Chapter 4 Building Construction


Two or more interconnected structural components combined to meet a specific function or design requirement. Typical assemblies are roof trusses, wall frames, and doors including their frames.


An open space between the roof and ceiling of a building; most commonly found in single and multifamily residential occupancies. Attics provide open spaces in which fires can burn undetected or spread throughout a structure.

Balloon frame

A type of wood-frame construction in which the studs in exterior walls extend from the basement or foundation to the roof. This type of construction allows fires to spread, often undetected, from the basement to the attic through the hollow walls.

Bar joist

A joist constructed of steel with bars in the vertical web space. A common structural component in office buildings and other commercial structures. Very high strength-to-weight ratio except when exposed to the heat of a fire then early failure is likely.


A horizontal structural component subjected to vertical loads. Typical beams are steel or wooden I-beams or large-dimensions wooden members.

Bowstring truss

A roof assembly with a curved (arched) top chord and a horizontal bottom chord. These assemblies are very strong except when exposed to direct flame contact when catastrophic failure without warning may occur.

Butterfly roof

A V-shaped roof in which the two sides slope toward a valley in the middle. An unusual type of roof that is rarely seen in cold climates where snow load is a factor


A beam that is unsupported at one or both ends. Typically used to support balconies on apartments and some office buildings

Oriented strand board (OSB) aka chipboard

A wooden structural panel formed by gluing and compressing wood strands together under pressure. This material has replaced plywood and planking in the majority of construction applications. Roof decks, walls, and subfloors are all commonly made of OSB.


The main structural members of a truss as distinguished from diagonals. Chords span the open space between the upper and lower diagonal member in a truss assembly.


An open space between the roof and ceiling of a commercial or industrial building. Usually found under flat or nearly flat roofs. In a fire, these spaces act in much the same way as attics.


A vertical supporting member. Columns may be wooden or steel posts. Steel posts often support lightweight roof assemblies, and if unprotected by surface insulation, steel posts may fail quickly in a fire.


Force that tends to push the mass of a material together. Bearing walls in a building are under compression from the weight of the roof and other materials above.


Horizontal layer of masonry units. A row of bricks is an example of a course.

Curtain board

Nonload-bearing interior wall extending down from a roof or ceiling to limit the horizontal spread of fire and heat. If curtain walls are penetrated by unprotected openings, fire can spread unchecked.

Curtain wall

Nonload-bearing exterior wall used as a weather barrier but not for structural support. On many high-rise buildings, the outside walls (often sheet glass in frames) are curtain walls.


Planks or panels of plywood or oriented strand board (OSB) that form the substrate of a roof assembly. In vertical ventilation through a roof, the decking must be removed from the ventilation opening to realize the full effect of the opening.


Gypsum wall board. A fire-resistive wall covering also called Sheetrock.


The edge of a pitched roof that overhangs an outside wall. Attic vents in typical eaves provide an avenue for an exterior fire to enter the attic.

Engineered I-beam

A wooden I-beam consisting of continuous wooden upper and lower chords separated by a web of OSB or similar sheet rock.

Fire door

A rated assembly consisting of a solid core door, door frame, and hardware. Fire doors are used to confine a fire to one or section of a building by closing a communicating opening when triggered by a fire. If fire doors are to function as designed, they

Fire load

Total potential heat release if a building and it's contents burned. The fire load of a fully stocked lumber yard is considerably higher than that of an empty building of the same dimensions.

Fire wall

A rated assembly that extends from the foundation to and through the roof of a building to limit fire spread. Fire walls are intended to confine a fire to one room or section of a building. If they are penetrated by openings not protected with fire doors,

Flat roof

A roof that is flat or nearly flat relative to the horizon. Many commercial buildings have flat roofs covered with tar and gravel or other weatherproof material. Flat roofs lend themselves to being opened for vertical ventilation.

Gable roof

A pitched roof characterized by square cut ends and sides that slope down from the ridge line to the eaves. These are the most common roof style on homes and other small buildings.

