Post Spacing vs Beam Size

Jump to the decision

When The Post Layout Is Fixed — Upsize The Beam

Existing footings, a patio door that cannot take a post in front of it, a stair opening you already framed, or a homeowner who will not accept another pier in the yard. The bays are the bays. The beam has to reach.

Run the calculator with planned spacing equal to the existing on-center. If it flags exceedance, step plies (2-ply → 3-ply) or depth (2x10 → 2x12) until the cell covers that bay, or leave the table and buy listed LVL. Every ply must bear on the post cap and be nailed to R507.5. A third ply that hangs in the air does not count.

When The Beam Size Is Fixed — Add Posts

The yard has 2-ply 2x10s. The budget does not have LVL. The beam is already cut. Then the table tells you the maximum bay, and you add posts until every bay is at or under that number.

That is usually cheaper in lumber and more expensive in holes, concrete, and the way the space under the deck feels. Each new post is also a new footing, a new post cap, and another frost-depth conversation. Do not skip the footing size guide because you “only added one post.” Tributary area on the neighbors changes too.

Cost And Complexity: Footing Count vs Lumber

Adding posts. More excavation, more concrete, more hardware, more things to plumb. You keep the cheaper beam. You lose clear space under the deck. You add failure points if post caps and lateral bracing are sloppy.

Upsizing the beam. More treated 2x, heavier lifts, maybe a third ply that has to bear on every post. You keep the open bay. You might still need bigger footings because the same load is sitting on fewer pads — wait, no: fewer posts means more tributary per pad. Adding posts usually shrinks tributary per footing. Upsizing the beam while keeping wide spacing can demand a larger pad. Run both numbers.

Neither direction is a midspan sister. Neither direction is “we always use double 2x8s around here.” Look up the cell, then pick the knob you still own.

When The Answer Is Call An Engineer

• Long cantilevers past what Table R507.5 and R507.6 allow you to convert.
• Hot tubs, outdoor kitchens, roof loads, or anything that is not 40 psf live / 10 psf dead.
• Multi-story decks, stacked posts, or a deck carrying a second-floor wall.
• Snow-country rows (R507.5(2)–(4)) that this calculator does not include.
• A bay you cannot shorten and a beam you cannot upsize far enough inside the sawn 2x table.
• Rot, split plies, or a missing stamp on lumber already in the air.

The calculator will tell you when planned spacing exceeds the prescriptive cell. That flag is the line. Past it, drawings beat hunches.

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