Floor Trusses vs Floor Joists Floor Trusses vs Floor Joists

Residential construction has seen a tremendous increase in the use of floor trusses over the past several years. Floor trusses are not to be confused with the traditional construction method of using floor joists. Although both are systems designed to provide structural support to a floor system, the two are markedly different in a number of ways.

Floor Joists

Floor joists are a part of the structural support of a floor. They support the weight of the building and absorb impact, providing stability and security to the overall floor system. Floor joists are typically cut on-site and are placed at regular intervals. Depending on the local building codes and the contactor’s preference, they may be placed at 12 or 24-inch intervals, although 16-inch spacing is typical. Sizing is also dependent upon local building code. A joist header runs perpendicular to the joist, capping it at the ends. Spliced across beams and other support structures, joists provide the basis for sub-flooring. Plywood sheets may be used to construct this sub-floor, placed perpendicular to the floor joists. This layer of plywood or other sheet product is referred to as decking. Alternatively, boards may be placed across the joists, either diagonally or at right angles. Joists are doubled at openings, such as a stairway and are capped with headers, again placed perpendicular to the joists.

Floor Trusses

A modern and economical alternative to floor joists, trusses are manufactured off-site according to the exact requirements of each project. They should not be altered in any way, but are to be used in accordance with the plans exactly. If floor trusses require lengthening or shortening, this may be done under some circumstances, but requires the professional advice of a qualified engineer.


In order to accommodate for size discrepancies, there are suitable methods of slightly increasing or decreasing the span of a floor truss. Any modifications other than those outlined below require the advice of an engineer. Floor trusses should never be cut, drilled, or have connector plates removed.

Increasing the Span

An increase may be accomplished by placing an additional timber to the ends of the floor truss. This solution is suitable only for small extensions. The timbers must be attached vertically to the end of the truss, one timber at a time. A large increase in span is generally impossible and necessitates ordering new trusses built to the correct specifications.

Decreasing the Span

Most trusses have small cord extensions which may be removed with no decrease in performance or integrity. This method is suitable for decreasing the floor truss span slightly. While a large decrease in span is possible, it is not economical. Using trim-able trusses allows for limited on-site trimming, and eliminates many span related issues from becoming major concerns.


Both floor trusses and floor joist systems provide suitable structural support. Floor trusses, though, are an economical alternative to joists. Because they are manufactured off-site according to the specifications of the project, the need for alterations is almost completely eliminated. Slight alterations can be made on-site if necessary, as outlined above. Floor joists, while customizable on-site, involve the possibility of human error and may be even less cost-efficient if such mistakes are made, necessitating additional material and labor to rectify miscalculations.

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