Floor Joist Spacing

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What Is Floor Joist Span and Spacing?

Floor joists are supporting members used in building floors that span over an open space and between load-bearing walls and structural beams. They are placed parallel and equidistant to each other, and connect to vertical members like studs and support beams which distribute weight across the rest of the framing.

Individual joists need to be able to support the anticipated load for the building over long periods of time. It is important to have joists at the right depth for a project, because they provide critical stability for a floor or ceiling system.

Joists are usually spaced one or two feet apart. The required joist depth gets larger depending on how far apart each joist is from each other. This limits stress on the joists and prevents deflection over time. A general rule for sizing joist depth is to take half of the intended joist span in feet, add two, and then use that number in inches. For example, a 12-foot span would require a joist span of 8 inches:

12 ft. / 2 = 6

6 + 2 = 8-inch joist span

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Blocking the spaces between joists with smaller pieces of wood can reduce this stress and increase overall stability in the floor framing. A floor frame can have more strength if it is built with engineered wood joists instead of the more common farmed lumber. Prefabricated joist options such as I-joists increase the overall depth and are made with higher quality wood.

I have an addition that is to be built for a homeowner who wants I-joists. I&#;ve gotten MUCH conflicting info between my Lumberyard and the Big box stores as to the right size joist.

The big boxes are spec&#;ing Boise Cascade I-joists. The homeowner says they want to feel the floor as solid as possible with as little hint of deflection as possible.

The structure in question here is 20ft 6&#; long x 28ft wide. I plan to space I-joists 12 o.c across the width of 28ft for a total of 26 I-joists. I&#;m using 11 7/8&#; I josits. The span that they must span is of course 26ft 6&#; (this span includes the foundation wall, outside to outside).

The owner plans for carpet on the floor, maybe wood. A possible large typical aquarium, typical furniture and chairs (this is a Sunroom). The upstairs floor, which mirrrors the first, will be a master BR consisting of typical BR furnishings like a nice heavy bed set.

I&#;ve been told I can get away with a BCI 90 which is a 3.5&#; top/bot cord of the joist. I also been told I can do it with a BCI MINIMALLY, or that I can use a BCI . One places says I&#;m told by the others wrongly. One place says I MUST use a 3.5&#; cord. Other places say I must use web stiffeners.

Any opinions out there on who is right? One place claims the mfgr says I must use 3.5&#;. I don&#;t know who to believe. The price difference is almost double between BCI and BCI 90.

If at first you don&#;t succeed, try using a hammer next time&#;everything needs some extra persuasion from time to time.  -ME

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