Contractor Job Estimator
$29.00
BuyBeam Span vs Joist Span
They are not interchangeable calculations even though both come from IRC deck tables
Both numbers live in IRC Chapter 5, both assume 40 psf live load in this cluster, and both get printed in feet-inches. That is where the similarity ends. Joists are Table R507.6. Beams are Table R507.5(1). The joist cell is how far a 2x can run between supports. The beam cell is how far a built-up 2x assembly can run between posts while carrying those joists. The joist calculator and the beam calculator are a sequence, not two names for the same lookup.
How beam columns work: beam guide. How joist tables work: joist guide. Inherited bounce: beam troubleshooting.
Run both tables on the same deck.Joists first, then the beam that carries them. Starting point only.
Open Beam Calculator →Jump to what you need
Different Load Path
Decking spans between joists. Joists span between a ledger and a beam (or between two beams). The beam spans between posts. Each member only has to carry what lands on it.
- Joist. Tributary width is half the spacing to each neighboring joist (typically 16" o.c.). The span is ledger-to-beam. Table R507.6 cares about species, size, spacing, and wet service. Plies are not a joist input.
- Beam. Tributary width is the joist span it supports (effective span after Table R507.5(5)). The span is post-to-post. Table R507.5(1) cares about species group, ply count, size per ply, and that effective joist span. On-center joist spacing does not appear as a beam-table column.
That is why a 2-ply 2x10 beam is not “twice a 2x10 joist.” The joist is a repetitive member sharing load with its neighbors. The beam is a single assembly taking the dump from a whole run of joists. Different tributary area. Different span direction. Different table.
Different Governing Table
IRC 2021 split deck framing into dedicated tables instead of telling you to reuse the floor-joist chart. R507.6 is joists (with a cantilever column this cluster does not use). R507.5(1) is beams at 40 psf live load. R507.5(2)–(4) are snow. Using the joist cell as a beam span is reading the wrong book, even when both say Southern Pine #2 2x10.
Joists → Table R507.6 (size × spacing). Beams → Table R507.5(1) (plies × size × effective joist span). Do not paste one cell onto the other member.
Floor joists inside the house are a third book (R502.3.1(2)). That is the floor vs deck joist comparison. Do not drag that number onto a deck beam either.
Same Stick, Two Jobs (IRC 2021)
Southern Pine #2, wet service. Joist values: Table R507.6 at 16" o.c., no cantilever. Beam values: Table R507.5(1) for a 2-ply member at a 12 ft effective joist span.
| Member | Table | What you look up | Example cell |
|---|---|---|---|
| 2x10 joist | R507.6 | Species, size, 16" o.c. | 14′-0″ joist span |
| 2-ply 2x10 beam | R507.5(1) | Species, 2-ply, 2x10, 12 ft joist column | 7′-4″ beam span |
| 2x8 joist | R507.6 | Species, size, 16" o.c. | 11′-10″ joist span |
| 2-ply 2x8 beam | R507.5(1) | Species, 2-ply, 2x8, 12 ft joist column | 6′-2″ beam span |
| 3-ply 2x10 beam | R507.5(1) | Same 12 ft column, one more ply | 9′-2″ beam span |
The 2x10 joist can run 14 feet. The 2-ply 2x10 beam under a 12-foot joist run can only go 7′-4″ between posts. If you copied 14′-0″ onto the beam, you overspanned it by almost double. That is the whole point of this page.
Which Calculator to Use When
Laying out a new deck: joist calculator for size and spacing, then beam calculator for ply count and post spacing, then deck stairs. Posts and footings follow the beam span you actually use.
Diagnosing bounce: stand in the middle of a joist bay and in the middle of a beam bay. Different spots, different tables. Joist troubleshooting vs beam troubleshooting.
Buying lumber: joists are repetitive 2x. Beams are 2-ply or 3-ply of the same size or deeper. Volume: board feet.
Have the joist span already?That number is the beam calculator's effective-span input, not its output.
Open Beam Calculator →FAQ
Can I use a joist span as a beam span if I double the joist?
No. Doubling is a 2-ply beam only after you look up Table R507.5(1) with the right effective joist column. It is not “twice the joist cell.”
Why does spacing show up for joists but not beams?
Joist spacing changes how much deck each joist carries. Beam load is driven by how far those joists run, which is already the table column.
Do I-joists and LVLs use these tables?
No. Manufacturer charts. These pages are sawn-lumber IRC rows only.
Any trade
Checkout opens here — you stay on this page.
Framing Layout Gear
Confirm the stamp and the bay. Then layout. The table does not replace a square or a level on an existing sag.
16×24 Framing Square
Mark post layout after the table says the beam reaches. Same square for the deck stairs off that frame.
View on Amazon →
Swanson Speed Square
Quick 90s and ply checks. The calculator still wants species from the stamp, not from memory.
View on Amazon →
48" Box Beam Level
Existing midspan bounce and sag are a field measurement first. Compare that to the tabulated beam span second.
View on Amazon →
7-1/4" Circular Saw
Do not cut a beam longer than the table allows and hope. Add a post, change plies, or get engineering.
View on Amazon →
Johnson Stair Gauges
When the deck also needs stairs, lock the square. Beam span and stringer math are separate.
View on Amazon →
Look Up the Table Cell
Species, plies, size, effective joist span. Starting point only — verify local code and the stamp.
Open Deck Beam Span Calculator →As an Amazon Associate, TestTalkHQ earns from qualifying purchases.