Deck Beam Span Chart

Built-up beam spans, southern pine No. 2

Read this table by joist span first. A beam doesn't have a single capacity — it depends on how much deck it carries. Find your joist span across the top, then read down to the beam size you're considering; the number is the maximum distance between post centers.

For Douglas fir-larch reduce these spans by about 5%, and for hem-fir or spruce-pine-fir by about 10%. If you can't confirm the species stamp on the lumber, use the conservative figure.

Choosing between more posts and a bigger beam

Every deck design is a trade between beam size and post count. A footing plus post plus hardware costs roughly $120-$220 installed; adding a third ply to a 16-foot beam costs about $60-$90 in lumber. That math usually favors the bigger beam — and fewer posts means a cleaner look under the deck and less obstruction if you plan to use the space below.

Building a built-up beam correctly

Nail or screw the plies together in two staggered rows — 10d nails or structural screws at 16 inches on center, top and bottom, from both faces. Crown all plies the same direction. Splice only over a post, and never splice two plies at the same post. If the beam sits in a wet location, laminate with a bead of construction adhesive rated for treated lumber and use stainless or hot-dip fasteners; ACQ treatment eats electroplated hardware.

Dropped versus flush beams

A dropped beam sits under the joists on post caps. It's the stronger and simpler option, and it lets you cantilever joists past it for a beam that's hidden from view. A flush beam sits in the same plane as the joists, with each joist carried on a hanger. Flush beams maximize headroom below the deck — necessary on low decks or where you want a clean ceiling — but every joist connection becomes a rated hanger, the beam does more work, and there's no cantilever option.

Where beams meet posts

This connection is the single most safety-critical joint on the deck after the ledger. Use a rated post cap — Simpson BCS, BC, or equivalent — filled with the specified nails. Do not rely on toe-nailing, and never carry a beam on lag screws through the side of a post; that puts the fasteners in shear and withdrawal simultaneously. For 6x6 posts, a notch with 3-1/2 inches of remaining post plus a through-bolt is acceptable in most jurisdictions, but a cap is stronger.

Next steps: confirm your joist size with the joist span chart, size footings with the footing calculator, and count posts with the post calculator.