Deck Beam Span Chart
Deck beams carry the combined load from all the joists above them and transfer it down through posts to footings. The maximum distance a beam can span between posts depends on three things: beam size (depth and number of plies), lumber species, and joist span (which determines tributary load). Tables below are derived from IRC Table R507.5 for residential deck construction. Values assume 40 psf live + 10 psf dead load — snow loads, hot tubs, and concentrated loads require engineered design beyond these prescriptive tables.
How to Read the Table
Find your joist span along the top (the distance your joists run from ledger to beam, or beam to beam). Find your beam size in the left column. The intersection gives you the maximum post spacing — the farthest apart your posts can be along the beam.
Example: if your joists span 10 feet and your beam is a 3-ply 2x10, the table shows a maximum post spacing of 9'-9". Posts along the beam can be no more than 9'-9" apart.
Southern Pine / Douglas Fir-Larch (#2 or better)
The two most common deck framing species. Southern Pine is standard pressure-treated lumber at most lumberyards; Douglas Fir is common in the western US.
| Beam Size | 6' Joist | 8' Joist | 10' Joist | 12' Joist | 14' Joist |
|---|---|---|---|---|---|
| 2-ply 2x6 | 6'-2" | 5'-5" | 4'-10" | 4'-5" | 4'-1" |
| 2-ply 2x8 | 8'-2" | 7'-2" | 6'-4" | 5'-10" | 5'-5" |
| 2-ply 2x10 | 10'-5" | 9'-1" | 8'-1" | 7'-5" | 6'-10" |
| 2-ply 2x12 | 12'-7" | 11'-0" | 9'-10" | 8'-11" | 8'-3" |
| 3-ply 2x6 | 7'-2" | 6'-4" | 5'-8" | 5'-2" | 4'-10" |
| 3-ply 2x8 | 9'-8" | 8'-6" | 7'-7" | 6'-11" | 6'-5" |
| 3-ply 2x10 | 12'-5" | 10'-10" | 9'-9" | 8'-10" | 8'-2" |
| 3-ply 2x12 | 15'-0" | 13'-1" | 11'-8" | 10'-8" | 9'-10" |
Hem-Fir / SPF (Spruce-Pine-Fir)
SPF and Hem-Fir are weaker than Southern Pine and have shorter allowable spans. Common in pressure-treated lumber at big-box stores, especially in the northern US and Canada.
| Beam Size | 6' Joist | 8' Joist | 10' Joist | 12' Joist | 14' Joist |
|---|---|---|---|---|---|
| 2-ply 2x6 | 5'-5" | 4'-9" | 4'-3" | 3'-10" | 3'-7" |
| 2-ply 2x8 | 7'-2" | 6'-3" | 5'-7" | 5'-1" | 4'-9" |
| 2-ply 2x10 | 9'-1" | 7'-11" | 7'-1" | 6'-6" | 6'-0" |
| 2-ply 2x12 | 11'-0" | 9'-7" | 8'-7" | 7'-10" | 7'-3" |
| 3-ply 2x6 | 6'-4" | 5'-7" | 5'-0" | 4'-7" | 4'-3" |
| 3-ply 2x8 | 8'-6" | 7'-5" | 6'-8" | 6'-1" | 5'-8" |
| 3-ply 2x10 | 10'-10" | 9'-6" | 8'-6" | 7'-9" | 7'-2" |
| 3-ply 2x12 | 13'-1" | 11'-6" | 10'-3" | 9'-4" | 8'-8" |
How to Size a Deck Beam
The process works in reverse from what you might expect:
Step 1: Determine your joist span.The distance from the ledger board (or house rim joist) to the center of the beam. For a freestanding deck, it's the distance between beams.
Step 2: Choose your post spacing.Posts are typically 6 to 8 feet apart for most residential decks. Closer spacing allows smaller beams; wider spacing requires larger beams. Post spacing is often determined by deck layout — you want posts to land at logical points like corners and stair locations.
Step 3: Find the beam size from the table. Cross-reference your joist span and desired post spacing. Always round up to the next beam size if your dimensions fall between table values.
Step 4: Check your footing size. Beam size determines the load on each post, which determines footing diameter. IRC Table R507.3.1 gives minimum footing sizes based on tributary area and soil bearing capacity.
