Deck Post Spacing: How Far Apart Support Posts Should Be

Ask ten deck builders how far apart posts should be and most will answer “it depends on the beam,” which sounds evasive but is exactly right. Deck post spacing is set by how far the beam can span between supports, and that depends on the beam size, the wood species, and how long the joists are that sit on it. For a typical residential deck, support posts usually land 6 to 8 feet apart, with heavier doubled or tripled 2×10 and 2×12 beams stretching that toward 9 to 10 feet when joist spans are short.

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The posts themselves are rarely the limiting factor. A 6×6 post can carry far more weight than the beam can deliver to it. What matters is keeping the beam from sagging or overstressing, and making sure each footing is large enough for the load it collects. Get those two things right and the layout follows.

Deck Post Spacing: Typical Ranges by Beam Size

Prescriptive deck guides and local codes publish beam span tables built around a standard design load of about 40 pounds per square foot of live load (people, furniture, snow in some areas) plus 10 pounds per square foot of dead load (the deck itself). The longer the joists that rest on a beam, the more weight it carries, so the shorter its allowable span between posts.

Here are typical ranges for common pressure-treated southern pine or similar species. They are general examples; your local code tables and the lumber grade govern the actual numbers.

Species matters. Southern pine generally spans a bit farther than hem-fir or spruce-pine-fir of the same size, and lower grades span less. Snow loads in northern regions can push the design live load higher, which shortens every span. Treat the table as a planning tool, then confirm with the span tables your building department accepts.

How Joist Span, Beam Size and Load Work Together

Picture the deck as layers. Decking sits on joists, joists sit on a beam (and usually a ledger at the house), and the beam sits on posts, which sit on footings. Each layer passes weight down to the next.

A beam carries half the joist span on each side it supports. If the joists run 12 feet from the ledger to a drop beam, that beam carries the load from about 6 feet of deck depth along its whole length. Make the joists longer and the beam carries more; to compensate, you either upsize the beam or move the posts closer together. That trade-off is the heart of deck support spacing.

Cantilevers add flexibility. Beams can typically extend past the end post by up to about a quarter of their span between posts, and joists can cantilever past the beam by up to about a quarter of their span, depending on local rules. Using those cantilevers lets you pull posts and footings back from the deck edge for a cleaner look, without adding posts.

Special loads change everything. Hot tubs, outdoor kitchens with masonry, planters full of wet soil and roof structures over the deck concentrate far more weight than the standard design load. Those areas need engineered framing and footings, not table values.

Deck Footing Spacing and Sizing

Deck footing spacing is the same as post spacing, since each post needs its own footing. The size of that footing comes from simple math: the area of deck each post supports, times the design load, divided by how much weight your soil can bear.

A worked example helps. Suppose posts are 8 feet apart along a beam, and the joists span 12 feet from the house. Each interior post supports 8 feet of beam times 6 feet of deck depth, or 48 square feet. At 50 pounds per square foot, that is about 2,400 pounds. On average soil rated around 1,500 pounds per square foot, the footing needs roughly 1.6 square feet of bearing area, which is about an 18-inch round footing. Tighter post spacing means smaller footings; wider spacing means bigger ones.

Footings must also reach below your local frost depth, which can range from a few inches in warm climates to 4 feet or more in the far north. Frost heave can lift shallow footings and twist a deck out of level over a single winter.

Tools and Materials

• String line, stakes and line level or a builder’s level
• Tape measure (a 100-foot reel is handy for squaring with the 3-4-5 method)
• Marking paint
• Post hole digger or a gas post hole auger
• Concrete mix, footing tubes and a wheelbarrow
• Post bases or anchors rated for exterior use, with corrosion-resistant fasteners
• 6×6 pressure-treated posts rated for ground contact
• Beam lumber, structural screws or through-bolts, and beam-to-post connectors

A gas post hole auger makes a huge difference on decks with six or more footings, especially in clay or rocky soil. A two-person model with a 10- or 12-inch bit can bore a frost-depth hole in minutes. Rent one for a day rather than digging a dozen holes by hand.

Step-by-Step: Laying Out Deck Posts

1. Confirm your framing plan. Choose joist size and spacing, then pick a beam size and look up its allowable span for your joist length. That span is your maximum post spacing.
2. Divide the beam length evenly. For a 20-foot beam with a maximum 8-foot span, three spans of about 6 feet 8 inches with four posts, or a short cantilever on each end with three posts, both work. Equal spans look better and share load evenly.
3. Call 811. Have underground utilities marked at least a few business days before digging.
4. Set batter boards and strings. Run a string for the beam line parallel to the house. Square it with the 3-4-5 method (or larger multiples such as 6-8-10).
5. Mark post centers. Measure along the string and mark each footing center with paint or a stake.
6. Dig and inspect. Bore holes to frost depth and the required diameter. Many jurisdictions require a footing inspection before concrete is placed.
7. Pour footings and set anchors. Place concrete, set post bases in line with the string, and let the concrete cure per the mix label before loading.
8. Set posts and beam. Cut posts to height, fasten the beam with rated connectors, and brace everything until joists tie the structure together.

Decking Post Distance Mistakes to Avoid

The most frequent errors with decking post distance, and with deck post spacing in general, come from guessing instead of checking span tables. Common problems include:

• Posts too far apart for the beam. A bouncy or visibly sagging beam is the usual sign. Adding a mid-span post and footing is the fix.
• Undersized footings. Wide spacing with small footings can settle unevenly. Size each footing to its tributary area.
• Notched 4×4 posts. Notching posts to hold the beam weakens them, and many codes now require 6×6 posts for most decks. Use full posts with rated connectors.
• Ignoring the ledger. If the deck is attached to the house, the ledger connection matters as much as the posts. A freestanding deck needs a second beam and footing line instead.
• Uneven spans. One long span next to short ones overloads that beam section. Keep spans equal when possible.

Troubleshooting an Existing Deck

Deck feels bouncy in the middle. Check whether bounce comes from joists (springy between beams) or the beam (sagging between posts). Joist bounce can be reduced with mid-span blocking; beam sag needs another post or a larger beam.

One post has sunk. The footing was too small, too shallow or on disturbed soil. Shore the beam with temporary supports and rebuild that footing.

Posts lean. Lateral bracing may be missing on taller decks. Knee braces or diagonal bracing between posts and beam, installed per local rules, help resist sway.

Costs and When to Call a Pro

Each footing typically costs anywhere from roughly $50 to $150 in materials for DIY work, more in deep-frost regions where holes are long. Contractors often price footings individually, and adding posts usually costs less than upsizing a long beam. Renting an auger generally runs about the price of a nice dinner per day.

Bring in a licensed contractor or engineer for decks higher than about 30 inches off the ground, multi-level designs, decks with hot tubs or roofs, sloped or unstable soil, and any project that needs a permit (which is most of them). Proper anchoring into framing and footings, wind and snow load design, and HOA approvals all fall under that review. A pro can confirm that deck post spacing, beam size and footing size work together under your local code before anyone digs a hole.