Introduction
Post material is a structural decision first and an aesthetic one second: the species, treatment, and size you choose determine load capacity, rot resistance, and what your inspector will accept. This guide compares every common option — 4×4 vs 6×6, pressure-treated grades, cedar and redwood, steel posts, connectors, and concrete types — so you can specify the right stock before you price it. Once you have chosen, the Deck Post Calculator converts your layout into exact counts and lengths.
4×4 vs 6×6: Which Post Size
| Factor | 4×4 (3.5 in / 89 mm actual) | 6×6 (5.5 in / 140 mm actual) |
|---|---|---|
| Best use | Low, ground-level decks | Elevated decks; the modern standard |
| Buckling resistance | Lower — height-limited | Much higher for tall posts |
| Connection quality | Adequate for light loads | Superior bearing for beams |
| Cost (8 ft PT) | $12-$20 | $28-$45 |
| Availability in long lengths | Excellent | Good; 12 ft+ can be special order |
The trend in residential codes and practice is unmistakable: 6×6 for anything elevated, 4×4 acceptable for low decks within span and height limits. Tall posts — generally over 8 ft (2.44 m) — push most designs into 6×6 territory regardless of deck size, so confirm with your local code before ordering. Do not choose 4×4 to save money on a tall deck — the cost guide shows the savings are small next to the structural compromise.
Pressure-Treated Lumber Grades
Pressure-treated southern yellow pine dominates structural post work. What matters is the treatment retention rating, stamped on every piece: UC4A (ground contact, general use) is the minimum for posts near or in the ground; UC4B (ground contact, heavy duty) for severe exposures; UC4C for extreme duty such as saltwater-adjacent sites. Above-ground-rated stock (UC3B) is not acceptable for posts within 6 inches of grade, no matter how dry the site looks today.
Buy straight, dry stock: sight down each post for twist and bow at the yard, reject anything with deep checks through the center, and remember that treatment does not make lumber waterproof — cut ends must be field-treated with preservative, post bases still need standoff connectors and drainage as described in the pro tips guide, and end-grain sealer on every cut adds years of service in wet climates.
Cedar and Redwood
Western red cedar and redwood resist decay naturally through extractives in the heartwood, and they look far better than treated pine where posts are visible — a real consideration for railing posts and pergola-style details. Expect to pay roughly double the pressure-treated price per post, with redwood pricing varying strongly by region (cheapest in the western US).
The trade-offs: cedar is softer and lower-strength than treated pine, so structural posts may need to size up or stay within tighter limits — verify against your local code rather than assuming equivalence. Use only heartwood grades for ground-adjacent use; sapwood cedar rots nearly as fast as untreated pine. For the classic combination, many builders use pressure-treated 6×6 structural posts with cedar railing posts and trim, getting strength where it counts and looks where it shows.
Steel Posts
Galvanized steel posts (typically 4×4 square tube or lally-style columns) appear on high-end and commercial decks and anywhere maximum strength in minimum footprint matters. They do not rot, they carry enormous loads, and they pair with welded or bolted beam seats. Downsides: cost is multiples of wood, connections need welding or engineered brackets, and most residential inspectors will want to see an engineer’s detail. For typical residential decks, steel is overkill — but on tall, heavy, or architecturally exposed frames it is worth pricing.
Post Bases, Caps, and Connectors
Connectors are structural components, not accessories, and inspectors check them against manufacturer specs. The essential set per structural post:
- Standoff post base — anchors the post to the footing while holding wood 1 inch (25 mm) above concrete for drainage. Sized to the actual post dimension (3.5 or 5.5 in).
- Beam-to-post cap or saddle — ties the beam to the post top against uplift and lateral loads. Through-bolting is the alternative where caps are not used.
- Through-bolts and structural screws — hot-dip galvanized minimum; stainless within a few miles of saltwater. Never substitute deck screws for structural connections.
- Railing post connectors and tension ties — where the guardrail design requires them, these are code-mandated hardware, not optional upgrades.
Buy connectors matched to your actual lumber dimensions and your exposure zone, and keep the manufacturer’s spec sheets for the inspector. Undersized or mismatched hardware is a common correction notice — the code compliance guide explains what inspectors look for.
Concrete Types for Footings
| Type | Best for | Notes |
|---|---|---|
| Bagged general-purpose mix (80 lb / 36 kg) | 1-6 footings | ~0.6 ft^3 (0.017 m^3) per bag; just add water |
| Bagged high-strength mix | Deep piers, heavy loads | Higher psi; worth it for tall decks |
| Fast-setting mix | Setting posts same-day | Do not use where long working time is needed |
| Ready-mix delivery | 7+ footings or ~1 yd^3+ | $150-$250/yd^3 typical; order 10% extra |
Whichever you choose, the volume math is identical — pi x r^2 x h per footing — and the formula guide derives it with worked examples. Add forming tubes for a clean pier above grade and gravel for the footing base, and keep spare bags on hand: running short mid-pour creates weak cold joints.
Related Guides
- Deck Post Complete Guide — the flagship overview with a full worked example.
- Deck Post Cost Guide — what each of these materials actually costs.
- Deck Post Pro Tips — protecting your material investment on build day.
- Deck Post Common Mistakes — material errors and their fixes.
- Deck Post FAQ — quick material questions answered.
Get Your Number
Material choice made? Turn it into a shopping list: enter your layout in the Deck Post Calculator to get exact post counts, lengths, and footing volumes for the stock you have specified, then take that list to the yard with the grade stamps and connector specs from this guide in hand. Right material, right count, no second trip.
Disclaimer: Results are planning estimates only. Verify quantities and prices with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
