Understanding Floor Joists and Optimal Spacing

Ensuring your floor joists are correctly sized and spaced is essential for your home’s structural stability. Inadequate joists can cause issues like sagging, uneven floors, or worse, collapse, posing safety risks and requiring expensive repairs. Besides flooring weight, factors like furniture, appliances, and foot traffic also affect the load on your floor joists. In this article, VINAWOOD will discover the fundamentals of floor joists, understand floor joist spacing, and explore their impact on your home and upcoming remodeling projects.

What is a floor joist and floor joist spacing?

Floor joists are horizontal structural elements, often made of 2×8 or 2×10 lumber, which bear the load of the floors within your dwelling. Arranged parallel to one another, they are usually spaced evenly along the floor’s expanse.

The beams are often obscured by layers of other materials such as flooring or ceilings. To pinpoint the location of your floor joists, you can employ a stud finder or search for lines of nails along your subfloor.

The floor joist spacing refers to the maximum distance that a structural member, like a joist, can cover from one end to the opposite end. Several factors contribute to determining the joist span, which include, but are not limited to, the distance to be covered, the width and thickness of the joist, the material used, and the spacing between the joists.

The dimensions and arrangement of floor joists vary based on multiple factors, such as the anticipated load they must bear, the distance they span, and the specifications outlined in building codes.

Floor joist spacing is always equal and connected to the sill plate
Floor joist spacing is always equal and connected to the sill plat

Components of a Floor Frame

After knowing what is a floor joist and floor joist spacing, you should know about the floor frame comprising various elements designed to provide support and stability to each layer of flooring. Below are the primary components of a floor frame:

  • Floor joist: Horizontal beams or slats responsible for carrying the weight of flooring materials.
  • Header joist: Frames the openings within the floor structure.
  • Blocking: Small wooden blocks placed between joists to reinforce and stabilize the structure.
  • Beam: Offers structural integrity and bears the load of the floor; typically constructed of lumber or steel.
  • Sill plate: Treated wood affixed atop the foundation of a building; floor joists are attached to the sill plate.
  • Subfloor: Panels of plywood oriented strand board (OSB) secured to the floor joists; underlayment and finished flooring materials are installed on top of the subfloor.
The Construction of a Floor Frame has 5 key factors
Components of a Floor Frame has 6 key factors

Types of Floor Joists

There are three primary types of floor joists, each with distinct characteristics in terms of cost, lifespan, and installation complexity.

  • Solid Lumber Floor Joists: This type is the most common and economical, crafted from mature trees. The species of wood chosen dictates the board size and limits span distances. Care must be taken as using younger trees may result in warped boards.
  • I-Joists (TJI Joists): These are more expensive compared to solid lumber floor joists but offer strength, lightness, and durability due to their composition of different materials. I-Joists typically feature a wood or laminated veneer top and a plywood center support. They provide long and continuous spans suitable for residential flooring and roofing.
  • Open-Web Floor Trusses: Constructed from cantilevered wood pieces, these joists are favored by builders, especially in commercial projects. Their natural openings make them ideal for accommodating pipes and wires without the need for cutting holes in the wood. However, they tend to be costly and require careful custom design and installation for each project.

>>> What To Select? Wood Vs. Engineered Lumber

Floor Joists has 3 types
The Importance Floor Joist Spacing And Size In Home 6

Floor Joist Spacing Chart

Joist span is the distance covered by the joist between supporting structures, like beams or foundation walls. Builders typically rely on pre-calculated tables to determine appropriate joist spans based on size, spacing and type of wood.

Consulting local building codes is crucial, as unique situations may necessitate different span recommendations. Accurate floor joist span calculations require input from a structural engineer or contractor.

The following table provides minimum floor joist sizes for joists spaced at 16 inches and 24 inches on-center (o.c.) for 2-grade lumber with a dead load of 10 pounds per square foot and a live load of 40 pounds, typical of residential construction:


Yellow Pine, Douglas Fir

Redwood Hemlock, Spruce

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.


9′ 9″

8′ 3″

8′ 8″

7′ 6″

7′ 6″

6′ 3″


12′ 8″

10′ 8″

11′ 0″

10′ 2″

10′ 5″

8′ 6″


16′ 0″

13′ 0″

14′ 6″

12′ 4″

12′ 9″

10′ 5″


18′ 6″

15′ 0″

17′ 6″

14′ 4″

14′ 9″

13′ 0″

Builders and contractors have flexibility in selecting lumber size and spacing based on specific needs. For instance, to address headroom concerns, they may opt for smaller joists placed closer together with a shorter span. Conversely, for longer spans such as framing a ceiling in an open-concept room, stronger lumber species and larger joists can be chosen.

What Are the Standard Sizes for Floor Joists?

Determining the appropriate size of floor joists for a specific flooring project involves considering various factors, such as the expected weight load and the span of the floor. While these factors influence the choice, there are standard sizes commonly used in construction projects.

In general, floor joists in America, Canada typically range from 2×6 inches to 2×12 inches in size. Here’s a breakdown of common sizes used in home construction:

  • 2×6: These joists are suitable for lighter-duty floors found in areas like bedrooms or attics. With a span capability of up to seven feet, they are ideal for moderate loads.
  • 2×8: Often employed for mid-range floors, such as those in living rooms or dining areas, these joists can span up to 10 feet. They offer a higher load-bearing capacity compared to 2×6 joists.
  • 2×10: These joists are utilized for heavier-duty floors like those in kitchens or bathrooms. With a span capacity of up to 14 feet, they boast an even greater load-bearing capacity than 2×8 joists.
  • 2×12: The largest standard-size joists, these are reserved for extremely heavy loads found in places like garages or storage areas. Able to span up to 18 feet, they possess the highest load-bearing capacity among the standard sizes.

It’s important to note that these sizes serve as general guidelines, and the appropriate choice may vary based on specific project requirements. Local building codes and regulations also dictate minimum standards for floor joist size and spacing. Therefore, it is the responsibility of a professional contractor to assess and determine the suitable size and spacing for a given project in accordance with local requirements.

Some factors to determine the correct joist span
Some factors to determine the correct joist span

The above article has provided you with information about floor joist spacing. VINAWOOD has also helped you with the Floor Joist Spacing Chart. Check out our website if you want more information about plywood types.