Fireground Support Operations (1st Edition)
Fireground Support Operations (1st Edition)
Chapter 8 - Vertical Ventilation
"By The Numbers"
▪ It is recommended that a clear space of 10 feet be around the operator of a cutting tool.
▪ An advantage of rubbish hooks for stripping roofing is that it has 2 hooks to grab the roofing (some have D-handles for more advantage).
▪ Considerations for vertical ventilation include: safety, 2nd means of egress, attack/protection lines ready, weather, loads/obstructions on roof, reading roofs, locating seat of fire, coordination with attack crews, using existing roof openings, cutting 1 large hole, enlarging original opening, roof construction, type, and condition, and elapsed time into incident.
▪ Monitors with solid sides have at least 2 opposite sides hinged at the bottom and are held closed at the top with a fusible link (open during fire).
▪ Clerestory windows are a row of windows installed on a vertical wall between 2 offset roof sections.
▪ A roof that decreases 5 inches vertically for each 1 foot horizontally is called a 5 in 12 roof.
▪ Roofs designed to withstand heavy snow loads might have a 12 in 12 pitch (45 degree angle).
▪ Ridge and rafters of gable roofs are often 2 x 6 inch if built on site and 2 x 4 inch if prefabricated.
▪ Rafters are commonly spaced 16 to 24 inch on center.
▪ Valley rafters are used where 2 ridgelines intersect.
▪ Hip roofs consist of a ridge beam with rafters running perpendicular to the ridge and hip rafters running from the ridge beam at a compound angle of 45 degrees laterally, and a lesser angle down to and beyond the outside walls.
▪ Shed roofs are 1/2 of a gabled roof.
▪ Shed roofs slope only from 1 side.
▪ Shed roofs may be constructed of mono-pitch trusses which have only 1 web member (collapse prone).
▪ Bridge trusses (heavy-duty) have sloping ends with the top chord shorter than bottom.
▪ Wood bridge trusses are usually made from 2 x 12 inch lumber, and vertical metal tie rods for support, while the joists are 2 x 6 inch or 2 x 8 inch covered with 1 x 6 inch sheathing.
▪ Bridge truss spacing may be as much as 16 to 20 feet on center.
▪ Mansard roofs have a double slope on each of the 4 sides.
▪ Mansard roofs pitch at 2 angles, instead of a constant angle.
▪ The difference in the mansard and modern mansard is they way the 4 sides meet in the middle.
▪ In a modern mansard roof, 4 steeply sloped sides rise to meet a flat top called a deck.
▪ Gambrel roofs are essentially a gable type with 2 different slopes on each side of the peak (lower slope steeper than upper slope).
▪ Rafters of sawtooth roofs are 2 x 8 inch lumber or larger, with wood or metal support bracing.
▪ Butterfly roofs resemble 2 opposing shed roofs that meet at the lower edges in the middle of the building.
▪ Tracking elapsed time into an incident at 5-, 10-, 15-, and 20-minute intervals is a way of tracking collapse hazards of roofs.
▪ Ventilation openings in roofs should at least 4 x 4 feet and in many cases may need to be larger.
▪ 1 large ventilation opening is better than several small openings.
▪ Flat roofs have a slight slope (2 in 12 pitch or less) from the front toward the rear.
▪ Inverted roofs differ from conventional flat roofs in that the main roof joists are set at the level of the ceiling, and a framework of 2 x 4 inch members is constructed above the main joists.
▪ Wooden deck roofs present a hazard if lightweight plywood or OSB decking is used (panels 3/8" to 5/8" offer little fire resistance).
▪ It is difficult and time consuming to cut concrete roofs over 4 inches thick.
▪ Poured gypsum roofs consist of bar joists or I-beams with gypsum-covered brackets tack welded to the joists and a layer of gypsum cement on top up to 2 1/2 inches thick with wire mesh reinforcement.
▪ Firefighters may have to cut 1 or more inspection (AKA smoke indicator) holes to find the seat of the fire.
▪ Heavy fire conditions can quickly burn through 2 x 4 inch support members.
▪ On flat roofs, the 1st cut for a ventilation hole should be made across the leeward end, followed by the parallel cuts (approx. 4 feet), then pull back covering.
▪ Louver vents on flat roofs can be accomplished by making a 4th cut to connect the parallel cuts.
▪ At least 2 sides of a roof monitor should be removed when used for ventilation.
▪ Typical arches are spaced on 16 to 20 foot centers.
▪ The chords of bowstring arch members are usually laminated 2 x 12 inch or larger lumber and rafters are usually 2 x 10 inches covered with 1 x 6 inch sheathing and roofing material.
