Floor & Roof Joist Systems

Joist Basics

• Joist systems use individual beams of wood or steel to create a support network for a floor or roof. The material used for a joist depends upon the amount of pressure placed on the beams – steel provides much greater strength than wood. In situations of particularly strong pressure, builders sometimes coat steel joists in concrete. Joist beams sit parallel to one another under an entire floor or roof surface at even intervals and support not only the floor or roof but also everything on top of it.

Floor Joists

• Floor joists run parallel across an entire surface of a floor, from wall to wall. In domestic settings, builders commonly use wood joists for floor systems. Industrial settings require the use of steel joists to support the weight of machinery and other heavy things. In addition to supporting everything on a floor, joists must help support the weight of walls sitting on floors. Joist systems installed in second-story floors or higher extend from walls and must take into account the load-bearing capacity of a wall. For instance, you must reinforce any joists extending from a wall over a window because the window cannot bear the same load as the surrounding areas of wall.

Roof Joists

• Roof joists provide the basic frame for a roof. Angled roof joists create a pitched roof, while flat roof joists create a flat roof. Builders often use angled joists because they direct weight away from joists and to the walls and foundation of a building, placing less stress on the joists themselves. Roof joist systems must possess the capability to support not only the weight of a roof but also anything that might end up on on that roof, from workers to accumulated snowfall.

Joists vs. Trusses

• Trusses and joists can resemble one another, particularly in roofs, though they differ in important ways. Trusses use beams assembled in a triangular shape to distribute weight in numerous directions to prevent the concentration of stress in one place. A joist consists of nothing more than a single beam in a joist system. A truss, on the other hand, consists of three or more beams assembled in a triangular shape. Furthermore, joists always assume the same basic form – a system of parallel beams. Though all joists assume a basically triangular shape, numerous derivations exist in truss formation and alignment.