Single-Ply Beams vs Multi-Ply Beams
The IRC deck beam tables only cover multi-ply built-up beams (two or three pieces of dimensional lumber fastened together). Single solid beams (like a solid 4x8 or 6x6) have different span values because their cross-sectional properties differ from built-up members. For solid timbers or non-standard loading (hot tubs, rooftop decks, long cantilevers), use our Beam Calculator to check bending, deflection, and shear for your specific span and load conditions.
Fastening Multi-Ply Beams
Multi-ply beams must be fastened together so the individual plies act as a single structural unit. Three common methods:
Structural screws (recommended).Products like GRK RSS, Simpson SDS, or FastenMaster HeadLOK are the easiest and fastest way to laminate multi-ply beams. They meet code, require no pre-drilling, and can be driven with a standard impact driver. Typical schedule: two rows staggered at 16" on center from both sides, using screws long enough to penetrate through the first ply and at least 1.5" into the second (for 2-ply) or through two plies into the third (for 3-ply). Check the screw manufacturer's ICC-ES evaluation report for the approved schedule — that's what your inspector will reference.
Through-bolting.1/2" carriage bolts or hex bolts at 24" on center in a staggered pattern, with washers on both sides. The traditional method, unambiguously code-compliant everywhere, but slower. Some jurisdictions still prefer through-bolting for 3-ply beams. If your inspector is old-school, this is the safe bet.
Nailing (IRC minimum).The IRC allows nailing from both sides in a staggered pattern — two rows of 10d (3") nails at 16" on center. Minimum code requirement. Works, but structural screws are superior in withdrawal resistance and are just as fast.
Pressure Treatment Requirements
All deck beams must be pressure-treated lumber or a naturally rot-resistant species (cedar, redwood). The treatment retention level for beams is typically UC4A (0.40 pcf for above-ground contact). Beams within 6 inches of the ground or in ground contact require UC4B (0.60 pcf) treatment.
Standard interior LVL is NOT acceptable for deck beams — the adhesives degrade with moisture cycling. If you need an engineered beam for a long deck span, use pressure-treated LVL (PWLVL) specifically rated for exterior exposure, or a treated glulam.
Common Deck Beam Mistakes
Undersized beams from using the wrong species.The difference between Southern Pine and SPF/Hem-Fir spans is significant — sometimes 2 feet or more for the same beam size. Always verify which species your lumber actually is. The grade stamp on the end of the board tells you species and grade.
Not accounting for cantilever. If your joists cantilever past the beam, the effective joist span for beam sizing purposes is measured to the far end of the cantilever, not to the beam. Cantilevered joists also create an uplift condition on the backspan side that must be addressed with proper connections.
Using a single 2x beam instead of a built-up beam. A single 2x10 on edge is not the same as a 2-ply 2x10. The IRC tables are for built-up beams only. A single 2x on edge has different section properties and lateral stability concerns.
FAQ
How far can a 2-ply 2x10 deck beam span?
For Southern Pine or Douglas Fir with joists spanning 8 feet, a 2-ply 2x10 beam can span up to 9'-1" between posts. With a 12-foot joist span, the maximum post spacing drops to 7'-5". SPF and Hem-Fir values are about 15% less.
Do I need a 3-ply beam or will 2-ply work?
For most residential decks with post spacing of 8 feet or less and joist spans under 12 feet, a 2-ply 2x10 or 2x12 is adequate. Three-ply beams are needed when post spacing exceeds 10 feet or joists span 14 feet or more. A 3-ply beam is also common when the beam serves as a guardrail post support, since the extra width provides a better connection for notched posts.
Can I use a steel beam for my deck?
Yes, but it requires engineering. Steel beams (typically W-shape or HSS tube) can span farther than wood beams with smaller cross-sections, useful for long clear spans or maintaining headroom below an elevated deck. Steel beams must be hot-dipped galvanized or painted for exterior use, and connections to posts and joists require engineered hardware.
Calculate Your Specific Deck
- Deck Joist Span Calculator for joist sizing
- Beam Calculator for custom beam sizing with deflection checks
- Concrete Calculator for footing volumes