Floor joists are an essential structural component in any home. They provide support for the floor system and need to be properly sized and spaced to carry the expected load without excessive deflection. Understanding the basics of how floor joist spans are determined is important for any homeowner, builder, or contractor.
What are Floor Joists?
Floor joists are horizontal structural members that run from wall to wall, or beam to beam, to support the floor above. They transfer the weight of the floor, as well as any live loads placed on it, to the rest of the structure. Joists are typically made of dimensional lumber, engineered wood products like I-joists, or sometimes steel.
The spacing between joists is typically 16″ or 24″ on center, meaning from the center of one joist to the center of the next joist. The ends of the joists rest on the wall framing or on beams. Joists are identified by their nominal dimensions in inches, such as 2×8, 2×10, etc. The actual dimensions are usually 1-1/2″ less than the nominal size.
How Joist Spans are Determined
The allowable span for a floor joist is based primarily on two factors:
- The spacing between the joists
- The size and grade of the joist lumber
Other considerations include the species of wood, if the joists are solid sawn or engineered, live and dead loads, and joist orientation.
Builders and contractors reference span tables provided by the American Wood Council and confirmed by building code approval bodies. These span tables list maximum allowable spans for joists under normal loading conditions.
Let’s take a closer look at the factors that influence floor joist spans:
Joist Spacing
Joist spacing has a direct impact on allowable spans. The closer the joists are spaced, the shorter the allowable span will be. This is because more closely spaced joists must each carry a larger share of the total load on the floor.
Typical joist spacing is 16″ or 24″ on center. A 16″ on center spacing means there is 16″ from the center of one joist to the center of the next. Some builders opt for 12″ OC joist spacing for a stiffer floor, while 24″ OC spacing helps reduce material costs.
Joist Size and Grade
Larger joists can span farther distances than smaller ones. A 2×12 floor joist will have a longer allowable span than a 2×8 joist made from the same grade of lumber.
The grade or quality rating of the lumber is also key. Joist spans are calculated using #2 grade lumber as a baseline. Higher grades like Select Structural or #1 can span slightly farther. Lower quality lumber will have a reduced span capacity.
Engineered wood I-joists typically have better span capabilities than solid sawn lumber. Their larger cross-sectional area and built-in stiffness provide added structural performance.
Wood Species
Softwood species like Southern Pine, Douglas Fir, and Hem-Fir are commonly used for joists. Their natural strength properties allow them to span adequate distances in typical residential settings.
Some hardwoods can also be used but may have reduced span capacities compared to softwoods. Always check span tables for the particular wood species you plan to use.
Live and Dead Loads
The expected dead and live loads on the floor influence allowable joist spans. Dead loads refer to the weight of the flooring, ceiling, and any permanent items. Live loads account for moving loads like people, furniture, and appliances.
Standard span tables are based on minimum code requirements of 10 psf dead load and 40 psf live load. Higher loads will result in reduced spans.
Joist Orientation
Joists should run perpendicular to the supporting walls or beams. Spans are calculated based on joists simply spanning the opening, not running parallel with walls.
Always check local building codes for specific requirements in your area. Many provide their own supplemental span tables. An engineer can also calculate custom joist spans for any non-standard conditions.
Reading Span Tables
Now that we’ve covered the basics of how joist spans are determined, let’s look at how to use the published span tables.
The tables are formatted to show the maximum allowable spans for different joist sizes, species, and grades. Here’s an example span table for 16″ OC Southern Pine joists:
| Joist Size | #2 Grade SPF | #1 Grade SPF |
|-|-|-|
| 2×6 | 10′ 2″ | 11′ 2″|
| 2×8 | 13′ 1″ | 14′ 4″|
| 2×10 | 15′ 8″ | 17′ 2″ |
| 2×12 | 18′ 0″ | 19′ 11″ |
To find the allowable span for a specific scenario:
- Identify the joist spacing – typically 16″ or 24″ OC.
- Find the row for the joist size you plan to use.
- Look in the column for the grade of lumber you will be using.
