Deck Post Size Requirements
When 4x4 posts are enough and when 6x6 is required. Height limits, tributary area rules, and bracing requirements per IRC R507.4.
Quick Answer: A 4x4 post is allowed up to 8 feet tall with tributary areas under about 40 sq ft. Above 8 feet, or at larger tributary areas, 6x6 is required. Maximum unbraced length is 8 feet for 4x4 and 14 feet for 6x6. Many jurisdictions now require 6x6 across the board regardless of height.
Post Size by Height and Tributary Area
Based on IRC Table R507.4. Post height measured from top of footing to bottom of beam. Tributary area in square feet (beam span / 2 x joist span / 2). Grade #2 Southern Pine, SPF, Hem-Fir, or Douglas Fir-Larch. Assumes 40 PSF live + 10 PSF dead load.
| Post Height | Minimum Post Size by Tributary Area | |||
|---|---|---|---|---|
| Up to 20 sq ft | 20 to 40 sq ft | 40 to 60 sq ft | 60 to 80 sq ft | |
| Up to 6 ft | 4x4 | 4x4 | 6x6 | 6x6 |
| 6 to 8 ft | 4x4 | 6x6 | 6x6 | 6x6 |
| 8 to 10 ft | 6x6 | 6x6 | 6x6 | 6x6 |
| 10 to 12 ft | 6x6 | 6x6 | 6x6 | 8x8 |
| 12 to 14 ft | 6x6 | 6x6 | 8x8 | 8x8 |
Values reflect the 2021 IRC R507.4 prescriptive table. Many jurisdictions require 6x6 minimum regardless of tributary area or height. Check your adopted code and local amendments before assuming 4x4 is acceptable.
How to Calculate Tributary Area
Tributary area is the slice of deck surface that loads a single post. It drives how much weight that post carries.
Measure the beam span on each side of the post
The beam span is the distance from this post to the next post on either side. For an end post, only one side applies and the tributary width is half of the adjacent span.
Take half the distance on each side
A post midway between two posts 10 feet apart has 5 ft on each side = 10 ft of tributary beam length. An end post with the next post 10 ft away has only 5 ft of tributary beam length.
Measure the joist span the beam carries
This is the distance from the ledger (or the other beam) to this beam. A 12-foot-deep deck attached to a house has 12-foot joists landing on the outer beam.
Take half the joist span
The beam carries half of each joist it supports (the other half is carried by the ledger or the opposite beam). A 12-foot joist contributes 6 feet to the beam tributary.
Multiply to get tributary area
Tributary area = (tributary beam length) x (half the joist span). Example: interior post between posts 10 ft apart, carrying 12-foot joists = 10 x 6 = 60 sq ft.
Unbraced Post Height Limits
Per IRC R507.4.1, deck posts have a maximum unbraced length. Beyond that, the post must be laterally braced or the deck must be engineered.
4x4 post
Maximum unbraced length: 8 feet from top of footing to bottom of beam. Above 8 feet, 4x4 is not allowed in the prescriptive table at all.
6x6 post
Maximum unbraced length: 14 feet. Above 14 feet requires engineered design, 8x8 posts, or lateral bracing that reduces the unbraced segment.
Lateral bracing on tall decks
Any deck with posts taller than 8 feet generally requires lateral bracing in at least two directions — typically diagonal knee-braces from post to beam, or X-bracing between adjacent posts. The unbraced length limit measures the longest segment of post without a brace, not the total post length. A 12-foot post with a knee-brace at the top is still within the 4x4 limit if the brace-to-footing distance is 8 feet or less. Most inspectors will flag tall unbraced posts regardless of the table, especially for decks more than 30 inches above grade.
Species and Grade Impact
Unlike joist and beam tables, R507.4 post sizes do not vary significantly between Southern Pine, SPF, Hem-Fir, and Douglas Fir-Larch at the common deck loads. The table above applies to any Grade #2 or better solid-sawn lumber in these species.
Southern Pine
Most 4x4 and 6x6 treated posts east of the Rockies. Takes treatment well, resists checking, standard ground-contact rating.
Douglas Fir / Hem-Fir
Common on the West Coast. Slightly stiffer than SPF; similar prescriptive sizing. Often sold as "HemFir" or "DougFir" at big-box stores in the West.
SPF (Spruce-Pine-Fir)
Common in the North. Lower density but adequate for prescriptive post sizes. Watch for ground-contact rating — not all SPF is treated to the same level as Southern Pine.
Post Base Hardware
IRC R507.4 requires approved hardware between the post and the footing. Direct contact between wood posts and concrete is not allowed — concrete wicks moisture into the end grain and rots posts from the bottom up.
Approved post base standard requirements
- Hot-dip galvanized or stainless for treated-lumber compatibility
- Lifts post a minimum of 1 inch above the concrete surface
- Anchored to footing with embedded bolt or cast-in-place stand-off
- Connected to post with the manufacturer-specified fastener pattern
- Minimum uplift capacity of 1,800 lb (Simpson ABU44Z) or 3,000 lb (ABU66Z)
Do not set posts directly in concrete
Setting a treated post directly into a concrete footing (with the post embedded 2 to 3 feet into the hole and concrete poured around it) is allowed in some jurisdictions but shortens post life significantly. The end grain stays wet against the concrete, and freeze-thaw cycles crack the concrete around the post. Above-grade post bases perform better for 30+ years and make replacement possible without demolishing the footing. Most modern decks use Simpson ABU or equivalent hardware, not direct embedment.
Typical Post Sizing Examples
10' x 12' attached deck, 3 ft above grade
4x4 posts are acceptable. Post height around 3 to 4 ft, tributary area around 30 sq ft. 6x6 is commonly used anyway for durability and easier hardware attachment.
