A floor joist span table shows the maximum clear distance a joist can span between supports based on lumber size, species, grade, and spacing. This reference uses IRC (International Residential Code) span data — the standard for residential construction in the United States — so you can quickly find the right joist size for any floor framing project.
Cells where the max span meets or exceeds your floor span will be highlighted.
Douglas Fir-Larch #2 — 40 psf Live Load
Maximum clear span between supports per IRC. Dead load = 10 psf.
| Joist Size | 12" OC | 16" OC | 24" OC |
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Species Comparison
Max span for selected joist size, 16" OC, 40 psf — all species and grades side by side.
How to Use This Table
- Select your lumber species, grade, and live load requirement above.
- Find your joist spacing (12", 16", or 24" on center).
- The value in each cell is the maximum clear span — the distance between supports.
- Choose a joist size whose listed span equals or exceeds your actual floor span.
- Enter your floor span in the "Highlight" box to instantly see which sizes work.
How to Use the Floor Joist Span Table
A floor joist span table tells you the maximum clear distance a joist can safely span based on species, grade, size, and spacing. Whether you're designing a new floor, planning an addition, or checking an existing structure, knowing the allowable span helps you pick the right lumber without guessing or over-building.
Step 1: Choose Your Lumber Species and Grade
Different species have different structural properties. Douglas Fir-Larch and Southern Pine are among the strongest common framing species. SPF (Spruce-Pine-Fir) is widely available in the northern US and Canada, and Hem-Fir is common on the West Coast. Within each species, #2 grade is the workhorse for residential framing — it balances cost and performance. Use #1 grade if you need to push spans a bit further. #3 grade is rarely used for joists as it significantly reduces the allowable span.
Step 2: Select Joist Spacing
Joist spacing — measured center-to-center — is typically 12", 16", or 24" on center (OC). Closer spacing means each joist carries less load, allowing a longer span for the same lumber size. 16" OC is the most common residential spacing. 12" OC is used when you need to maximize spans or add stiffness. 24" OC reduces lumber cost but requires larger joists for the same span.
Step 3: Choose the Right Live Load
The floor joist span table offers two live load options. Use 40 psf for standard living areas, bedrooms, kitchens, and most residential floors — this is the IRC default for occupied living space. Use 30 psf only for attic sleeping areas or spaces explicitly allowed by IRC code at that reduced load. When in doubt, design for 40 psf. Dead load (the weight of the floor structure itself) is taken as 10 psf in both cases, giving 50 psf or 40 psf total design load respectively.
Step 4: Read the Maximum Span
Find the row for your joist size (2×6, 2×8, 2×10, 2×12) and the column for your spacing. The value shown is the maximum clear span — the inside distance from face of support to face of support. Your actual floor span must be less than or equal to this value. If your span exceeds the maximum for a given size, move up to the next larger joist or decrease the spacing.
Using the Span Highlighter
Enter your floor span in feet in the "Highlight" box. All cells where the maximum span meets or exceeds your floor span will be highlighted in the table, instantly showing you every joist size and spacing combination that works for your project.
Important Notes and Limitations
These spans are based on IRC Table R502.3.1(1) and related provisions for solid sawn lumber. They assume normal duration loading and dry service conditions (interior, climate-controlled). For cantilevered joists, point loads, notched joists, or engineered lumber, consult a structural engineer. Always verify spans with your local building department, as local amendments to the IRC may apply.
Frequently Asked Questions
Is this floor joist span table free to use?
Yes, completely free with no account required. All data is built into the page and works offline — nothing is sent to any server.
Is my data private?
Yes. Everything runs entirely in your browser. No data ever leaves your device.
What load assumptions does this floor joist span table use?
The table uses 40 psf live load + 10 psf dead load (50 psf total), which is the IRC standard for bedroom and living areas. A 30 psf live load option covers sleeping-only areas, per IRC Table R802.4.1 and related floor framing tables.
What size floor joist do I need to span 15 feet?
It depends on species, grade, and spacing. For example, Douglas Fir-Larch #2 at 16" OC: a 2×10 spans 19'1" and a 2×8 spans 15'0". For a 15-foot span at 16" OC, a 2×8 DF-L #2 works marginally, but a 2×10 is the safe choice. Use this table to compare species and sizes for your exact situation.
Can I use these spans for roof rafters?
No. This table covers floor joists only. Rafter spans depend on roof pitch, snow load, and wind load, which require separate IRC rafter span tables. The principles are similar, but the load assumptions and allowed spans differ significantly.
What is the difference between 40 psf and 30 psf live load?
Live load is the weight of people and furniture. The IRC requires 40 psf for living areas, kitchens, and bedrooms. Attic areas with limited access and certain sleeping-only rooms may qualify for 30 psf. When in doubt, use 40 psf — it results in shorter maximum spans (more conservative) and is the standard for most residential construction.
Does lumber grade affect joist span?
Yes, significantly. #1 grade lumber has fewer knots and defects, allowing slightly longer spans than #2. #3 grade requires substantially shorter spans. Most residential framing uses #2 because it balances cost and performance. Always confirm the grade stamp on your lumber before using a span table.
How do I read a floor joist span table?
Select your lumber species, grade, and joist spacing (how far apart the joists are, measured center-to-center). The table shows the maximum clear span — the distance between supports — for each lumber size. Choose a size whose maximum span equals or exceeds your actual span. A longer listed span means the joist is adequate; a shorter span means you need a larger size or closer spacing.