Proper Spacing for Floor Joists

Floor joists in a construction project.

Properly spacing floor joists when framing an addition or whole structure is largely a factor of building codes. There are three basic measurements used for floor joist spacing. There are also several floor joist sizes and types used. The right spacing depends on the code requirements, as well as the type of structure it is and the weight it will bear. In this article, you'll start by understanding what floor joists are and why they are used. From there, the size and spacing options will be discussed.

Floor Joists

Floor joists are horizontally oriented beams that make a frame on which to build a raised floor. Raised floors are superior to floors built on a slab, mainly for the reason that they provide a space through which to install other components. Electrical and plumbing lines, as well as an area for insulation, are more easily accommodated with a raised floor. Raised floors are also better able to withstand the constant wear and tear a floor experiences.

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Integral Part of Framing

Structures are built able to bear a tremendous amount of weight. Floor joists, especially on the first floor, must be durable enough and properly spaced in order to function effectively. While they do not support the full weight of the home or other structure, joists are instrumental in ensuring that the building is soundly framed. Floor joists installed on second or third (or higher) levels double as ceiling joists for the level below. In buildings with basements, the first level of floor joists serves the same purpose.

Floor Joist Sizes

In the past, 2x4s or 2x6s were sometimes used as floor joists. This size is no longer acceptable when framing a structure. In general practice, 2x8s, 2x10s or 2x12s are used when framing a raised floor instead. The exact size necessary may be stipulated by the building code required for a particular structure. The weightier the structure, the more durable the floor joists must be. For most homes, 2x8s are suitable. For bigger structures, manufactured I-shaped joists are used. These more closely resemble girders used for framing the largest buildings.

Proper Spacing

As with the joist size, the building code for your structure will stipulate the proper spacing between floor joists. The code itself is determined by the engineering requirements of a particular building. Normally, floor joists are spaced 16 inches apart on center. This means from the center of one upright joist to the center of the next. Given that 2x8s are actually 1.5 inches wide, it works out to be 14.5 inches between each joist. With some structures, floor joists are required to be 12 or 24 inches apart on center.

To find the actual distance between the inside edges of each adjacent floor joist, subtract 1-¾ inches from either 12 or 24 inches. Still, when framing a raised floor, it's easier to think of floor joist spacing in terms of 12, 16 or 24 inches on center because each joist is positioned along the header according to its center point.

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