Deck Beam Span Tables
How far posts can be spaced based on beam size, joist span, and lumber species. Per IRC R507.6.
Quick Answer: Beam span depends on both beam size and joist span. A 2-ply 2x10 beam carrying 10-foot joists in Southern Pine spans about 7' 7" between posts. Same beam in SPF: 6' 5". Larger beams or 3-ply construction allow wider post spacing.
Beam Span Tables
From IRC R507.6. Grade #2 lumber, 40 PSF live + 10 PSF dead load, single-span beam (no cantilever adjustment). Spans shown are maximum center-of-post to center-of-post.
Southern Pine #2
| Beam Size | Max Beam Span (post-to-post) by Joist Span | |||
|---|---|---|---|---|
| 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | |
| 2-ply 2x8 | 7' 7" | 6' 8" | 6' 0" | 5' 6" |
| 2-ply 2x10 | 9' 8" | 8' 6" | 7' 7" | 6' 11" |
| 2-ply 2x12 | 11' 9" | 10' 3" | 9' 3" | 8' 5" |
| 3-ply 2x8 | 9' 5" | 8' 4" | 7' 5" | 6' 9" |
| 3-ply 2x10 | 12' 1" | 10' 8" | 9' 6" | 8' 8" |
| 3-ply 2x12 | 14' 9" | 12' 9" | 11' 6" | 10' 5" |
SPF #2
| Beam Size | Max Beam Span (post-to-post) by Joist Span | |||
|---|---|---|---|---|
| 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | |
| 2-ply 2x8 | 6' 8" | 5' 10" | 5' 2" | 4' 9" |
| 2-ply 2x10 | 8' 2" | 7' 2" | 6' 5" | 5' 10" |
| 2-ply 2x12 | 9' 11" | 8' 8" | 7' 9" | 7' 1" |
| 3-ply 2x8 | 8' 2" | 7' 2" | 6' 4" | 5' 10" |
| 3-ply 2x10 | 10' 4" | 9' 0" | 8' 1" | 7' 4" |
| 3-ply 2x12 | 12' 7" | 11' 0" | 9' 10" | 8' 11" |
Hem-Fir and Douglas Fir-Larch values fall between Southern Pine and SPF. For those species, consult the full IRC R507.6 table or AWC's Prescriptive Residential Wood Deck Construction Guide (DCA 6).
How Beam Span Works
Three variables decide how far apart your posts can go:
1. Beam size
Larger cross-section = longer span. 2x12 spans significantly further than 2x8 at the same ply count.
2. Ply count
3-ply beams carry more load than 2-ply. Jump in capacity is roughly 25-30%.
3. Joist span it carries
Longer joists = more load per foot of beam = shorter beam span allowed. This is why longer-joist decks need more posts or larger beams.
Multi-Ply Beam Fastening
A 2-ply beam is not just two 2x10s laid next to each other — they must be fastened together to act as one piece. Per IRC R507.6:
2-ply beam
Fasten with 10d or larger nails in two rows, staggered, at 16" on center along the length. Or use structural screws at the same spacing. Do not under-fasten — this is one of the most common inspection failures for built-up beams.
3-ply beam
Through-bolt with 1/2" bolts at 24" on center (staggered), or use approved structural screws (SDS or equivalent). The middle ply must be clamped between the two outer plies tightly.
Engineered (solid) beam
LVL, PSL, or glulam beams are single pieces and do not need inter-ply fastening. They do require approved connections at posts and any splice locations per manufacturer spec.
Typical Deck Framing Examples
12' x 12' attached deck, Southern Pine
Ledger to beam: 12' joists (2x10 @ 16" OC). Beam: 2-ply 2x10. Post spacing: ~6' 11" = one post in the middle, one at each corner. Three posts total.
14' x 20' attached deck, Southern Pine
14' joists (2x12 @ 16" OC). Beam: 2-ply 2x12. Post spacing: ~6' 10" = three posts (2 interior + 2 ends). Or 3-ply 2x12 for wider spacing (~9' between posts).
10' x 16' freestanding deck, SPF
Two beams, 10' joists between them. Beams: 2-ply 2x10. Post spacing: ~6' 5" = two interior posts per beam + corners. 6 posts total.
Small 8' x 12' deck, any species
8' joists (2x8). Single beam 2-ply 2x8. One post in the middle, plus corners. Simplest deck framing.
Frequently Asked Questions
What is a deck beam span?
The beam span is the horizontal distance between the posts that support the beam. Each joist that rests on the beam transfers its load to the beam, so beam span depends on both the beam size and the joist span it carries. A beam carrying 12-foot joists is under more load than one carrying 6-foot joists at the same beam span.
Do I need a 2-ply or 3-ply beam?
Two-ply (two 2x boards fastened together) is standard for most residential decks. Three-ply is used when the span and load exceed 2-ply capacity — usually long spans with large joists. Solid beams (a single piece of engineered lumber) are another option and often match 3-ply performance in a cleaner package.
How are beam plies fastened together?
Per IRC R507.6, multi-ply built-up beams are fastened with nails or bolts at specified spacing. Typical: 10d nails in two rows at 12 inches on center for 2-ply, staggered. For 3-ply, through-bolts or structural screws at 16 to 24 inches on center, with the middle ply sandwiched. Always check manufacturer specs for the fastener being used.
Can I use a single 2x12 as a beam?
Only for very short spans and light loads. Code generally does not allow a single 2x member as a beam on decks because it lacks the redundancy of multi-ply construction. Use at least 2-ply 2x8 or larger for anything structural.
What is the difference between a flush beam and a drop beam?
A drop beam sits below the joists, with the joists resting on top of it. This is the common and easier method. A flush beam sits at the same level as the joists, with joists hung into its face using joist hangers. Flush beams create a flat ceiling under the deck and are used when there is a finished space below or when aesthetic matters. They require more labor but the spans are the same.
How far can the beam cantilever past a post?
IRC R507.6 allows the beam to cantilever past the outermost post up to one-quarter of the adjacent span. A 10-foot span between posts allows 2.5 feet of cantilever past the end post. This is useful for decks where you want a cleaner edge without a visible post at the corner.
What species of lumber is in my beam?
Check the grade stamp on each board. Most pressure-treated 2x10 and 2x12 lumber sold in the US east of the Rockies is Southern Pine; west is typically Douglas Fir-Larch or Hem-Fir. Grade #1 and #2 are most common; #1 gives slightly longer spans. Mixing species within a single built-up beam is not allowed by code.
Joist Span Tables
How to size the joists that rest on the beam.
Ledger Board Code
Connection between deck and house.
All Deck Codes
Complete IRC R507 reference.
Values from the 2021 International Residential Code Section R507.6. Cross-reference AWC DCA 6 for additional species and configurations. Local amendments may apply. Not engineering advice.