A 4×4 wooden post mounts to concrete with a metal post base: an anchored standoff base on an existing slab or a cast-in-place base set before the pour.
A post sitting flat on a slab rots from the bottom: concrete holds water against the end grain while the rest of the post looks solid. A metal post base breaks that contact, gives the post something structural to bolt to, and lets you level it before it becomes permanent. On cured concrete you drill and anchor; on a fresh pour you embed the base. Either way, the post fastens to a base with the manufacturer’s specified fasteners.
Attaching A 4×4 Post To Existing Concrete: Which Base Fits?
An adjustable standoff base is the common pick for a cured slab; it lifts the post off the concrete and can be leveled after the anchor is set.
Simpson Strong-Tie’s ABU44Z attaches wooden posts to existing concrete and carries a 1-inch standoff to keep moisture out of the wood. The other workhorse is the Simpson Strong-Tie EPB elevated post base, which allows 1 inch to 2½ inches of clearance above the concrete, 2 inches on the EPB44A, keeping the post bottom dry. To line up models before drilling, our tested roundup of the best 4×4 post base for concrete puts sizes and standoffs side by side. Simpson’s EPB and PB families do not share installation requirements, so anchor diameter, hole depth, and fastener type change with the model.
From Drill Bit To Post: The Install Order
On an existing slab the sequence is mark, drill, clean, set the anchor, level the base, then fasten the post.
| Install Step | What You Do | Required Spec |
|---|---|---|
| Mark the location | Lay out the post line and check it square. | Square before any hole goes in |
| Drill the hole | Bore with a masonry bit sized for your anchor. | 7/8″ diameter, 4″ deep minimum on the EPB adhesive install |
| Clean the hole | Blow or brush out every trace of dust. | Nothing loose left inside |
| Set the anchor | Fill the hole halfway with anchoring adhesive, then push the threaded rod in. | Rod embedded 3½” minimum |
| Place the base | Slide the base over the rod and level it. | 1″ to 2½” standoff; 2″ on the EPB44A |
| Fasten the post | Stand the post in the base and drive the listed fasteners. | 0.162″ x 3½” nails; ½” bolts only on PB66 and PB66R |
| Let it cure | Keep load off the connection. |
Dust is the first detail that decides whether the anchor holds: adhesive bonds to concrete, not grit, so clean the hole before anything goes in. The second is patience, since loading the base early quietly loses holding power. You want a base sitting flat and level on the rod, post plumb, before you drive a single fastener.
For a fresh pour, the EPB cast-in-place detail wants a nut and bearing plate on the embedded end, at least 4 inches from the top of the concrete to the top of that plate, with the base fully engaging at least three threads, and the threaded rod’s center at least 3 inches from any concrete edge.
Three Mistakes That Cut The Rated Load
- Substituting fasteners. Simpson’s connectors call for the specified fasteners, and load capacity depends on that exact pattern and type. A deck screw where the sheet calls for a 3½-inch nail leaves you guessing.
- Ignoring sidecover. The PB line needs 2 inches minimum sidecover for full load; the EPB cast-in-place detail wants 3 inches from the rod’s center. Set a post tight against a slab edge and the published numbers stop applying.
- Loading too early. Adhesive and fresh concrete both need time. Give the connection its cure window before nailing pickets to it.
Line up four things, a base sized for your concrete, the fasteners printed for that model, a gap between wood and slab, and the full cure time, and the connection carries what it is rated to carry.
FAQs
Can I Set A 4×4 Post Directly On The Concrete?
No. Wood sitting flat on concrete keeps moisture against the end grain, where rot starts. A metal post base lifts the post off the slab and gives you a structural connection to fasten into. Even a short standoff helps: Simpson’s ABU44Z uses a 1-inch standoff for exactly that reason.
What Size Hole Do I Drill For A Post Base Anchor?
It depends on the model. Simpson’s EPB adhesive installation calls for a 7/8-inch-diameter hole at least 4 inches deep, filled halfway with anchoring adhesive, with the threaded rod embedded 3½ inches minimum. Other bases take different anchors, so read the installation sheet for the exact model before the bit touches the slab.
How Soon Can I Put Weight On The Post?
Loading early is the fastest way to reduce how much the base will hold, and it is a mistake you cannot see afterward.
References & Sources
- Simpson Strong-Tie. “EPB Elevated Post Base.” Source for the 1″ to 2½” standoff, the 7/8″ x 4″ adhesive anchor hole, and the cast-in-place embedment detail.
- Simpson Strong-Tie. “PB Post Base.” Source for the 0.162″ x 3½” nail specification, the ½” bolt option on PB66 and PB66R, and the 2″ sidecover rule.
- Simpson Strong-Tie. “Cast-in-Place Post Bases.” Explains how these connectors brace the bottom of wood posts and set base height before embedment.