12' x 20' attached deck, 6 ft above grade
6x6 posts required. Post height around 6 to 7 ft, tributary area around 48 sq ft on interior posts. 4x4 is not adequate at this combination.
14' x 24' elevated deck, 10 ft above grade
6x6 posts required. Post height around 10 ft, tributary area around 56 sq ft. Add lateral bracing in two directions. Footings sized accordingly (larger diameter for the concentrated load).
Walkout basement deck, 12+ ft above grade
6x6 minimum, or upgrade to 8x8 or concrete-filled steel. Lateral bracing required. Many builders specify concrete-filled steel posts (Fortress, Titan) at this height for durability and cleaner aesthetics.
Small low deck (under 24" tall)
4x4 fine. Short posts, small tributary area. Still use ground-contact treated lumber and post base hardware.
Check for local 6x6 minimum amendments
A growing number of jurisdictions have adopted local amendments requiring 6x6 as the minimum deck post size regardless of height or tributary area. The reasoning: 4x4 posts twist and check over time, make proper post-to-beam hardware connections awkward, and contribute to a disproportionate share of failed deck inspections. Before buying 4x4 posts based on the prescriptive table, confirm with your building department that 4x4 is still allowed locally. When in doubt, use 6x6 — the cost difference per post is modest and inspection passes more easily.
Frequently Asked Questions
What is tributary area for a deck post?
Tributary area is the portion of deck surface whose load is carried by a single post. It is calculated as (half the beam span on each side of the post) multiplied by (half the joist span on each side). A post halfway between two posts 8 feet apart, carrying 12-foot joists, has a tributary area of 8 ft x 6 ft = 48 sq ft. Larger tributary areas concentrate more load on the post, which is why taller posts with wider spacing need 6x6 lumber.
When must I use 6x6 posts instead of 4x4?
Per IRC R507.4 and Table R507.4, 6x6 posts are required when any of these apply: (1) post height exceeds 8 feet from the top of footing to the bottom of the beam, (2) tributary area exceeds roughly 48 square feet at heights above 6 feet, or (3) your local jurisdiction has amended the code to require 6x6 across the board. Many jurisdictions have done exactly that, since 6x6 gives more room for proper post-to-beam hardware and does not twist the way 4x4 does over time.
Can I use 4x6 deck posts?
Generally no for structural deck posts. IRC R507.4 Table lists 4x4, 6x6, and 8x8 as the prescriptive sizes. 4x6 is occasionally used for railing posts or short guard posts but is not a standard beam-support option in the prescriptive table. If you want to use 4x6, you need engineered design or a jurisdiction that has explicitly approved it.
Can deck posts be notched to hold the beam?
Yes, but with limits. A 6x6 post can be notched to create a pocket for a built-up beam, reducing the post cross-section to approximately 3.5 inches where the beam sits. The notch must not remove more than 50 percent of the post cross-section, and the beam must be positively attached with bolts or structural screws, not just toe-nailed. Many inspectors prefer an unnotched post with a saddle connector (like Simpson ABU or CBSQ) on top because it is simpler to verify. 4x4 posts generally should NOT be notched because the remaining cross-section is too small.
Do tall posts need lateral bracing?
Yes. IRC R507.4.1 limits unbraced post length to 8 feet for 4x4 and 14 feet for 6x6. Any post taller than these limits needs lateral bracing — typically cross-bracing between adjacent posts, a knee-brace between post and beam, or attachment to a structural element like the house. Decks with posts taller than 8 feet are also required to have lateral bracing in at least two directions to resist wind and crowd-induced sway, even if the individual post is within its unbraced length limit.
Can I use a concrete-filled steel post?
Yes, but not under prescriptive code. Concrete-filled steel tube posts (like Fortress or Titan Post Anchor systems) are common on tall decks and elevated walkways. They are specified by the manufacturer and must be installed per their engineering. The IRC prescriptive path covers solid-sawn 4x4, 6x6, and 8x8 wood only. If you go with steel, bring the manufacturer cut-sheet to your permit application so the plan reviewer can verify the product approval.
Do deck posts need to be pressure-treated?
Yes. IRC R317.1.2 requires any wood within 6 inches of exposed ground or embedded in concrete to be pressure-treated with a ground-contact rated preservative. For deck posts, that means Ground Contact (GC) rated lumber — not the lighter "above ground" treatment. The stamp should read "Ground Contact," "GC," or the retention level (e.g., 0.40 PCF for southern yellow pine). Using above-ground-rated treatment below grade is a common reason decks rot out in 10 to 15 years instead of 30 to 40.
What post base hardware is required?
IRC R507.4 requires deck posts to be anchored to their footings with approved hardware that resists uplift, lateral load, and prevents direct contact between the post and concrete. Common approved bases: Simpson ABU44Z (for 4x4), ABU66Z (for 6x6), or equivalent from MiTek, USP, or OZ-Post. The base lifts the post about 1 inch off the concrete so water drains away and the end grain stays dry. Direct burial of treated posts in concrete is also allowed in some jurisdictions but shortens post life and is harder to replace — most builders and inspectors prefer the above-grade post base.
Beam Span Tables
How beam size sets your post spacing.
Footing Depth
Frost line and footing size under your posts.
Joist Span Tables
Joist size drives the load your posts carry.
Ledger Board Code
House-side connection that replaces a post line.
All Deck Codes
Complete IRC R507 reference.
Values from the 2021 International Residential Code Section R507.4 and Table R507.4. Your jurisdiction may have adopted a different edition, local amendments, or require 6x6 minimum regardless of table values. Cross-reference AWC DCA 6 for additional configurations. Not engineering advice.