Gable wall

A wall rising to meet a gable roof at the end of a building. These walls are found only at the ends of gable roofs and they often include an attic vent near the top of the wall.

Gambrel roof

A roof characterized by a single ridge line from which roof sections on both sides of the ridge descend at two different pitches. These roofs are common on barns and other farm structures. Because of the differing angles of the slopes, gambrel roofs can m


A horizontal structural member used to support beams or joists. Girders are almost always of larger dimensions than the members they support.

Glue-lam beam

A wooden structural member composed of relatively short pieces of lumber glued and laminated together under pressure to form a long, extremely strong, beam. Because of the mass of most glue-lam beams, they resist fire extremely well compared to other mate

Gusset plate

Wooden or metal plate used to connect structural members that are butted together; most often used in the construction of trusses. Many metal gusset plates are simply pressed into the wood and are subject to early failure if the plates warp from the heat

Gypsum board

Interior finish material consisting of calcinated gypsum, starch, water, and other additives sandwiched between two sheets of specially treated paper; see drywall.

Header course

Course of bricks laid with the ends facing outward. Because the ends of the bricks are smaller than the sides, a header course is easy to identify. Header courses are only used in unreinforced masonry, and this makes that type of construction easy to iden

Hip roof

A pitched roof in which the ends are all beveled so that there are no gable walls. A common roof style on many newer residences. Unlike gable roofs, in hip roofs the attic vents are only under the eaves or on the roof.


Abbreviation for heating, ventilation, and air conditioning. Unless properly protected with automatic fire dampers, the ductwork associated with these systems can allow smoke and fire to spread throughout a building.

Interstitial space

An accessible or inaccessible space between layers of building materials; an attic or cockloft sometimes used to house HVAC and other machinery. Like attics and cocklofts, unless properly protected, these spaces can allow fire to burn undetected or to spr


Horizontal structural members used to support a ceiling or floor. Drywall materials are nailed or screwed to the ceiling joists, and the subfloor is nailed or screwed to the floor joists.

Lamella arch

An arch constructed of short wooden members connected in a specific geometric pattern. While rare in modern construction, these roof assemblies can still be found in many older buildings.

Mansard roof

A roof characterized by steeply sloped facets surrounding a flat or nearly flat center section. Many remodeled buildings have false mansard roofs that consist of a fascia added to an existing flat roof. In some cases, the fascia forms a concealed space in


A mixture of sand, cement, and water used to bond masonry units into a solid mass. The joints between bricks are filled with mortar. Mortar joints are sometimes the easiest to penetrate when a masonry wall must be breached.

Open web joist

A joist constructed with a web composed of materials such as bars or tubes that do not fill the entire web space. These are very common building assemblies because of their strength compared to their cost. When exposed to fire they lose their strength qui

Parallel chord truss

A truss constructed with the top and bottom chords parallel. These trusses are used as floor joists in multistory buildings and as ceiling joists in buildings with flat roofs.


A wall at the edge of some roofs. Most parapet walls range from a few inches to a few feet in height, but they can be high enough to require a ladder to reach the roof from the top of the wall. At night, parapet walls can be significant trip hazards for f

Party wall

A wall shared by two adjoining buildings; usually a load bearing wall that is also a fire wall. The failure of a roof assembly attached to a party wall can affect the structural integrity of the adjoining building.


The ratio of rise-to-span of a roof assembly. The steeper the pitch, the greater the slip hazard unless roof ladders are used.

Pitched roof

A roof that is sloped (pitched) to facilitate runoff. Pitched roofs range from those that are flat to those that are extremely steep, such as are common on some churches.


The top or bottom horizontal member of a frame wall. The sole plate is nailed or screwed to the subfloor, and the top plate is what the roof assembly rests on.

Platform construction

Frame-type construction in which each floor interrupts the exterior studs forming an effective fire-stop at every floor. This is the most common type of construction used to frame modern residences and other small buildings.


