Bowstring Truss — Lightweight truss design noted by the bow shape, or curve, of the top chord.
Chimney Effect — Created when a ventilation opening is made in the upper portion of a building and air currents throughout the building are drawn in the direction of the opening; also occurs in wildland fires when the fire advances up a V-shaped drainage swale.
Churning — Movement of smoke being blown out of a ventilation opening only to be drawn back inside by the negative pressure created by the ejector because the open area around the ejector has not been sealed. Also called recirculation.
Forced Ventilation — Any means other than natural ventilation. This type of ventilation may involve the use of fans, blowers, smoke ejectors, and fire streams. Also called Mechanical Ventilation.
Horizontal Ventilation — Any technique by which heat, smoke, and other products of combustion are channeled horizontally out of a structure by way of existing or created horizontal openings such as windows, doors, or other holes in walls.
Hydraulic Ventilation — Method of ventilating a fire building by directing a fog stream of water out a window to increase air and smoke movement.
Kerf Cut — A single cut the width of the saw blade made in a roof to check for fire extension.
Louver Cut or Vent — Rectangular exit opening cut in a roof, allowing a section of roof deck (still nailed to a center rafter) to be tilted, thus creating an opening similar to a louver. Also called center rafter cut.
Lamella Arch — A special type of arch constructed of short pieces of wood called lamellas.
Leeward Side — Protected side; the direction opposite from which the wind is blowing.
Mushrooming — Tendency of heat, smoke, and other products of combustion to rise until they encounter a horizontal obstruction. At this point they will spread laterally until they encounter vertical obstructions and begin to bank downward.
Natural Ventilation — Techniques that use the wind, convection currents, and other natural phenomena to ventilate a structure without the use of fans, blowers, or other mechanical devices.
Negative-Pressure Ventilation — Technique using smoke ejectors to develop artificial circulation and to pull smoke out of a structure. Smoke ejectors are placed in windows, doors, or roof vent holes to pull the smoke, heat, and gases from inside the building and eject them to the exterior.
Positive-Pressure Ventilation (PPV) — Method of ventilating a confined space by mechanically blowing fresh air into the space in sufficient volume to create a slight positive pressure within and thereby forcing the contaminated atmosphere out the exit opening.
Purlin — Horizontal member between trusses that supports the roof.
Pyrolysis (Pyrolysis Process or Sublimation) — Thermal or chemical decomposition of fuel (matter) because of heat that generally results in the lowered ignition temperature of the material. The pre-ignition combustion phase of burning during which heat energy is absorbed by the fuel, which in turn gives off flammable tars, pitches, and gases.
Roof Covering — Final outside cover that is placed on top of a roof deck assembly. Common roof coverings include composition or wood shake shingles, tile, slate, tin, or asphaltic tar paper.
Roof Ladder — Straight ladder with folding hooks at the top end. The hooks anchor the ladder over the roof ridge.
Stack Effect — Phenomenon of a strong air draft moving from ground level to the roof level of a building. Affected by building height, configuration, and temperature differences between inside and outside air.
Thermal Column — Updraft of heated air, fire gases, and smoke directly above the involved fire area.
Thermal Layering (of Gases) — Outcome of combustion in a confined space in which gases tend to form into layers, according to temperature, with the hottest gases are found at the ceiling and the coolest gases at floor level.
Trench Ventilation — Defensive tactic that involves cutting an exit opening in the roof of a burning building, extending from one outside wall to the other, to create an opening at which a spreading fire may be cut off.
Vertical Ventilation — Ventilating at the highest point of a building through existing or created openings and channeling the contaminated atmosphere vertically within the structure and out the top. Done with holes in the roof, skylights, roof vents, or roof doors.
Windward Side — The side or direction from which the wind is blowing.