The Difference Between Joists & Rafters
- Joists are the parallel horizontal beams that run across an open space, joining opposite walls in a building to support both the ceiling below and the floor above. Joists are traditionally made from timber. They are 2 inches wide and come in a variety of depths from 8 to 12 inches. The choice depends on the load-bearing requirement, which is strongly influenced by the length of the span that the beam must bridge.
- In many modern building projects, traditional timber joists are replaced by ones manufactured from alternative materials. Steel "I-profile" beams are used in the construction of steel-framed buildings. These have the advantage of spanning greater distances without bowing. Timber "I-joists" are also available. These are manufactured with laminated or dimensional timber forming the flanges of the "I" and plywood forming the center, or web. Pre-stressed concrete beams are used in some construction projects. These are combined with innovative flooring methods that can help speed building work.
- Rafters are the building components that run diagonally from the wall plate to the apex of a roof structure. Rafters form the foundation for attachment of the roof boards, onto which the final roof covering is fixed. In traditional timber, they are 2 inches wide and 8 to 10 inches deep.
- In many modern construction projects, prefabricated roof trusses are used. These triangulated structures, with a high degree of strength and stability due to the geometry used, form both the joist and rafter components of a building. These structures are typically manufactured offsite to a detailed plan and delivered complete, which helps to speed construction.