A wooden structural panel formed by gluing and laminating very thin sheets of wood together under pressure. Plywood is still used in some applications but has been replaced in construction by OSB


Beams that span from a ridge board to an exterior wall plate to support roof decking. While it is important to cut away roof decking during vertical ventilation operations, cutting rafters can seriously weaken a roof and should be avoided whenever possibl

Rated assembly

Two or more construction components combined to form an assembly that has a specific fire-resistance rating. A fire door is an example of a rated assembly as well as a wood frame wall covered with a specified thickness of gypsum drywall


Shoe for reinforcing bar. These steel bars are placed in concrete forms before the cement is outed. When the concrete sets (hardens) the rebar within it adds considerable strength.

Reinforced concrete

Concrete that has been poured into forms that contain an interconnected network of steel rebar

Sawtooth roof

A roof with a profile of vertical and sloping surfaces that resemble a saw blade. These roofs are common on older industrial buildings, but many are still in existence. The vertical walls in these roofs usually include many windows to allow light in. In s


Plywood, OSB, or wooden planking (sometimes called sheeting) applied to a wall or roof over which a weather resistant covering is applied. Most sheathing is relatively easy to penetrate for forcible entry or ventilation.

Shed roof

A pitched roof that slopes in one direction only from the ridge


Degradation of concrete due to prolonged exposure to high heat. Water trapped within the concrete is vaporized by the heat and expands, causing the concrete to break apart. While spalling concrete can sound like fun fire, in most cases it is relatively ha


A vertical structural member in a frame wall. Stud walls are the assemblies to which wall coverings are nailed or screwed. Studs can be made of either wood or light-gauge steel.


Force that tends to pull the mass of a material apart. Tension is what causes some roof assemblies to pull away from walls and fall inward.


A wooden or metal structural unit made up of one or more triangles in a flat plane. Because of the inherent strength of the triangles within its structure, when a truss is intact it is much stronger than the individual members of which it is made.

Load bearing wall

A wall that is used for structural support

Nonload bearing wall

Wall, usually interior, that supports only its own weight

Partition wall

Interior non load bearing wall that separates a space into rooms

Green wood

Wood with high moisture content

Fire wall

Fire rated wall with a specified degree of fire resistance, built of fire resistive materials and usually extending from the foundation up to and through the roof of a building, that is designed to limit the spread of a fire within a structure or between


Bricks blocks stones and unreinforced and reinforced concrete products

Cantilever walls

Walls that extend beyond the structure that supports them

Veneer walls

Walls with a surface layer of attractive material laid over a base of common material.

Fire resistive construction

Another term for type 1 construction; construction that maintains it's structural integrity during a fire.

Non combustible construction

Another term for type II construction; construction made of the same materials as fire-resistive construction except that the structural components lack the insulation or other protection of type I construction

Ordinary construction

Another term for type III construction; construction that requires that exterior walls and structural members be made of non combustible or limited combustible material.

Wood frame construction

Another term for type V construction; construction that has exterior walls, bearing walls, floors, roofs, and supports made completely or partially of wood or other approved materials of smaller dimensions than those used for heavy timber construction

Heavy timber construction

Type IV construction that requires that exterior and interior walls and their associated structural members be made of non combustible or limited combustible material

Heavy fire loading

Presence of large amounts of combustible materials in an area or a building

Roof covering

Final outside cover that is places on top of a roof deck assembly

Collapse zone

The area extending horizontally from the base of the wall to one and one half times the height of the wall

Lightweight steel truss

Structural support made from a long steel bar that is bent at a 90 degree angle with flat or angular pieces welded to the top and bottom

Lightweight wood truss

Structural supports constructed of 2x3 inch or 2x4 inch members that are connected by gusset plates

Gusset plates

Metal or wooden plates used to connect and strengthen the intersections of metal or wooden truss components roof or floor components into a load-bearing unit.

Gang nail

Form of gusset plate. These thin steel plates are punched with acutely V-shaped holes that form sharp prongs on one side that penetrate wooden members to fasten them together

Rain roof

A second roof constructed over an older roof