Common sense tells us that large floor joists are able to carry more load, and spacing joists closer together also increase the load-bearing capacity of a floor. But larger is not always better when builders are constructing a home or adding a room addition. The extra two inches of vertical distance when a floor is framed with 2 x 10 joists rather than 2 x 12s can be quite important, for example. So the challenge to builders is to choose joists that are appropriate to the load they carry while maximizing space.
Figuring load capacities and picking the right floor joist sizes and spacing is a more complicated task than you might imagine since there are many variables at play:
- Wood species
- Grade of the lumber
- Width and thickness of the boards
- Spacing between joists
- Load placed on the floor
- Length that the joists span
Different wood species have different strength characteristics, with some having much higher bending strength than others. In general, species that are slow-growing have more growth rings per inch and are therefore considerably stronger than fast-growing trees. This is also true of trees within the same species—when conditions cause slow growth, the lumber from the tree will be stronger.
Common species used in-house framing include:
- Southern yellow pine and Douglas fir have high bending strength.
- Hemlock, spruce, and redwood have medium bending strength.
- Western red cedar, Eastern white pine, and ponderosa pine have low bending strength.
The fewer defects contained in a piece of lumber, the stronger it will be. Higher grades of lumber (clear, select, or 1) have fewer flaws, and will, therefore, be stronger. 2-grade lumber is a common choice for construction framing. While not as strong as the higher grades, the flaws in 2-grade lumber are generally not enough to seriously weaken the boars. Avoid using 3 or 4-grade lumber for structural framing applications. If you are hand-picking boards, examine them for knots and other flaws.
The strength of a given joist board is most dramatically affected by the board's top-to-bottom width. The width is considerably more important than the thickness of a board. For example, a joist made from doubled 2 x 6s can span a distance about 25 percent more than a single 2 x 6, but a 2 x 12 can span about 80 percent more than a 2 x 6, even though it has the same amount of wood as a doubled 2 x 6.
Joist span is also governed by the weight placed on the floor. Floor loads are described using two measurements: dead load and live load.
Dead load for residential construction is generally considered to be about 10 pounds per square foot. The dead load is calculated by adding together the weight of the building materials and dividing by the square footage.
The term live load refers to the total load carried by the floor, including furnishing, occupants, and other objects being stored. For residential floors, the live load is usually considered to be 30 to 40 pounds per square foot, although this varies depending on the location within the home. First-floor live loads have higher requirements than second-floor live loads (40 pounds per square ft. vs. 30 psf). A room used solely for sleeping might need to carry only 30 psf, whereas a garage floor over a basement would need 50 psf or higher. An inaccessible attic space, on the other hand, might have a live load of only 20 psf.
Joist Span Tables
Joist span refers to the measurement covered by the joist between supporting structures, such as beams or foundation walls. Builders generally use pre-calculated tables to tell them appropriate joist spans for each lumber species, size, and spacing. But local building codes should always be consulted, since unusual situations may call for different span recommendations. True floor joist span calculations can only be made by a structural engineer or contractor.
This sample table gives minimum floor joist sizes for joists spaced at 16 inches and 24 inches on-center (o.c.) for #2-grade lumber with 10 pounds per square foot of dead load and 40 pounds of live load, which is typical of normal residential construction:
|Western red cedar
Eastern white pine
|Joist Size||16″ o.c.||24″ o.c.||16″ o.c.||24″ o.c.||16″ o.c.||24″ o.c.|
|2×6||9′ 9″||8′ 3″||8′ 8″||7′ 6″||7′ 6″||6′ 3″|
|2×8||12′ 8″||10′ 8″||11′ 0″||10′ 2″||10′ 5″||8′ 6″|
|2×10||16′ 0″||13′ 0″||14′ 6″||12′ 4″||12′ 9″||10′ 5″|
|2×12||18′ 6″||15′ 0″||17′ 6″||14′ 4″||14′ 9″||13′ 0″|
Builders can adjust their choice of lumber size and spacing depending on circumstances. For example, where head-room is an issue, they can choose smaller joists and space them closer together with a shorter span. Or, where a long span is needed, such as when framing a ceiling above an open-concept room, larger joists made with a stronger lumber species can be chosen.