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How far apart deck posts can be for your beam and joists, how many you need, and the load on each footing.
With these inputs:
| Result | Value |
|---|---|
| Maximum post spacing | 7 ft 5 in |
| Posts on this beam | 3 posts, 6 ft 5 in apart |
| Beam overhang past each end post | 1 ft 7 in |
| Load per post (middle posts) | 1,920 lb |
| Round footing for that load | 16 in diameter |
| Beam | 2-ply 2×10 |
| Joist span it carries | 12 ft |
| Tributary area per post | 38 sq ft |
| End posts carry | about 1,440 lb each |
Spans for southern pine No. 2 at 40 psf live + 10 psf dead load (the table on our beam span chart); reduce about 5% for Douglas fir and 10% for hem-fir or spruce-pine-fir. A freestanding deck has a beam on each side; each carries the same load as the ledger deck's beam. Footing sizes assume a round footing on undisturbed soil at or below your frost depth — your building department has the final word.
Post spacing is set by the beam: a bigger beam spans further between posts, and a longer joist span puts more weight on it, which pulls the posts closer together. This reads the beam span table for your beam and joist length, lays out the fewest posts that stay inside it (letting the beam overhang its end posts if you want), and works out the load each footing carries so you can size the footings to match.
Most decks end up with posts 6–8 ft apart. The limit comes from the beam: a doubled 2×10 carrying 12 ft joists can span about 7 ft 5 in between posts, a tripled 2×12 about 10 ft 8 in. Longer joists put more load on the beam and pull the posts closer.
Post size does not set the spacing — the beam does. Post size is about height: 6×6 is the usual choice and required by many jurisdictions, while 4×4 posts are limited to short decks. Use the deck post calculator to check post size for your height.
An attached 12×16 deck with a doubled 2×10 beam carrying 12 ft joists needs three posts along the 16 ft beam if the beam overhangs the end posts — about 6 ft 5 in apart with 1 ft 7 in of overhang at each end — or four posts set right at the ends. A freestanding version doubles that, with a row under each beam.
Divide the load on the post by the soil bearing capacity. A middle post carrying 2,400 lb on 1,500 psf soil needs about 1.6 sq ft of footing — an 18 in round footing. Stronger soil or closer posts allow smaller footings.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.