What Size Beam Does My Deck Need?

A deck beam carries every joist that lands on it down to the posts. The bigger the area it supports and the farther it has to reach between posts, the beefier it has to be. IRC Table R507.5 ties those together: you look up the beam by the joist span it supports, and it tells you the maximum distance allowed between posts.

Beam span is driven by the joist span

This trips people up. A 3-ply 2x10 Southern Pine beam can span about 13 ft between posts when it carries 6-ft joists, but only about 10 ft when it carries 12-ft joists — same beam, more load, shorter reach. Always size the beam against the actual joist span it supports, not the deck's width.

Two-ply vs three-ply

Built-up beams are made by nailing or bolting 2x stock together. Adding a third ply to a beam of the same depth buys a meaningful jump in span — often the cheapest way to delete a post and a footing. A 3-2x10 reaches noticeably farther than a 2-2x10.

Don't forget the connections

• Built-up beams must be fastened together per IRC R507.5.2 (a specific nail or bolt pattern).
• Posts to a dropped beam need a rated post cap, not just toe-nails.
• Where the beam sits on notched posts, the notch and bolts must follow R507.4.

Enter your deck in the calculator and it returns the recommended beam, the maximum post spacing, and how many posts your deck width needs — all from the R507.5 table.