- The table will give the maximum span for that joist under normal loading conditions.
Always double check that your planned joist spacing, size, grade, and species match what is listed in the title of the span table.
Span Calculations Step-by-Step
Let’s run through an example to see how joist span calculations are done in real-world practice:
Scenario:
A floor will use 2×10 Douglas Fir joists spaced 16″ on center. What is the allowable span if the joists are #2 grade?
Step 1) Consult the span table for 2×10 Douglas Fir joists, 16″ OC spacing.
Step 2) Locate the row for 2×10 joists and the column for #2 Douglas Fir.
Step 3) The maximum span shown is 15′ 5″.
Therefore, with 2×10 #2 Douglas Fir joists at 16″ OC, the allowable span is 15′ 5″.
This process can be repeated for any joist spacing, size, grade, and species to quickly look up permissible spans. Always double check your inputs against the span table title for accuracy.
Common Mistakes to Avoid
While joist span tables simplify the process of determining allowable lengths, some key mistakes can lead to undersized joists:
- Assuming wider joist spacing allows longer spans – Incorrect, this reduces allowed spans.
- Failure to account for non-standard live/dead loads – Get engineering input.
- Using lower grade lumber than listed in the tables – Upgrade to the required grade.
- Mismatching units – Spans are listed in feet/inches – Avoid mixing up units.
- Running joists parallel to the span direction – Orient joists perpendicular.
- Using unapproved or outdated span tables for your area – Check local codes.
With an understanding of how joist spans are calculated and limiting simple mistakes, you can determine proper joist sizing for your floor framing needs.
FAQs
Can joists span farther if they are engineered I-joists?
Yes, engineered wood I-joists like LVL or LSL can span longer distances than solid sawn lumber of the same depth. Their composite structure gives them higher strength and stiffness ratings to handle longer spans under normal loads. Always check manufacturer specs.
What if my joists spans are longer than the allowable tables?
For joist spans greater than the published tables, speak to a structural engineer. They can analyze the floor system and loads to potentially approve longer custom spans with different joist sizes.
Can I use a MMA beam to support longer joist spans?
Yes, manufactured wood beams like LVL or Paralam can support joists over longer spans than typical wall framing. An engineered design is required to ensure the beam is sized appropriately to carry the floor load.
How do I calculate joist spacing and spans for a cantilever?
Cantilevered spans involve extra considerations like the overhang length. Work with an engineer to calculate the allowable cantilever distance and any changes to joist spacing or sizes to account for the concentrated load.
Should joists lengths be specified as minimums or maximums?
Joists should be specified based on the maximum required length as calculated from approved span tables. Avoid treating joist span values as minimum acceptable lengths during construction.
Key Takeaways:
- Floor joist spans are determined by the joist spacing, size, wood grade, species, and loading conditions.
- Span tables provide maximum allowable spans for common scenarios based on building code requirements.
- Values are given in feet and inches for various joist dimensions, grades of lumber, and typical 16″ or 24″ OC spacing.
- Ensure you match the parameters of your project to the titles of the span table.
- Use the tables to lookup permitted joist spans and sizes for your specific floor framing.
- Avoid common mistakes like overspanning, spacing joists too far apart, and using under-sized lumber grades.
- Talk to an engineer if your joist spans exceed table values or for non-standard floor system designs.
With the basics of using joist span tables covered, you can properly size and space floor joists for reliable structural performance. Check your local building codes for any additional span requirements in your area. Precision and planning from the start will result in a quality floor system supported for the long run.
Conclusion
Properly sizing and spacing floor joists is a critical part of any residential or commercial construction project. While advanced technical analysis can be done, builders can rely on published span tables for many typical floor framing scenarios. By understanding the factors that influence joist spans and how to correctly use the tables, reliable floor systems can be efficiently designed and constructed. Always check local building codes and work with structural engineers when exceeding the normal conditions of the prescriptive tables. With this foundational knowledge, contractors and DIYers can meet code requirements, minimize costs, and achieve the necessary structural integrity of floor framing.