Repair or Replace a Rotted Deck Post? The Foot Decides
What this comes down to
- A post standing on a concrete pier still counts as ground contact under treatment standards.
- Deck posts rot in the bottom few inches first, because that is where water sits longest.
- Most of a post's strength is gone before the surface looks bad enough to worry anybody.
- Post size is set by how much deck it carries, so a 4x4 is undersized more often than not.
- A rotted foot can be spliced out, but only where sound wood starts well above grade.
- Several posts failing at once is a drainage fault, and a new post will rot the same way.
A soft deck post is not automatically a replacement. What settles it is where the sound wood starts. The other half is whether the concrete under it still works. Both are answered with an awl and a tape.
What follows covers why posts fail at the foot and what the decay has taken. It then sizes the post and runs the three repair depths. It also covers what nobody can judge until the frame is held up.
Can a rotted deck post be repaired?
Often yes, and the test is where sound wood begins. A post soft only in its bottom few inches can be cut back and spliced. A post soft at the beam is past repair, whatever the foot looks like.
An awl settles it faster than a visual check. Pushed hard into the post at grade, it either stops or sinks. Where it sinks, keep probing upward until it stops. That height is the whole decision.
The reason the height matters is what a splice has to do. It carries the deck straight down through a joint. That joint needs sound wood on both sides of it, and air under the foot.
Rot that reaches above the splice line changes the job. So does rot in the beam above it. That is structural rot repair rather than a post swap, and it is quoted that way.
Why do deck posts rot at the foot first?
Because the foot is the one part of a post that stays wet. Rain runs down the face and stops at the concrete. Soil, leaf litter and mulch bank up against it and hold that water in place.
Treatment standards are blunt about this. The American Wood Protection Association sets what ground contact means. Its standard puts sawn deck posts in that category by name.
The useful part is a footnote most homeowners never see. A member sitting less than six inches above final grade counts as ground contact. That holds where it rests on concrete with no moisture break.
A post bolted flat to a pier is not above ground in any sense the standard recognizes. Above-ground stock in that spot was always going to fail early, and most decks are built that way.
What has the rot already taken?
More than the surface shows, and that is the part worth knowing. Decay is fungal growth eating the wood from inside. Strength goes long before the outside of the post looks wrong.
The USDA Forest Service's Forest Products Laboratory sets out how decay works in its wood handbook. Fungi need moisture, mild warmth and air. Wood denied any one of them lasts centuries.
That cuts both ways on a deck. It explains why a post under a dry overhang outlives one standing in a flower bed. It also explains why drying a rotted post out puts none of the lost strength back.
So a post that sounds hollow under a hammer is not an early finding. Softness at grade under paint that still holds its shape is how this normally presents. By then it is well advanced.
How is a deck post supposed to be sized?
By how much deck it carries, which is the part that surprises people. The International Code Council publishes post height tables for decks rather than a single rule. Height and floor area are read together.
The numbers move sharply. A southern pine 4x4 is good to fourteen feet where it carries twenty square feet of deck. The same 4x4 tops out at six feet two inches at a hundred and sixty.
Height is measured from the underside of the beam down to the top of the footing. That is not the same as the height above the soil, and the difference matters on a sloping yard.
Notching is the other trap. A post cut to let the beam sit in it is sized on what is left after the notch. That is how a 4x4 ends up a post that was never big enough.
What are the repair depths, and what does each one fix?
There are three, and they are set by what has failed rather than by budget. The table below runs them in order. The fourth line is where a post repair stops being the answer at all.
| The repair | What comes out | What it restores | When it is not enough |
|---|---|---|---|
| Foot spliced out | The bottom of the post only | Bearing, with a joint above grade | Soft wood above the splice line |
| Post replaced, footing kept | The whole post | Bearing, and a proper standoff | A pier that has heaved or crumbled |
| Post and footing replaced | Post, base and concrete | Bearing, depth and drainage | Posts failing in more than one bay |
| Frame rebuilt above it | Posts, beam and joists | The whole substructure | Nothing: this is the last rung |
Each row needs the deck held up while the work happens. A screw jack and cribbing take the load off the post. That temporary support is most of the labor at any of the three depths.
