A joist is a horizontal structural member used in
framing to span an open space, often between
beams that subsequently transfer loads to vertical members. When incorporated into a
floor
A floor is the bottom surface of a room or vehicle. Floors vary from simple dirt in a cave to many layered surfaces made with modern technology. Floors may be stone, wood, bamboo, metal or any other material that can support the expected load ...
framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal
diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.
Joists are either made of wood,
engineered wood
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of woo ...
, or steel, each of which has unique characteristics. Typically, wood joists have the
cross section
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
of a
plank
Plank may refer to:
*Plank (wood), flat, elongated, and rectangular timber with parallel faces
*Plank (exercise), an isometric exercise for the abdominal muscles
* Martins Creek (Kentucky), the location of Plank post office
* ''The Plank'' (1967 fi ...
with the longer faces positioned vertically. However, engineered wood joists may have a cross section resembling the Roman capital letter ""; these joists are referred to as
-joists. Steel joists can take on various shapes, resembling the Roman capital letters "C", "", "L" and "S".
Wood joists were also used in old-style
timber framing
Timber framing (german: Holzfachwerk) and "post-and-beam" construction are traditional methods of building with heavy timbers, creating structures using squared-off and carefully fitted and joined timbers with joints secured by large wooden ...
. The invention of the circular saw for use in modern
sawmill
A sawmill (saw mill, saw-mill) or lumber mill is a facility where logs are cut into lumber. Modern sawmills use a motorized saw to cut logs lengthwise to make long pieces, and crosswise to length depending on standard or custom sizes (dimensi ...
s has made it possible to fabricate wood joists as
dimensional lumber
Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
.
Strength characteristics
Joists must exhibit the strength to support the anticipated load over a long period of time. In many countries, the fabrication and installation of all framing members including joists must meet
building code
A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...
standards. Considering the cross section of a typical joist, the overall depth of the joist is critical in establishing a safe and stable floor or ceiling system. The wider the spacing between the joists, the deeper the joist will need to be to limit stress and
deflection
Deflection or deflexion may refer to:
Board games
* Deflection (chess), a tactic that forces an opposing chess piece to leave a square
* Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers
Mechanics
* Deflection ...
under load. Lateral support called
dwang
In construction, a noggingFleming, Eric. ''Construction technology: an illustrated introduction''. Oxford: Blackwell, 2005. 123, 160. Print. or nogging piece (England and Australia), dwang (Scotland, South Island, New Zealand, and lower/central N ...
, blocking,
[Emmitt, Stephen, R. Barry, and Christopher A. Gorse. ''Barry's advanced construction of buildings''. 2nd ed. Chichester, U.K.: Wiley-Blackwell, 2010. 3-94. Print.] or strutting
increases its stability, preventing the joist from buckling under load. There are approved
formulas
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
for calculating the depth required and reducing the depth as needed; however, a rule of thumb for calculating the depth of a wooden floor joist for a residential property is to take half the span in feet, add two, and use the resulting number as the depth in inches; for example, the joist depth required for a 14‑foot span is 9 inches. Many steel joist manufacturers supply load tables in order to allow designers to select the proper joist sizes for their projects.
Standard dimensional lumber joists have their limitations due to the limits of what farmed lumber can provide. Engineered wood products such as
I-joist
An engineered wood joist, more commonly known as an I-joist, is a product designed to eliminate problems that occur with conventional wood joists. Invented in 1969, the I-joist is an engineered wood product that has great strength in relation to ...
s gain strength from expanding the overall depth of the joist, as well as by providing high-quality engineered wood for both the bottom and the top chords of the joist. A common saying regarding structural design is that "deeper is cheaper", referring to the more cost-effective design of a given structure by using deeper but more expensive joists, because fewer joists are needed and longer spans are achieved, which more than makes up for the added cost of deeper joists.
Types
![Lipman Wolfe Warehouse-2](https://upload.wikimedia.org/wikipedia/commons/8/8c/Lipman_Wolfe_Warehouse-2.jpg)
In
traditional timber framing there may be a single set of joists which carry both a floor and ceiling called a single floor (single joist floor, single framed floor) or two sets of joists, one carrying the floor and another carrying the ceiling called a double floor (double framed floor). The term binding joist is sometimes used to describe beams at floor level running perpendicular to the ridge of a gable roof and joined to the intermediate
posts. Joists which land on a binding joist are called bridging joists. A large beam in the ceiling of a room carrying joists is a
summer beam. A ceiling joist may be installed flush with the bottom of the beam or sometimes below the beam. Joists left exposed and visible from below are called "naked flooring" or "articulated" (a modern U.S. term) and were typically planed smooth (wrought) and sometimes
chamfer
A chamfer or is a transitional edge between two faces of an object. Sometimes defined as a form of bevel, it is often created at a 45° angle between two adjoining right-angled faces.
