Deck Beam Span Calculator

What a beam does and why size matters

The beam is the horizontal member that runs perpendicular to your joists and carries their load down into the posts. Undersize the beam and the deck bounces, sags, and eventually fails — the number-one structural mistake on DIY decks. Oversize it and you're spending money on lumber you don't need. The calculator above returns the maximum allowable span for common built-up beams based on the joist length they'll support.

How the span table works

Beam capacity is a function of three things: lumber size, ply count (how many 2× members are nailed or bolted together to form the beam), and the tributary joist length feeding into it. A single joist carries load to two supports — the ledger and the beam — so a beam supporting joists that span 12 feet has a tributary width of 6 feet. Longer joists mean shorter allowable beam spans between posts.

Beam sizing quick reference

• 2-ply 2×8: 6–9 ft between posts, best for small ground-level decks.
• 2-ply 2×10: the residential standard, 8–11 ft between posts.
• 3-ply 2×10: stiffer and longer spans, 10–13 ft, ideal for wider decks.
• 2-ply 2×12: deep and stiff, 10–13 ft between posts.
• 3-ply 2×12: maximum span, 12–16 ft, used on large or elevated decks.

Built-up beams versus solid timbers

Almost every residential deck uses a built-up beam — multiple 2× members bolted or nailed together. It's cheaper and easier to handle than a solid timber. For 2-ply beams, use 10d galvanized nails at 16" OC top and bottom; for 3-ply, use 1/2" through-bolts every 24" with washers on both sides. Ring-shank nails or structural screws (Simpson SDW or GRK RSS) also meet code and hold better long-term.

Where beams fail in the real world

Rot is the enemy. Never notch a post into a beam — always run the beam across the top of the post and attach with a Simpson post cap. Also install cap flashing over the top of any beam that will be exposed to sky-facing rain, since built-up beams trap water in the seams and rot from the inside out. Confirm your post count and post size once you've locked in the beam using our deck post calculator.

Code compliance

The numbers above match AWC DCA-6, the prescriptive residential deck code adopted by most US jurisdictions. Local codes can differ — hot tub or roof loads always require a stamped engineered design. When in doubt, size up one lumber grade and confirm with your building inspector before you order.