the roof decking should not be trusted.

Why are pre-engineered lightweight roof trusses vulnerable to early failure in fire conditions?

Because of the reduced mass of their component members and the complete interdependence of those components.

What fasteners connect joints in lightweight wood trusses, and what is their typical spacing?

Gang-nail gusset plates connect truss joints, and the trusses are spaced $24\text{ to }48\text{ inches}\,(610\text{ to }1220\text{ mm})$ apart from center to center.

How do the chords of a bowstring truss interact structurally?

The curved top chord is under compression (trying to straighten out) while the horizontal bottom chord is under tension (maintaining the shape of the top chord).

How can a partial failure of a wood bowstring truss lead to wall collapse?

If a truss fails only partially and remains in tension against the supporting wall, it generates an outward horizontal force that pushes the wall outward.

Why do parapet walls on buildings with bowstring trusses present an operational safety concern?

Parapet walls create the external appearance of a flat roof, concealing the presence of bowstring trusses and necessitating preincident planning.

Which two common pitched roof truss configurations are shown in this diagram?

Pratt-style and Fink-style trusses.

What structural role do unprotected interior tie rods play in arch-supported roofs?

They connect the two ends of the arc to resist outward horizontal thrust; if they fail in a fire, the arches spread outward and cause the entire roof to collapse.

What are the primary functions of a roof deck?

To separate the structure from weather, support weatherproofing materials, transmit live and dead loads to supporting members, and serve as the base for roof coverings.

What materials are used to construct roof decks?

Plywood or manufactured wood structural panels, wood planks, corrugated steel, precast or poured-in-place gypsum, precast or poured-in-place concrete, and cement planks containing wood fiber.

How are roof coverings attached to cast-in-place concrete decks that lack nailability?

Via wood nailing strips embedded in or attached to the concrete at regular intervals, drilled and anchored, or placed between rigid insulation panels.

What temperature extremes must roof covering materials typically withstand?

Temperatures ranging from over $100^\circ\text{F}\,(38^\circ\text{C})$ in the summer to below $0^\circ\text{F}\,(-18^\circ\text{C})$ in the winter.

What is the primary fireground concern when multiple layers of roofing materials exist on a building?

The extra thickness and varying materials impede roof cutting and ventilation operations, requiring extra resources.

What are the three core component layers of a flat roof covering assembly?

Roof deck, thermal insulation, and roofing membrane.

What is the function of a vapor barrier in a roof assembly, and what materials are used to make it?

It reduces the diffusion of interior moisture into the insulation layer; it is made of plastic, aluminum foil, kraft paper laminated with asphalt, or asphalt-saturated roofing felt.

Which rigid roof insulating materials are flammable, and which are fire-resistant?

Polystyrene foam board and polyurethane foam board are flammable. Cellular glass board and perlite board (expanded volcanic glass granules with binder) are fire-resistant.

What flat roof covering system and layered components are illustrated in this diagram?

A built-up tar-and-gravel roof, consisting of rafters, roof decking, tar paper (felt), roofing tar, and pea gravel.

What are built-up membranes, and how long is their typical service life?

They consist of overlapping layers of roofing felt saturated with bitumen (tar or asphalt) cemented with hot bituminous roofing cement; they usually last for $20\text{ years}$.

What materials are single-ply membranes made from, and what installation hazard can cause a roof fire?

They are made of EPDM synthetic rubber, PVC, or CPE ($0.03\text{ to }0.10\text{ inches}\,[0.75\text{ to }2.5\text{ mm}]$ thick). Heating the underside with a propane torch can ignite the membrane, causing liquefied burning material to drip through decking holes.

Where are fluid-applied membranes primarily used, and what liquid materials are applied?

On curved roofs such as domes; they are applied in several coatings using neoprene, polyurethane, silicone, or butyl rubber.

What are the benefits of using a gravel wear course on a built-up roof?

It protects the membrane from mechanical abrasion and increases the roof's resistance to fire spread from flying brands of adjacent buildings.

What are the dimensions and fire behavior of modern asphalt shingles?

Commonly $12 \times 36\text{ inches}\,(300 \times 900\text{ mm})$; they are fundamentally combustible, dripping and running under fire while producing characteristic heavy black smoke.

What firefighter hazards are unique to solar shingles?

They resemble standard shingles, remain electrically energized (cannot be shut off), and cause electric shock, slippery footing, and reduced vertical ventilation options.

What is the manufacturing distinction between wood shingles and wood shakes?

Wood shingles are thin, tapered slabs sawn from tree trunks. Wood shakes are manually or machine-split from wood and are thicker than shingles.

Why have several jurisdictions prohibited the use of wood shingles and shakes?

They pose a severe fire potential; flying brands ignite them easily, and they produce airborne embers that spread fire across entire neighborhoods.

What are curved clay tiles called, and what fire vulnerability do they exhibit?

They are called mission tiles (Spanish tiles); because they do not lie flat, flying brands can blow beneath them and ignite the combustible roof deck.

What are the weight and lifespan characteristics of slate roof coverings?

Slate has a life expectancy of $150\text{ years}$ and weighs $8\text{ to }36\text{ pounds per square foot}\,(45\text{ to }200\text{ kg/m}^2)$, requiring heavier framing and decking.

How do decking requirements differ between corrugated metal roofs and flat metal roofing sheets?

Corrugated metal sheets are stiff enough to be installed without decking (supported directly by purlins/beams), whereas flat sheets require solid decking and an underlying layer of roofing felt.

What architectural feature and hazardous concealed space are illustrated in this diagram?

A rain roof (overbuild) installed over an existing roof structure, creating an uninspected void space.

What firefighting hazards are created by rain roofs?

Fire in the void space is difficult to detect and extinguish, two roofs impede rapid ventilation, narrow voids present an entrapment hazard, and the rain roof may not support operational loads.

What are cool roofs and brown roofs?

Cool roofs are energy-efficient reflective systems; brown roofs are roofs designed with intentionally enhanced biodiversity.

What is the difference between a solar cell, a solar module (panel), and a solar array?

A cell is a single collector unit; a module (panel) is a series of cells in a self-contained unit; an array is a complete system of interconnected panels or shingles.

Why must firefighters treat rooftop photovoltaic (PV) wiring as live even after building power is shut off?

The energy feed from PV cells cannot be shut off; as long as light strikes the cells, they continue to generate electricity.

What dead load is added to a building by a vegetative green roof?

Depending on soil depth ($\text{inches to feet}$, or $\text{millimeters to meters}$), the dead load ranges from $20\text{ to }150\text{ pounds per square foot}\,(110\text{ to }830\text{ kg/m}^2)$.

What life safety warning did NIOSH release concerning translucent corrugated roof panels?

They are lightweight thermoplastic panels that constitute a severe fall hazard and caused a firefighter line-of-duty death (LODD); personnel must never stand or walk on them.

How can rooftop penthouses benefit firefighting operations?

When constructed as stairway enclosures, penthouses provide rapid internal building access directly onto the roof level.

What caution must firefighters exercise around automatic smoke and heat skylights?

They use fusible links and may open with great force and without warning under fire conditions; they cannot support firefighter weight and can be obscured by darkness, smoke, or snow.

What are the structural definitions of a collar tie and a rafter tie?

A collar tie is a horizontal member in the top third of the framing that braces rafters against wind uplift. A rafter tie is a horizontal member that connects rafters to prevent walls from spreading outward.

What are purlins and ridge boards in roof framing?

A purlin is a horizontal member positioned between trusses to support the roof. A ridge board (ridgepole) is the highest horizontal member in a pitched roof to which the upper ends of the rafters attach.