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What Is a Floor Joist Span?

By Troy Holmes
Updated May 16, 2024
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Floors are specifically designed to support the weight of people and furniture in homes and buildings, and part of the way they do this is with floor joists, which are horizontal supporting beams that run beneath the floor. A floor joist span is the length a floor joist goes across a room. This length has a maximum safety capacity that is based on the width and material used to make the floor joist.

Floor joist spacing is the distance a floor joist board can be separated from an adjacent floor joist. This spacing is combined with a floor joist span to determine the maximum capacity load of a floor. Each type of wood has a specific span and spacing requirement. Carpenters that build decks and floors typically use joist span tables to determine the maximum length and spacing of floor joists. There are different span tables available for each type of wood. These tables help carpenters design floors for buildings.

The elevation of an area can also affect what kind of floor joist span is needed. For example, an elevated deck that is designed to support the weight of many people would have a different span than a small, first floor room. The floor joist span for decks also depends on the floor dimensions and type of material used to build the deck.

This can also be affected by things placed on the floor or deck. For instance, adding a jacuzzi to a deck would necessitate a heavier and shorter floor joist span than that of a standard deck. A full Jacuzzi can add thousands of pounds weight to a deck. This requires a heavy-duty joist design that can support the additional weight.

The joist span of elevated floors is also smaller than the span of a floors built on the ground. Elevated floors hold more hanging weight and require a stronger foundation. These types of floors have shorter, thicker wood requirements to support heavier hanging weight.

Each type of floor joist has a predetermined amount of bend or flexibility. This is dependent on the span and material makeup of the floor joist. Homes that have second and third levels require stronger joists on the upper level floors. This helps reduce cracks that happen when people walk on the upper levels.

The joists of attics are specially designed to provide structural support for buildings. These joists are called ceiling rafters. The floor joist spacing of rafters depends on the length and height of the roof. Each type of roof design has a specific floor joist span requirement.

Installing floor joists requires a moderate level of carpentry experience. These joists are installed with special joist hangers that are attached to the sides of buildings. It is important that floor joists are installed and inspected by a professional. These joists are critical because they protect people from falling between the floor beams.

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Discussion Comments
By Soulfox — On Oct 03, 2014

@Markerrag -- That is true for one story homes, but two story homes have wooden joists out of necessity. I have seen steel joists in some homes with second floors, but those are somewhat rare.

By Markerrag — On Oct 03, 2014

As stout as these things are, they can fail over time as all sorts of things can ruin the integrity of wood (moisture, termites, etc.) It is wise, then, to inspect joists as often as is practical and have any damage repaired as quickly as possible.

One of the great things about homes built on concrete slabs is that you really don't have to worry about floor joists. A hunk of concrete is a lot more durable than a bunch of wooden joists, after all.

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