Deck Support Posts: 4×4 vs 6×6, Spacing & Max Height

A deck can have perfect boards and a gorgeous railing and still feel wobbly underfoot, and the cause is almost always below the beam. Deck support posts carry the beam, the joists and everything on the deck down to the footings, so their size, height and spacing decide whether the structure feels rock solid or sways when someone walks across it.

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The quick answer: use 6×6 posts for nearly every deck that supports a beam. A 4×4 can work only on very low decks with light loads, and many building departments no longer accept 4x4s for structural posts at all. Typical post spacing along a beam is 6 to 8 feet, set by the beam size and joist span. Taller decks need larger posts, bracing and engineered review once they climb past roughly 8 to 14 feet, depending on post size.

Post replacement on an existing deck and guard posts that hold the railing are different jobs. Here the focus is on sizing and laying out structural posts for a new or rebuilt deck frame.

What Deck Support Posts Actually Do

The load path on a typical attached deck runs from the decking into the joists, from the joists into a ledger on the house side and a beam on the outer side, from the beam into the posts and from the posts into concrete footings. Each decking support post therefore carries a slice of the deck area: the post spacing along the beam times roughly half the joist span.

Posts fail in two ways. Short, stout posts rarely crush, but tall, slender posts can buckle sideways under load long before the wood itself crushes. That is why post height matters as much as post size, and why tall decks need bracing.

4×4 vs 6×6 Posts

A 4×4 actually measures about 3.5 by 3.5 inches. A 6×6 measures about 5.5 by 5.5 inches, which gives it more than twice the cross-section and far greater resistance to buckling.

• 4×4: Acceptable in some jurisdictions only for very low decks (a few feet off grade) with modest tributary area. It is too narrow to notch for a beam while leaving enough wood, so beams must sit on top with a post cap. Many inspectors now reject 4x4s for beam support entirely.
• 6×6: The standard. It can be notched to carry a doubled or tripled 2x beam, it resists buckling at greater heights and it gives room for proper fasteners.
• 8×8 or engineered columns: For very tall decks, heavy loads such as hot tubs or second-story decks with long beam spans.

Use lumber rated for ground contact for posts, even when they sit on concrete, because the bottom end stays damp. Select straight posts with the crown and checks noted, and seal any cut end with a brush-on preservative.

Deck Post Height Limits

Post height is measured from the top of the footing (or post base) to the underside of the beam. Prescriptive deck codes give a maximum height by post size and by the area each post supports. As a general picture:

• 4×4 posts are typically limited to about 6 to 8 feet, and less as the tributary area grows.
• 6×6 posts typically reach about 14 feet in prescriptive tables.
• 8×8 posts can go a bit taller, but decks that high usually need an engineer.

Treat those as ceilings, not targets. A 6×6 at 10 feet will work under most tables, but it will feel more flexible than one at 4 feet unless you add bracing. When you plan deck post height, also think about frost depth: deeper footings do not change post height, but a post buried in soil instead of sitting on a footing is a rot risk most codes no longer allow.

Post Spacing and Beam Span

Post spacing equals beam span. The larger the beam, the farther apart the posts can go. Joist span matters too, because longer joists put more load on the beam.

• A doubled 2×8 beam often spans around 5 to 6 feet between posts with moderate joist spans.
• A doubled 2×10 beam commonly spans about 6 to 8 feet.
• A doubled or tripled 2×12 beam can reach roughly 8 to 10 feet or more.

Your local deck code has a beam span table; look up your species and grade, the joist span and the beam size together. Most residential decks land at 6 to 8 feet of deck support spacing along the beam, with posts no more than a short cantilever (often about 2 feet) in from each end of the beam.

Fewer posts look cleaner, but each one then carries more load, needs a larger footing and pushes you toward a bigger beam. On a 16 foot wide deck, three posts with a doubled 2×10 beam is a typical, economical layout.

Connecting Posts to Footings and Beams

At the bottom

Set posts on concrete footings that reach below the local frost line and are sized for the load. Fasten them with a metal post base that holds the post off the concrete by about an inch so end grain can dry. An adjustable post base is handy: it lets you fine-tune position after the concrete cures, which fixes the small layout misses that happen with poured footings. Anchor the base with a cast-in bolt or a properly sized concrete anchor.

At the top

Either notch a 6×6 so the beam sits on a shelf and through-bolt it, or set the beam on top and use a rated post cap. Simply bolting a beam to the side of a post without bearing (side-mounted beams hanging on bolts) is not allowed in current prescriptive codes because it puts the entire load on the bolts.

Bracing Tall Posts

Bracing turns a set of tall, flexible posts into a stiff frame. Once posts get past roughly 2 feet tall on a freestanding deck, or past a few feet on an attached one, diagonal or knee bracing helps a lot. Common approaches:

• Knee braces: 2×4 or 2×6 pieces at about 45 degrees from post to beam, bolted at each end, in both directions on corner posts.
• X-bracing: Diagonal boards between posts, useful on tall decks.
• Attached design: A ledger tied to the house with tension ties provides lateral stiffness for the house side.

Bolt braces rather than nailing them, and keep at least 6 inches of clearance to grade so they do not wick moisture.

Tools and Materials

• 6×6 ground-contact rated posts (or 4×4 only where explicitly allowed)
• Concrete, form tubes or footing forms, rebar where required
• Galvanized or stainless post bases and post caps, or bolts for notched connections
• String line, batter boards, tape measure, 4 foot level, post level
• Circular saw, handsaw or reciprocating saw for notches, drill, socket set
• Gloves, eye protection and hearing protection

Step-by-Step Layout

1. Get a permit and call 811 to mark utilities before digging footings.
2. Lay out the beam line with batter boards and string, and square it to the house using a 3-4-5 triangle.
3. Mark post locations along the line at your chosen spacing, keeping end posts inside any allowed beam cantilever.
4. Dig and pour footings to the required depth and diameter. Set anchor bolts or plan for drilled anchors.
5. Install post bases, then stand posts long and brace them plumb in two directions.
6. Mark the beam height from the ledger with a level or laser, cut the posts and cut notches if used.
7. Set and fasten the beam, then add bracing before joists go on.

Troubleshooting a Wobbly or Uneven Frame

• Deck sways side to side: Add knee braces or diagonal bracing, and check that the ledger has lateral ties to the house.
• Beam sags between posts: The span is too long for the beam size. Add a post and footing or upsize the beam.
• Post bottom is dark or soft: It is wicking water. Lift it onto a standoff base and treat the end grain; replace if rot is deep.
• Post twisted out of plumb: Wet lumber dried unevenly. Bracing helps, and a badly twisted post should be swapped.
• Footing cracked or sunk: Undersized or too shallow for frost. That footing needs to be replaced, usually with temporary shoring.

Cost Ranges

Treated 6×6 posts typically cost tens of dollars each depending on length, with 4x4s noticeably cheaper. Post bases and caps add a modest amount per post, while concrete and forms are often the largest per-post material cost. A contractor installing footings and posts for a standard deck usually charges somewhere in the range of a few hundred dollars per post including the footing, more on steep slopes or rocky soil.

When to Call a Pro

Hire a licensed deck contractor or structural engineer for decks above one story, decks on steep slopes, any deck carrying a hot tub, or when your soil, frost depth or wind and snow loads are unusual. Footings must be anchored correctly into undisturbed soil, loads must be calculated for your climate, and permits and HOA approval should be in hand before you dig. A pro can also confirm whether your jurisdiction allows any 4×4 posts at all.