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Steel support used to hold a floor or ceiling.


A joist is used as a support structure between the floor and a girder. Often times the joist will be used on a wood based structure.

A joist is a construction term that describes the horizontal framing structure in the frame of a property. Its purpose is to stiffen the sheathing of a property, thereby strengthening the structure. They are typically found between beams, sometimes in groups of two or three joists depending on the size of the structure.  It’s specifically meant to strengthen the open space portion of a structure where weight and loads oftentimes are greatest where there are no pillars or other mechanisms to strengthen the building.

Types of Joists

Joist are typically made up of wood or steel, with the overwhelming majority of them being wood in residential structures and metal in commercial buildings. They can be made up of real solid wood or engineered wood. There are various designs of joists which are based on the type of material being used, the purpose of the joist, and the structure itself. Each type of joist has unique features and choosing the right one depends on the type of building and building safety requirements.

Purpose of Joists

Joists must have the necessary strength to endure extremely heavy weight loads. Building code standards require joists to be able to handle weight loads heavier than they almost would ever need to. The greater the depth of a joist, the stronger the support system will be. When spacing between joists increase, the closer the joists need to be to one another. Engineers and builders have specific formulas that calculate the number of joists needed and the distance that they need to be from one another. There are various lateral support mechanisms that increase the structures integrity including strutting, blocking, and dwangs.

There are different methods that allow joists to be attached to supporting beams. When a joist sits on top of a supporting beam, it is referred to as being lodged. Another common method is when a joist rests on top of the foundation, followed by a sill sitting on top of the joist.