Concrete Post Base: Types, Sizing, and Installation Done Right

Bury a wood post in concrete and you have built a moisture trap that will rot it off at grade in 10-15 years. A concrete post base — the galvanized hardware that connects a post to a footing while holding it up out of the water — is the fix, and modern codes effectively require one for decks. The category covers two families: wet-set anchors placed into fresh concrete, and bolt-down bases fastened to cured footings or slabs. This guide walks through the hardware types (mostly the Simpson Strong-Tie catalog, since it dominates the aisle), how to size bases and footings for fences, decks, and pergolas, frost-depth rules, and the install sequence for both methods.

Why Elevate the Post at All

End grain drinks water like a straw, and the ground-contact zone — where oxygen, moisture, and soil organisms all meet — is where posts fail. A proper post base does three jobs: it transfers load into the footing, it resists uplift and lateral kick-out (wind on a pergola, people leaning on a deck rail), and it holds the post bottom at least 1 inch above the concrete on a standoff plate so the end grain can dry. Code language reflects this: the IRC requires deck posts to bear on footings with hardware providing lateral restraint and uplift resistance, and inspectors in most jurisdictions want a 1-inch minimum standoff wherever posts are exposed to weather or splash.

Wet-Set Bases: Cast Into Fresh Concrete

Wet-set anchors have legs or a fin that you push into the concrete while it is still plastic, locking the base into the footing permanently.

• Simpson PB series (PB44, PB66): The economy wet-set base for 4×4 and 6×6 posts, about $6-$15. Adequate for fences and light structures; minimal uplift rating and only a nominal standoff.
• Simpson CBSQ / CB column bases: Heavy wet-set column bases with a 1-inch standoff plate, rated for serious uplift and download — the pick for pergolas and covered structures. CBSQ66 runs $30-$45.
• Simpson EPB / PPB elevated bases: A rod-mounted cup that floats the post a full inch or more above grade — good where splash is constant.

Wet-set pros: strongest possible connection to the footing, no drilling, cheap. Cons: zero forgiveness — the base must be positioned, aligned to the string line, and plumbed during the pour’s finishing window, and a misplaced anchor means grinding it off. String lines and a template jig are not optional on multi-post layouts.

Bolt-Down Bases: Fastened to Cured Concrete

Bolt-down (post-installed) bases attach with a wedge anchor, Titen HD screw anchor, or epoxy-set threaded rod into hardened concrete — the method for existing slabs and for anyone who prefers to pour first and position later.

• Simpson ABA series: Adjustable base with a 1-inch standoff, around $12-$20 for ABA44Z; the workhorse for deck posts on new footings.
• Simpson ABU series: The beefier uplift-rated version (ABU44Z, ABU66Z, $18-$35); most deck inspectors are happiest seeing ABU-class hardware under structural posts.
• ABW / adjustable-width bases: Accommodate the true-vs-nominal lumber lottery and slight anchor misplacement — about 3/8 inch of side-to-side adjustment that saves layouts.
• Decorative bases: Simpson’s black powder-coated Outdoor Accents line and similar Ozco hardware trade a small premium ($25-$50) for exposed-structure looks on pergolas.

Anchor the base with a 1/2-inch or 5/8-inch Titen HD or wedge anchor embedded per the load table — typically 3-4 inches minimum into concrete, drilled with a hammer drill and an SDS bit, hole blown clean before the anchor goes in. Use hot-dip galvanized or stainless hardware against treated lumber; modern ACQ treatment corrodes bare zinc electroplate quickly.

Sizing: Base, Post, and Footing for Each Job

• Fences: Frost-line concrete footings 8-12 inches in diameter; a PB44 wet-set or direct burial (fences are the one place burial persists, since a rail-and-picket fence is sacrificial). Gate posts deserve 6×6 and an upgraded base.
• Decks: Footing diameter comes from tributary load — a typical 12-inch round footing on 1,500-PSF soil carries about 1,175 lb, while an 18-24 inch footing or bell handles common beam loads of 3,000-6,000 lb. Post base: ABU44Z/ABU66Z on a wet-set anchor bolt or Titen HD. 6×6 posts are code-preferred above 8 feet of height and simply good practice everywhere.
• Pergolas: Light download, big uplift and lateral demand — wind treats a pergola roof like a sail. Use CBSQ or ABU bases, 6×6 posts, and 12-16 inch diameter footings at frost depth; on an existing patio slab of at least 4 inches thickness, bolt-down bases with epoxy anchors are acceptable for open-top pergolas in most jurisdictions, but attached or roofed structures need real footings.

Frost Depth: The Rule That Overrides Everything

Footings must bear below the local frost line — 12 inches in the Gulf South, 36 inches through much of the Midwest and Northeast, 48+ inches in Minnesota and northern New England — because freezing soil heaves anything bearing above it by an inch or more per season. Frost heave is why fence posts lean and why floating deck footings walk out of level. Call your building department for the local depth; pour footings to that depth with the top 2-6 inches above grade, slightly crowned so water sheds away from the hardware. Tube forms (Sonotube, $10-$25 each) make clean cylinders; bell-bottom forms like Bigfoot add bearing area where soils are soft.

Installation Steps, Both Methods

1. Layout. String lines off batter boards; mark each footing center with a stake. Check diagonals for square on decks and pergolas.
2. Dig and form. Auger or hand-dig to frost depth, set tube forms plumb, and backfill around them. Bottom of hole on undisturbed soil — never loose fill.
3. Pour. An 8-inch x 36-inch tube takes about 1.2 bags of 80-lb concrete mix; a 12-inch x 36-inch tube about 2.8 bags. Rod the concrete to remove voids.
4. Set the anchor. Wet-set: push the base or a 5/8-inch J-bolt into the center while plastic, align to the string, plumb it, and let cure 24-48 hours. Bolt-down: cure 7 days ideally, then drill and set the anchor, and bolt the base down snug.
5. Install the post. Cut square, seat on the standoff plate, fasten through the base with the specified connectors — Simpson SD structural screws or 16d HDG nails in every hole, not two token screws. Plumb in both directions and brace until the structure ties it in.

Retrofitting Bases Under Existing Posts

Replacing a rotted buried post — or upgrading an old direct-bearing post to proper hardware — is a manageable weekend job. Temporarily support the beam with a screw jack or a doubled-2×6 T-brace, cut the old post free, and assess the footing: sound concrete of adequate size gets a drilled-in bolt-down base (Titen HD or epoxy anchor); crumbling or undersized footings get dug out and re-poured at frost depth with a wet-set anchor. Cut the new post 1 inch shorter than the old bearing height to account for the standoff plate, set it, fasten every hole, and lower the jack gradually. Budget $40-$80 in materials per post and about two hours each. The same sequence rescues pergola posts heaved out of plumb by frost — with the addition that the replacement footing finally goes below frost line, fixing the cause along with the symptom.

Mistakes That Shorten the Life of the Work

• Burying a post in concrete for a deck or pergola — rot plus a code violation on structures.
• Filling half the base’s fastener holes; ratings assume every hole is used.
• Landing anchors near the footing edge — keep 1.75-inch minimum bolt edge distance, more per the load table.
• Electro-galvanized hardware against treated lumber; insist on HDG (Z-suffix) or stainless.
• Ignoring standoff height in grade-level splash zones, which quietly re-creates the buried-post problem the base was meant to solve.

Hardware for a whole 10-post deck costs less than one sheet of decking. Spend the extra $10 per connection on uplift-rated, properly anchored bases, and the posts will outlast everything standing on them.