Chamfers are frequently used in machining, carpentry, fu ...
ed or
beaded.
Connections to supporting beams
Joists may join to their supporting beams in many ways: joists resting on top of the supporting beams are said to be "lodged"; dropped in using a butt cog joint (a type of lap joint), half-dovetail butt cog, or a half-dovetail lap joint. Joists may also be tenoned in during the raising with a soffit tenon or a tusk tenon (possibly with a housing). Joists can also be joined by being slipped into mortises after the beams are in place such as a chase mortise (pulley mortise), L-mortise, or "short joist". Also, in some Dutch-American work, ground level joists are placed on a foundation and then a sill placed on top of the joists such as what timber frame builder Jack Sobon called an "inverted sill" or with a "plank sill".
![Interieur stal, overzicht balklaag - Bocholtz - 20399966 - RCE](https://upload.wikimedia.org/wikipedia/commons/d/d5/Interieur_stal%2C_overzicht_balklaag_-_Bocholtz_-_20399966_-_RCE.jpg)
Joists can have different joints on either ends such as being tenoned on one end and lodged on the other end.
A reduction in the under-side of cogged joist-ends may be square, sloped or curved. Typically joists do not tie the beams together, but sometimes they are pinned or designed to hold under tension. Joists on the ground floor were sometimes a pole (pole joist, half-round joist, log joist. A round timber with one flat surface) and in barns long joists were sometimes supported on a sleeper (a timber not joined to but supporting other beams).
Joists left out of an area form an opening called a "well" as in a stairwell or chimney-well. The joists forming the well are the heading joist (header) and trimming joist (trimmer). Trimmers take the name of the feature such as hearth trimmer, stair trimmer, etc.
Shortened joists are said to be ''crippled''. The term
rim joist
Rim may refer to:
*Rim (basketball), the hoop through which the ball must pass
**Breakaway rim, a sprung basketball rim
*Rim (coin), the raised edge which surrounds the coin design
*Rim (crater), extending above the local surface
*Rim (firearms), ...
is rare before the 1940s in America; it forms the edge of a floor. The outermost joist in half timber construction may be of a more durable species than the interior joists. In a barn, loose poles above the drive floor are called a
scaffold
Scaffolding, also called scaffold or staging, is a temporary structure used to support a work crew and materials to aid in the construction, maintenance and repair of buildings, bridges and all other man-made structures. Scaffolds are widely use ...
. Between the joists, the area called a joist-bay, and above the ceiling in some old houses is material called
pugging
A pugmill or pug mill is a machine in which clay or other materials are mixed into a plastic state or a similar machine for the trituration of ore. Industrial applications are found in pottery, bricks, cement and some parts of the concrete and as ...
, which was used to deaden sound, insulate, and resist the spread of fire.
![Joist Hanger](https://upload.wikimedia.org/wikipedia/commons/4/4d/Joist_Hanger.jpg)
In
platform framing
Framing, in construction, is the fitting together of pieces to give a structure support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called ''mass wall ...
, the joists may be connected to the rim joist with
toenailing Toenailing or skew-nailing is the driving of a nail at a roughly 45-degree angle to fasten two pieces of wood together, typically with their grains perpendicular. A common example is toenailing a wall stud to a sole plate in stud framing.
Toenails ...
or by using a joist hanger.
See also
*
Beam (structure)
A beam is a structural element that primarily resists Structural load, loads applied laterally to the beam's axis (an element designed to carry primarily axial load would be a strut or column). Its mode of Deflection (engineering), deflection is ...
*
Framing (construction)
Framing, in construction, is the fitting together of pieces to give a structure support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called ''mass wall ...
*
Girder
A girder () is a support beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a stabilizing ' ...
*
Purlin
A purlin (or historically purline, purloyne, purling, perling) is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
Pu ...
*
Rafter
A rafter is one of a series of sloped structural members such as wooden beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck and its associated ...
*
Truss
A truss is an assembly of ''members'' such as beams, connected by ''nodes'', that creates a rigid structure.
In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assembl ...
References
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