The bottom row hands the job over. Where posts, beam and joists are all going, a rebuild priced is the honest comparison. The shared labor makes that gap narrower than it looks.
One warning sits across all four rows. Posts failing in more than one bay is almost never four bad posts. It is water arriving the same way at each of them, and the water has to be dealt with.
What has to be rebuilt when a post comes out?
Two connections, and both are usually worse than the post was. The post carried load down to the concrete and held the beam above it. A replacement has to do both again properly.
At the bottom, the fix is separation. Simpson Strong-Tie makes a standoff base that holds the post clear of the concrete. The standoff is a full inch, which is what code asks for.
That inch is the whole point. It lets the end grain dry between rainfalls instead of sitting in a puddle. End grain drinks water far faster than the face of a board.
At the top, the beam connection gets checked while access is open. A post toe-nailed to a beam was never right. The same visit is the cheapest time it will ever be to fix.
What cannot be judged from a photograph?
Where the sound wood starts. An awl finds it in a minute and a picture never shows it. That single height decides whether this is a splice or a full post.
What the footing is actually doing. A pier can look square and still have lifted. Only a level across two posts and a probe at the base settle it.
How much deck the post carries. Tributary area is a measurement, not a guess. It is what says whether the post was ever the right size for the span above it.
Where the water is coming from. Grade, downspouts and planting beds all put water at a post foot. A new post in the same conditions has the same future.
All four get checked before a figure is written. The deck inspection carries no charge alongside a repair estimate. The work is then done by the same crew that took the measurements.
Rotted deck post questions
Can you repair a rotted deck post instead of replacing it?
Yes, where sound wood starts above grade. The rotted foot is cut away and a new section spliced in. The deck sits on cribbing while the joint is made. Where softness reaches the beam, the post is replaced instead. An awl finds that height in about a minute.
How does Perfect Work decide whether a deck post can be spliced?
Perfect Work Remodeling & Repairs probes the post with an awl from grade upward. The height where the awl stops sinking is the splice line. A level then checks whether the pier has moved. A splice is offered where sound wood starts clear of the soil and the concrete is sound.
Why did my deck post rot when the boards are still fine?
Because the boards dry after rain and the post foot does not. Soil, mulch and leaf litter bank against the base and hold water there. Treatment standards treat a post on concrete as ground contact for that reason. Decking sits in a far easier position than the post under it.
Is a 4x4 deck post good enough?
It depends entirely on how much deck sits on it. Published tables size a post by height and by the floor area it carries. A southern pine 4x4 reaches fourteen feet under a light load and about six feet under a heavy one. Notching for the beam reduces it further.
What does a deck post estimate from Perfect Work include?
The visit, the probing and the written figure are not charged for. Perfect Work Remodeling & Repairs prices it as one number. That covers the temporary support, the timber, the base and the concrete. Where a second post turns out to be soft, the job is requoted before anything is cut.
Do I need a permit to replace a deck post?
Usually yes, because a post is structural. Most DuPage County towns treat replacing a beam, joist or post as work that needs review. That holds on an existing deck where nothing else changes. The permit desk for the address is the authority, and it is worth a phone call first.
Getting a straight answer on a soft deck post
A soft post is a short conversation once somebody has probed it. The splice line, the pier and the tributary area are the three numbers. All three come off the deck itself in one visit.
A visit from Perfect Work Remodeling & Repairs starts at the post foot with an awl and a level. The beam connection is checked while the access is open. The ledger connection gets the same look.
Where the frame is the problem rather than the post, that is said before anything is cut. Soft joists found on the same visit are covered in how joists fail and are quoted separately.
Call Perfect Work at 630-297-7888 · Book a deck check