What is a Roof Joist?

Mary McMahon
Mary McMahon

A roof joist is a structural member which provides support for the roof and distributes its weight so that it is even. Some roof designs lack joists, depending on how they are designed, using careful manipulation of other structural members to achieve the desired level of strength and stability. Because the roof joist network is integral to the roof and the rest of a structure, joists should never be cut, removed, or adjusted without advice from a professional.

Construction worker on roof.
Construction worker on roof.

A roof joist can be made from wood, metal, concrete, and a variety of other materials, as long as the material is strong and very sturdy. Joists attach to beams, lying parallel to each other and being aligned horizontally. Roof supports which are angled are known as rafters. The network of joists connects to the rafters, distributing the weight of the roof to the beams the roof joists are attached to, and thus to the walls of the structure, which eventually connect with the foundation.

The even distribution of weight across a roof joist network is intended to prevent the roof from collapsing. Since the weight of a roof is fairly static, it is easy to design for, but engineers do need to think about issues like the accumulation of snow in cold climates, layers of roofing materials added over the lifetime of the structure, and changes in roofing materials when they determine roof joist placement. Switching from composition shingle to slate, for example, is going to generate a substantial change in the weight of the roof.

One disadvantage of roof joists is that they can tend to limit headspace in attics and upper stories, depending on how a structure is designed. If space is at a premium, different support structures may be used to ensure that upper stories are more usable. An engineer can also design custom variations, such as an irregular distribution of roof joists, which makes it possible to fit in a skylight, a loft, and other features.

Roof joists vary in size, depending on the material used and the requirements of the job. As a general rule, roof joists and other structural members are easy to identify, because they are larger than surrounding building materials and clearly provide support. While many systems are designed to be redundant so that they will continue to function if one or more structural supports becomes compromised, it is not a good idea to assume that it will be safe to cut through, move, or temporarily remove any structural support. An engineer can examine the structure and provide advice about which kinds of modifications are possible.

Joists are also used in flooring, where they are known as floor joists or simply joists.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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