How to use the deck beam calculator
- Species: from the grade stamp. Southern pine is stronger, so it gets its own column. Douglas fir, hem-fir, SPF, redwood and the cedars share one conservative set of values.
- Beam size: a built-up beam of two or three 2× boards nailed or screwed together, or a solid 4× timber.
- Joist span: how far the joists run from the ledger (or another beam) to this beam. Longer joists put more deck area on the beam, so the beam can't span as far. The calculator rounds your joist span up to the next table column (an 11' joist span uses the 12' column).
- Post spacing: the distance between post centers, which is the beam span. The result shows whether it fits and the smallest double or triple beam that would.
Why joist span changes beam span
A beam carries half of the joist span on one side (the ledger carries the other half). Double the joist span and the beam carries roughly double the load per foot, so it has to be shorter between posts or bigger. That's why a (2) 2×10 in hem-fir can span 8'-1" with 6' joists but only 4'-5" with 18' joists.
Beam overhangs and splices
- Overhang: a beam may cantilever past its end post by up to ¼ of the adjacent span. A beam with posts 8' apart can extend 2' past the last post.
- Splices: joints in built-up beam plies must land over a post. The table assumes each span acts on its own (simple span).
- Fastening and posts: built-up plies must be nailed, screwed or bolted together, and the beam must be connected to each post with a notch or rated post cap. DCA 6 calls for 6×6 posts. Follow its details or your local code.
Which table is this?
The values come from AWC DCA 6, the free deck guide most building departments hand out. This calculator uses its 2015-IRC edition because it's the clearest published copy. The 2018 and 2021 IRC split the species into three groups and allow up to a few inches more span on some beams. If you're building under a newer code, this calculator may suggest one more post than you strictly need. It will never suggest fewer.
FAQ
How far can a double 2x10 deck beam span?
It depends on the joist span it carries. In southern pine, a (2) 2×10 spans about 10'-1" with 6' joists, 7'-9" with 10' joists and 7'-1" with 12' joists. In hem-fir, SPF or Douglas fir the same beam spans 8'-1", 6'-3" and 5'-8".
How far apart should deck posts be?
Post spacing equals the beam span, so it comes from the beam table. For common decks with a (2) 2×10 or (2) 2×12 beam, posts end up about 6 to 8 feet apart. Bigger beams or shorter joists allow wider spacing.
Can a deck beam overhang a post?
Yes. A beam can cantilever past its end post by up to one quarter of the adjacent beam span. With posts 8' apart, that is a 2' overhang.
Is a 3x10 the same as a double 2x10?
In the DCA 6 table they share the same span values. A solid 4× beam is stronger than a double 2× of the same depth and has its own row.
Do I need an engineer for my deck beam?
Prescriptive tables often satisfy the building department for simple decks, but beams carrying hot tubs, roofs, heavy snow or joists from both sides need an engineered design. Always check the math and confirm with your building department or a licensed engineer.