Fire Service Ventilation (7th Edition)-Chapter 4 Numbers



Fire Service Ventilation (7th Edition)

Chapter 4-Vertical Ventilation

"By The Numbers"

▪ A clear space of at least 10 feet should be observed in all directions of personnel using cutting equipment.

▪ Burning bars operate by a torch feeding oxygen and up to 200 amperes of electrical power to an exothermic cutting bar that produce temperatures in excess of 10,000oF.

▪ Cutting bars range in size from 1/4 to 3/8 inch in diameter and from 22 to 36 inches in length.

▪ A roof that slopes 5 inches for each foot horizontally from the ridge is said to be a "five-in-twelve" roof.

▪ A roof that slopes 12 inches for each foot horizontally from the ridge , a twelve-in-twelve roof, is the same as a 45 degree angle.

▪ Ridge rafters on gable roofs are usually 2x6 inches or larger and are spaced at 16 to 24 inch on center.

▪ In hip roof construction, hip rafters run at a 45 degree angle from the ridge beam, down and beyond the walls at the corners.

▪ In bridge truss roofs, when constructed of wood, trusses are usually made from 2x12 inch lumber.  Joists are usually 2x6 or 2x8 inch and are covered with 1x6 inch sheathing.

▪ In sawtooth roofs, rafters are usually 2x8 inches or larger and use wood or metal supports for bracing.

▪ The growing use of 2x4 inch trusses with gusset plates and 1/2 or 3/8 inch plywood decking can present great hazards to firefighters.

▪ Many fire departments have their dispatchers announce (over the radio), the elapsed time of an incident at 5, 10, 15, and 20 minutes.

▪ Flat roofs usually have a "two-in-twelve" pitch to permit drainage.

▪ In inverted roofs, the main roof joists are set at the level of the ceiling and a framework of 2x4 inch members is constructed above the main joists.

▪ Wooden deck roofs commonly use lightweight 3/8 to 5/8 inch plywood decking which offers little fire resistance.

▪ It is difficult to cut concrete roofs that are over 4 inches in thickness.

▪ Poured gypsum roofs are usually covered with a layer of gypsum cement up to 2 1/2 inches thick to which mesh reinforcement is added.

▪ At least 2 sides of roof monitors should be removed.

▪ Ribbed arch roofs are usually constructed of 2x12 inch or larger wooden members.

▪ The chords of arch members in bowstring arch roofs are usually laminated 2x12 inch or larger lumber.  Rafters are usually 2x10 inches and are covered by 1x6 inch sheathing.

▪ Lamella roof framework usually consists 2x12 inch wooden members bolted together at intersections with steel gusset plates and covered by 1x6 inch planking covered with composition roofing material.

▪ In modern construction, heavy timber, laminated beams, and 1x6 inch sheathing have given way to 2x4 inch lumber and 1/2 inch plywood.

▪ Panelized roof construction consists of laminated beams of various sizes (commonly 6x36 inch) that span the length or width of the building.

▪ Beams in panelized roofs may be bolted together to form lengths in excess of 100 feet and be spaced from 12 to 20 feet apart.

▪ Wooden purlins in panelized roofs (usually 4x12 inches) with metal hangers are installed on 8 foot centers between and perpendicular to the beams.

▪ Wooden joists in panelized roofs (usually 2 inches by 4 inches by 8 feet) are installed with metal hangers on 2 foot centers between and perpendicular to the purlins.

▪ In panelized roofs, sheets of 1/2 inch plywood (4x8 feet) are nailed to the wooden joists.

▪ In open web trusses, steel tube members are made from 1 to 2 inch cold-rolled steel tubing with ends pressed flat into a semi-circular shape and a hole punched through each end.

▪ In open web trusses, spans of up to 70 feet are attainable using a single 2x4 inch or two 2x3 inch members as the top and bottom chords.

▪ In open web trusses,  joining 2x4 inch lumber with mitered and glued finger joints, a continuous member of 20 feet or more is possible.

▪ Normal spacing of joists in open web truss construction is 2 feet on center.

▪ Metal gusset plate truss construction usually consists of 2x4 inch lumber held together by gusset plates.

▪ Gusset plates with 3/8 inch prongs are most common.

▪ Spans of up to 55 feet are possible using 2x4 inch lumber and gusset plates.

▪ The most common spacing for metal gusset plate truss construction is 2 foot on center and usually covered with 1/2 inch plywood sheathing.

▪ Wooden I beams consist of a top chord, bottom chord, and a 3/8 inch plywood or chipboard stem.

▪ Wooden I beam chords are usually made from 2x4 inch lumber, however, 2x3 inch lumber may also be used.

▪ Common spacing for wooden I beams is 2 foot on center.

▪ Shingles or shakes are usually nailed to wooden 1x4 inch or 1x6 inch planked sheathing with a spacing of about 1 inch.

▪ Rolls of composition shingles are most commonly found in 36 inch widths.

▪ One form of urethane/isocyanate foam is 4x8 foot sheets of foam insulation.

▪ Lightweight metal roof coverings are usually 18 to 20 gauge steel panels set over a wood or metal substructure.

▪ Steel clad roofs either consist of 1/4 inch thick by 2 inch wide strips laid at right angles to form a "grid" which leaves small openings from 6 to 8 inches square, or 4x8 foot sheets of steel, ranging from 1/8 to 1/4 inch thick welded together to form a continuous layer of steel.

▪ A 1/4 inch steel plate weighs about 10 pounds per square foot.

▪ Roof covering may be cut with a chain saw or rotary saw (depending on type of roof) at a 45 degree angle to any exterior wall.  The blade will usually encounter a rafter before the cut is 3 feet long.

▪ If rafter direction cannot be determined, cut a small 8 to 10 inch triangular cut at the known rafter location to determine direction.

▪ One 8x8 foot ventilation hole is equal to four 4x4 foot holes.

▪ When cutting parallel to rafters, the hole will be approximately 4 feet wide and 4 or more feet long.

▪ Trench cuts are made approximately 4 feet wide across the entire width of a building.

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