▪ Lamella roof framework is 2 x 12 inch wood members bolted together at intersections with steel gusset plates with 1 x 6 inch planking and composition roofing material on top.
▪ In many modern buildings, heavy timber and 1 x 6 inch sheathing have given way to laminated beams and 2 x 4 inch lumber covered with 1/2 inch plywood or OSB, regardless of building size.
▪ Panelized roofs consist of laminated beams, commonly 6 x 36 inches, that span the length or width of the building.
▪ Beams in panelized roofs may have lengths well in excess of 100 feet and may be spaced 12 to 14 feet apart with wooden purlins, usually 4 x 12 inches with metal hangers installed on 8 foot centers between and perpendicular to the beams.
▪ Wooden joists in panelized roofs, usually 2 x 4 inches by 8 feet, are installed with metal hangers on 2 foot centers between and perpendicular to purlins.
▪ Plywood or OSB (4' x 8' x 1/2") is nailed over the panelized roof framework (composition shingles on top).
▪ A 3-layer, laminated insulation is used on the underside of panelized roof decking.
▪ Wooden parallel roof trusses are held together with gusset plates (AKA gang nails) with 3/8 inch prongs most common.
▪ Web members on combination wood/metal trusses are usually 1 inch cold-rolled steel tubing with ends pressed into a semicircular shape (punched to receive connecting pins).
▪ Normal spacing of combination wood/metal trusses is 2 feet on center.
▪ Bar joists can support metal deck roofs for spans up to 144 feet.
▪ The most common pitched roof trusses are prefabricated of 2 x 4 inch lumber held by gusset plates at intersections.
▪ In pitched roof trusses, spans of up to 55 feet are possible using 2 x 4 inch components.
▪ The most common spacing for pitched roof trusses is 2 to 4 feet on center with 1/2 inch sheathing of plywood or OSB.
▪ Wooden I-beams consist of a top and bottom chord and a solid 3/8 inch plywood or OSB web.
▪ Chords of wooden I-beams may be 2 x 3 inch, 2 x 4 inch, or wooden laminate called "micro-lam".
▪ Roof coverings consist of 1 or more layers of underlayment (AKA substrate) as a vapor barrier.
▪ Roof coverings are classified in NFPA 203, Guidelines on Roof Coverings and Roof Deck Construction.
▪ Wood shingles/shakes are usually nailed to 1 x 4 inch or 1 x 6 inch planked sheathing with a space of about 1 inch between planks (AKA skip or spaced sheathing).
▪ Most common composition roofing is 36 inches in width.
▪ Melted roofing tar is "hot mopped" onto 1 or more layers of tar paper and is topped with pea-sized gravel or crushed slag to complete tar and gravel roofs.
▪ Urethane/isocyanate foams may be in the form of 4 x 8 foot sheets of foam or foam sealed in 1 or more layers of a roof covering.
▪ Concrete, clay, or ceramic tile roofs have tiles which are usually flat, interlocking pieces that hook over 1 x 2 inch battens (AKA furring strips) nailed to sheathing or rafters.
▪ Slate roofs are most common on churches (usually nailed to roof sheathing).
▪ Light-gauge metal/fiberglass roofs consist of aluminum, fiberglass, or 18- to 20-gauge steel panels over wood or metal substructure (may be corrugated, ribbed, or shaped to simulate tiles/shakes).
▪ Light-gauge metal/fiberglass roofs are prone to early collapse.
▪ One type of steel clad roof uses 1/4 inch thick by 2 inches wide steel strips laid side-by-side with a 2nd layer at right angles welded at intersections (leaves small openings of 6 to 8 inches), then lag bolted to the joists below.
▪ Another type of steel clad roof consists of 4 x 8 foot sheets of steel (1/8 to 1/4 inch thick) welded together to form a continuous steel layer.
▪ Steel clad roofs tend to promote flashover/backdraft, impede ventilation, and increase collapse potential.
▪ A 1/4 inch steel plate weighs about 10 lbs./ft.2.
▪ Most roof assemblies employ a parallel rafter system spaced from 12 to 24 inches on center, spanning the shortest distance between bearing walls (exception = panelized roofs).
▪ When sounding a roof fails, a chain/rotary saw can be used to cut diagonally, 45 degrees, to any exterior wall and blade will usually encounter a rafter before cut is 3 feet long. If rafter direction is still unknown, then cut parallel to sidewall 2 to 3 feet. Complete triangle cut to determine spacing.
▪ One 8 x 8 foot hole is equal to four 4 x 4 foot holes.
▪ With rafter spacing of 2 feet on center, a center-rafter cut (AKA cutting the rafters) will yield a 4 x 4 foot opening.
▪ Trench vents are approximately 4 feet wide and span the width of the building.
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