How to set deck posts
A post is only as good as the three decisions under it: how deep the hole is, whether the post touches the concrete, and whether it is plumb when the concrete sets. Get those right and the deck above is straightforward.
Hole depth and diameter
Depth comes from frost. In a region with a 36″ frost depth, the bottom of the footing must be at 36″ or deeper. Diameter is usually 12″ for a 4×4 post carrying a deck beam.
A footing that is 12″ in diameter and 36″ deep holds about 2.4 cubic feet of concrete — roughly four 60 lb bags. The post hole calculator does that arithmetic with the post volume subtracted.
The gravel base question
A 4–6″ layer of compacted gravel at the bottom of the hole does two things: it gives the concrete a firm, drained bearing surface, and it breaks the capillary path that pulls groundwater up into the post.
It does not replace depth. Gravel above the frost line is still in the frost zone, and the concrete sitting on it will still heave.
Post in concrete or on a bracket
This is the decision that determines how long the post lasts.
- Post embedded in concrete. Strong, and it rots. Concrete holds moisture against the wood, and the post fails just above the concrete line, where nobody looks until the deck moves.
- Post on a standoff bracket. The post sits above the concrete on a metal base, so water drains away and air reaches the end grain. The connection is a bracket, which has to be rated for the uplift and lateral load. This is the correct detail in most modern codes and it is what a post base is for.
If you embed, use ground-contact rated lumber and slope the concrete so water runs off the top of the pier rather than pooling around the post.
Plumb, and how to keep it
Plumb the post on two faces at 90° to each other, not just one. A post that is plumb front-to-back and leaning side-to-side passes a single check and fails the deck.
Brace it while the concrete cures. Two braces at 90° from the post to stakes in the ground is standard, and they come off after 24–48 hours. Concrete reaches about 70% of its strength in a week; do not load the post before then.
Height: cut after, not before
Do not cut posts to final height before setting them. Set them long, then run a string line or a laser at the beam height and cut in place. The footing depth, the anchor position and the concrete surface all vary, and cutting first means shimming later.
A beam that sits on top of the posts is stronger than one bolted to their sides. If the design has the beam on top, cut the posts to the underside of the beam; if the beam is flush, the post is notched — and a notch at the top of a post is the weakest point in the assembly.
Rot prevention, in three details
- Standoff base. Keeps the end grain out of the concrete.
- Post cap or sloped top. Water runs off instead of soaking into the end grain, which is where a post absorbs most of its water.
- Ground clearance. Keep the bottom of the deck framing, and any skirt, above the soil line so splash does not wick up.
What this guide will not design
- It does not size the footing. Bearing area depends on the load and the soil.
- It does not give your frost depth. That is in local code; see frost depth.
- It does not cover posts carrying a roof, which need larger footings and uplift connections.
- It does not cover helical or screw piles, which are a different system with manufacturer installation requirements.
Questions people actually ask
Should deck posts be set in concrete or on a bracket?
On a metal standoff bracket is better for longevity, because the post end grain stays dry and above the concrete. Posts embedded in concrete rot at the concrete line, which is where the failure is hidden.
How deep should a deck post hole be?
Below the local frost depth, typically 30 to 48 inches across most of the US, and 12 inches in diameter for a 4×4 post carrying a beam.
Do I need gravel under a deck post?
Four to six inches of compacted gravel helps drainage and gives a firm bearing surface, but it does not substitute for depth below the frost line.
How long before I can build on a new footing?
Wait 24 to 48 hours to remove the bracing and about seven days before loading it with the beam. Concrete reaches roughly 70 percent of its design strength in a week.