A
joist, in
architectureArchitecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...
and
engineeringEngineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, is one of the horizontal supporting members that run from wall to wall, wall to beam, or beam to beam to support a
ceilingA ceiling is an overhead interior surface that covers the upper limit of a room. It is generally not a structural element, but a finished surface concealing the underside of the floor or roof structure above....
,
roofA roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....
, or
floorA floor is the walking surface of a room or vehicle. Floors vary from simple dirt in a cave to many-layered surfaces using modern technology...
. It may be made of
woodWood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
,
steelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, or
concreteConcrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
. Typically, a
beamA beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...
is bigger than, and is thus distinguished from, a joist. Joists are often supported by beams and are usually repetitive.
The wider the span between the supporting structures, the deeper the joist will need to be if it is not to deflect under load.
Lateral supportLateral support may mean:* Lateral support, a type of support to help prevent sideways movement* Lateral and subjacent support, a legal term...
also increases its strength. There are approved
formulasEuler–Bernoulli beam theory is a simplification of the linear theory of elasticity which provides a means of calculating the load-carrying and deflection characteristics of beams. It covers the case for small deflections of a beam which is subjected to lateral loads only...
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 half the span in feet plus two 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.
Engineered wood products such as
I-joistAn engineered wood joist, more commonly known as an I-joist, is a product designed to eliminate many of the problems that occur with using conventional wood joists. Invented in 1969, the I-joist is a man made engineered wood product which has incredible strength in relation to its size and weight...
s gain strength from the depth of the floor or the height of each joist. A common saying in the industry is that
deeper is cheaper, referring to the lower quality but often more cost effective joists designed for joist depths of 14 inches or more.
Bandsill is another term for joist used by construction workers and home inspectors in the southeastern United States.
See also
- Rafter
A rafter is one of a series of sloped structural members , that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.-Design:...
- Truss
In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...
- Purlin
In architecture or structural engineering or building, a purlin is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc...
- Girder
A girder is a support beam used in construction. Girders often have an I-beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams...
- Bandsill
- Framing (construction)
Framing, in construction known as light-frame construction, is a building technique based around structural members, usually called studs, which provide a stable frame to which interior and exterior wall coverings are attached, and covered by a roof comprising horizontal ceiling joists and sloping...