Why is my deck post rotting at the base?

What's actually happening at the base of your post

The bottom of your deck post is staying wet, and wood that stays wet feeds fungus. That's the whole story in one sentence.

The rot almost always starts in the lowest 6 inches, right where the post meets soil, concrete, or a footing. That zone sees the most water and gets the least sun and air.

If you catch it while the softness is shallow, you're looking at a cheap afternoon fix. If the probe sinks in past a quarter inch, the post has lost real strength and needs to come out.

The good news: base rot is one of the most preventable deck problems. It usually comes down to a missing piece of hardware and some bad drainage, both of which you can fix for good.

Why deck posts rot at the base

in most cases it's water contact that never dries. The post either sits in dirt, sits in concrete, or sits flat on a footing with no gap for drainage.

Pressure treatment helps, but it's not magic. The chemicals protect the outer layers, and any field-cut end exposes untreated wood at the core. Set that raw end in a puddle and it rots from the inside out.

Grading and downspouts matter more than people think. If runoff pools at the base or a gutter dumps within 3 feet, the wood never gets a chance to dry between rains.

Splash-back is the sneaky one. A post next to a patio or walkway takes a daily soaking from rain bouncing off the slab, and the sun rarely reaches that shaded lower foot.

Once decay starts, it can travel. Rot at a wet beam connection works its way down into the post, so a bad base sometimes points to a bigger problem up top.

Step by step: what to check and fix

Start with the probe test at ground level. That single check tells you whether you're cleaning up a cosmetic issue or replacing structure.

Next, figure out how the post is anchored. A galvanized standoff base an inch above the concrete is what you want to see. A post buried in dirt or embedded in a slab is the problem itself.

Dig back the soil and mulch a few inches so you can see the real base and the footing. What looked like a healthy post at grade is often gone underground.

Check the beam and the top connection too. If those are soft, the fix just got a lot bigger, and you should stop and call a pro.

• Surface rot, low deck, firm core: clean the base, pull soil back, fix drainage, and treat the wood.
• Deep rot, load-bearing post: shore the beam, cut out the post, repair or extend the footing, set a new ground-contact post on a standoff base.
• Rot in the beam or ledger: stop and get a licensed contractor.
• Always seal cut ends with copper naphthenate before final assembly.

2026 cost breakdown

That covers a ground-contact 4x6 or 6x6 post, a galvanized standoff base, a bag or two of concrete, gravel, and a can of copper naphthenate.

If you already own a jack and hand tools, that's your whole cost. The wood and hardware are the bulk of it.

A licensed pro handling the same job typically charges $400–$4000 for one post. That reflects shoring the load safely, footing work, hauling the old post, and a warranty on the fix.

Costs climb fast if the footing has to be dug and re-poured, if the deck is high off the ground, or if a beam repair gets folded in. Multiple rotted posts or a permit and inspection push you toward the top of the range or beyond.

For context, ignoring it until a section sags can turn into $2,000 to $6,000 of reframing. A $120 Saturday fix is cheap insurance.

Can you DIY this, or should you call someone?

This is genuinely DIY-friendly if the deck is low, the post is easy to reach, and you're comfortable shoring a load. Replacing a single post on a 2-foot-high deck is a solid weekend project.

The line I draw is height and load. Once a deck is more than about 6 to 8 feet off the ground, or the post carries a long beam over a walkout, the shoring gets dangerous and I'd hand it to a pro.

If the rot has reached the beam, ledger, or multiple posts, that's not a repair anymore, it's structural work. Get a licensed contractor and, in some cases, an engineer's sign-off.

Be honest about your footing skills too. A post is only as good as what it sits on, and a poorly poured footing just resets the clock on the whole problem.

How long can you put this off?

Base rot is rated low urgency here for a reason: if it's shallow and the post is still firm at the core, you have months, not days.

That changes the moment a probe plunges in or you see the deck settling near that post. Active structural rot doesn't pause for your schedule.

The risk isn't usually a sudden collapse from one post. It's the slow shift of load onto neighboring posts and connections, which spreads the damage and the bill.

My rule: shallow rot on a firm post can wait until the next dry stretch, but fix the drainage now so it stops getting worse. Deep rot on a load-bearing post gets dealt with before people gather on the deck again.

Deck age and lumber treatment matter

If your deck went up before 2004, the posts were likely CCA-treated, which held up remarkably well in ground contact. Those decks are aging out now, so even the good lumber is reaching the end of its run.

Decks built from 2004 on use ACQ or copper azole treatment. Safer to handle, a little less durable in wet ground, and very sensitive to whether the right retention rating was used.

Here's the trap: lumber rated for above-ground use looks identical to ground-contact lumber but rots years sooner in the dirt. Always check the end tag or stamp when you buy a replacement post.

If you don't know your deck's age or how it was built, the base of the post will tell you. A buried, unbracketed post is old-school construction and worth upgrading to a standoff base while you're in there.

How we assess base rot

We diagnose from the load path down, not from the cosmetics. The first question is always what the post carries and how the failure would move if the base gives.

Probe depth is the deciding measurement. Under a quarter inch is weathering, past that is structural, and that single reading drives the whole recommendation.

Cost ranges reflect 2026 US material and labor pricing for a single-post repair, with the understanding that footing work and height drive the real number.

Our DIY boundary is set by safety, not skill-shaming. Shoring a live load is where good intentions get people hurt, so we push high or heavily loaded posts to a licensed pro every time.