Short answer: Posts rot at the base because the bottom of the post is in contact with soil or sits in a pocket that holds water, so the end grain stays wet and fungi eat it from the bottom up. Pressure-treated lumber slows this; it does not stop it. The same wood-to-ground contact is also a direct path for subterranean termites, which is why a termite inspection flags it even before there is visible damage.
What is actually going on
Wood rots when it stays above about 20% moisture for long stretches, and the end grain of a post soaks water up like a bundle of straws. A post base has three ways to stay wet:
- Buried or ground-contact. The post goes into the dirt, or the concrete footing was poured at or below grade and mulch or soil has built up around it. This is the common one.
- A cup on top of the footing. The post sits flat on a concrete pier with no standoff base, so rain runs down the post and pools under it. The footing itself becomes the puddle.
- Wrapped or boxed. A trim wrap, PVC sleeve or stone veneer around the bottom of a porch column traps water against it, and the rot is invisible until the column punches through the decking.
The detail that gets skipped. A post is supposed to sit on a footing that projects above grade, in a galvanized standoff base that holds the end grain about an inch off the concrete so it can drain and dry. The cheap way is to pour the footing flush with the ground (or skip the sonotube that would have raised it) and set the post straight on it. It passes inspection, it looks fine for five years, and then the bottom four inches are gone. On Daniel Island, where most of the housing stock is from the last twenty-five years, we see this on decks and porches constantly: builder-grade footings poured to grade, mulch beds later raised over them, and pressure-treated posts rotting from the bottom while the rest of the deck looks new.
Why the termite inspector cares
Subterranean termites live in the soil and need moisture. Wood touching soil is a highway with no checkpoint: they do not have to build a mud tube across a foundation where you would see it. That is why the annual bond inspection lists "wood-to-ground contact" as a conducive condition, and why some bonds will not cover damage while it exists. Fixing the post base is often required to keep the bond, and it is always cheaper than the framing repair the termites would otherwise cause. A rat slab (a thin concrete slab poured over the crawlspace floor) is the related fix for a house where the framing itself is close to bare earth; it removes the soil contact and the moisture at the same time.
How to check every post in ten minutes
- Look at the base. Can you see the footing above the ground and daylight under the post? Good. Is the post disappearing into mulch, soil or a wrap? Suspect it.
- Probe it. Push a screwdriver or awl into the post about an inch above the ground, on the side facing the weather. Sound wood stops it hard. If it sinks in, the post is rotting. Check all four sides; rot is usually worst on the uphill or the shaded side.
- Tap it. A hollow sound low on the post with solid sound higher up is rot or termite galleries.
- Look at the connection. Is the beam bolted through the post, or is the post just notched and nailed? A rotting post with a proper bolted connection is a post replacement. A rotting post with a nailed connection is a deck that can drop.
- Check the ledger while you are there. Where the deck attaches to the house, look for staining, soft wood and missing flashing. Ledger failures are what collapse decks; a rotten post usually just sags.
The honest DIY version
Pull mulch and soil back from every post so the footing is exposed and the post base can dry. If a post is sound but sitting flat on concrete, a jack, a temporary support and a standoff base is a doable repair for a confident DIYer on a low deck. Do not do it yourself on a raised deck, a porch that carries a roof, or anything where the post is already soft: the load has to be picked up safely before the post comes out, and the new post has to be sized, based, and connected properly or it is the same repair again in five years.
When it is serious
- The post is soft on more than one side, or the probe sinks more than half an inch: the post is carrying load on rotten wood. Keep people off that section until it is supported.
- The post carries a porch roof or a second-storey deck.
- Mud tubes on the post, the pier or the foundation nearby: get the termite company out before the repair, because the framing behind it may also be involved.
- A loose handrail on the same deck. See deck repair: a loose rail is usually a rotted post or a rotted rim, not a loose screw.
What the repair actually looks like — and who does it
A pest company treats the soil and inspects. What is left is the carpentry: temporary shoring, cutting out the rotted post, raising the footing (a new pier or a cap so it stands above grade), a galvanized standoff base, a new post with a through-bolted connection, and replacing any joist or rim that was eaten with it. If the wrap or veneer caused it, it goes back on with a drainage gap this time. This is deck and porch repair work, and it is exactly the kind of job we do: the post, the framing it touches, and the reason it rotted, in one visit. If the termite damage has reached the house framing, it becomes crawlspace and structural repair, and we handle that too.
Charleston and York County notes
Lowcountry rot is faster: high humidity, sandy soil that holds a post base wet, and a year-round termite season. Any post base within an inch of grade on the coast is on a clock. In York County the freeze-thaw is mild but the clay holds water against a buried post for weeks after rain, and a footing poured to grade in a mulch bed does exactly the same thing it does on Daniel Island. In both places the fix is the same: get the wood off the ground and let the end grain dry.
Sources
- American Wood Council — Prescriptive Residential Wood Deck Construction Guide (DCA 6): awc.org/publications/dca-6/
- Clemson Cooperative Extension — subterranean termite control: hgic.clemson.edu/factsheet/subterranean-te…
General guidance for homeowners in Charleston and York County, South Carolina, written from what we see on jobs. Conditions vary house to house; anything structural, gas or electrical is licensed work in South Carolina and should be looked at in person.
Frequently Asked Questions
Why do pressure-treated posts still rot?
Pressure treatment slows fungal decay; it does not make wood waterproof, and the cut end of a post is the least protected part. A treated post in constant ground contact or standing in a puddle on a footing will still rot from the bottom, just more slowly.
Should deck posts touch concrete?
No. The post should sit in a galvanized standoff base that holds the end grain about an inch above the footing, and the footing should stand above the soil so water drains away. A post set flat on concrete at grade is the most common cause of base rot.
Why did my termite inspection flag wood-to-ground contact?
Because subterranean termites reach wood most easily where it touches soil, with no visible mud tube. It is listed as a conducive condition, and some bonds will not cover damage while it remains. Fixing it usually means raising the post base or removing the soil contact.
Can a rotted deck post be repaired instead of replaced?
Rarely well. Cutting off the rotted section and splicing a new base works on a low, lightly loaded post but leaves a joint in the load path. For a porch column or a raised deck, replace the post and fix the footing detail that rotted it.
What is a rat slab and do I need one?
A thin concrete slab poured over a crawlspace floor. It separates framing from bare earth, keeps the ground moisture down and removes the termite path. It is worth considering when framing sits close to the soil or the crawlspace stays damp; most decks do not need it, but the house next to the deck sometimes does.