Soft Rot in Deck Posts: Identification, Repair or Replace Decision Guide

Safety First: This is a structural decision with real liability. If you’re unsure after reading this guide, replace the post or get a qualified pro to assess it. A sagging or compromised support can cause partial collapse.

Understanding Soft Rot: What It Is and How It Differs

Soft Rot vs. Brown Rot vs. White Rot: Critical Differences

Soft rot: fungi digest cellulose inside the wood first, creating spongy, stringy wood that dents easily with a screwdriver. Often slow, persistent, and common in moist, ground-contact areas.
Brown rot (“dry rot”): wood becomes brittle, brown, cubical cracking; rapid loss of strength.
White rot: wood fibers look bleached/fibrous; slower to destroy load capacity than brown rot but still serious.

In my field inspections, soft rot almost always shows up as localized mushiness that “gives” under moderate hand pressure—especially at the base 0–6 inches above grade.

The Fungal Biology Behind Soft Rot

Needs moisture (>20%), oxygen, and moderate temperatures.
Spores are everywhere; control moisture, not just fungus.

Why Deck Posts Are Particularly Vulnerable

Splash-back, wicking from soil, and trapped water at post-to-concrete interfaces.
End grain at the base behaves like a straw, pulling water up into the post.

Common Locations: Where Soft Rot Attacks First

Ground-Contact Areas: The Primary Failure Zone

The most frequent failure is the bottom 6–12 inches of the post. Freeze–thaw + soil moisture = classic rot pocket.

Wood–Concrete Junctions

Water wicks through concrete into end grain. Posts embedded in concrete are high-risk.

Post Bases and Caps

Base brackets that trap water and uncapped post tops that absorb rain speed up decay.

Hidden Areas Behind Skirting

Poor airflow behind skirting or planters keeps posts wet longer after rain.

Early Detection: Inspection Techniques That Work

Visual Indicators: Discoloration and Surface Changes

Look for dark staining, swelling at the base, fungal mycelium, or peeling finishes. Check fastener areas for rust stains—a moisture clue.

The Screwdriver Test: Proper Technique

Press a standard screwdriver straight into suspect wood with moderate hand pressure (no hammering):

≤ 1/8 in. dent: likely sound.
> 1/4 in. dent: significant soft rot.
≥ 1/2 in. penetration: replacement is mandatory.

My standard: if I can push past 1/4″ without straining, I flag the post. Past 1/2″? I treat it like a failure in progress.

Moisture Meter Assessment

≤ 15%: typical target for exterior framing at rest.
> 20%: active decay risk.
> 28%: likely decay conditions.

Probing for Depth of Penetration

Probe on all four faces and the bottom (if accessible). Map the depth to estimate percent of cross-section affected.

Assessing Structural Integrity: The Critical Decision Point

Load-Bearing Capacity Evaluation

Identify what the post supports: beam span, tributary area, and attachments (stairs, hot tub, roof). Heavier loads reduce your margin.

Percentage of Cross-Section Affected

Strength drops nonlinearly as you lose section. A seemingly “small pocket” can erase a big chunk of capacity if it’s at the extreme fiber.

Location of Rot: Base vs. Mid-Post vs. Top

Rot at the base is more dangerous due to combined compression + shear and connection eccentricity.

Post Size and Load Considerations (4×4 vs. 6×6)

A 4×4 has little redundancy; even 20–25% loss is serious. 6×6 tolerates slightly more, but don’t get complacent.

Early in my career I learned the 25% rule the hard way: if more than 25% of a post’s cross-section is compromised, it’s not structurally sound—period. I never “epoxy my way out” of that threshold anymore.

Rot Severity Scale (1–10)

Use this to classify what you’re seeing and align to action:

1–2: Surface discoloration, ≤12% dent, moisture ≤18% → Monitor
3–4: Local softness, 1/8–1/4″ dent, moisture 18–22% → Minor repair
5–6: 1/4–3/8″ dent, localized pocket at base, moisture >20% → Repair or replace based on % section
7–8: 3/8–1/2″ dent, multiple faces, hollow-sounding base → Replace
9–10: >1/2″ penetration, crushable fibers, visible deformation → Replace immediately + shore now

I once investigated a partial collapse in Olympia (2018): post had >60% rot—obvious to a trained eye, but the owner didn’t know the signs. Three injuries. That case cemented my hard lines on replacement.

Decision Matrix: Repair vs. Replace Criteria

Repair If: Less Than 25% Cross-Section Affected

Rot is localized, not at critical connection points.
Moisture is controlled (you can fix the source).
You can fully excavate and consolidate remaining fibers.

Replace If: Structural Compromise Detected

≥25% section loss anywhere, or any rot at the bearing interface.
Screwdriver >1/2″ penetration.
Observable settlement, cracking, or beam deflection.

Gray Areas: When to Get Professional Assessment

Heavy tributary loads (roof, hot tub, long spans).
Multiple posts with symptoms.
Historic or specialty structures.

Insurance and Liability Considerations

If an inspection report notes rot and you ignore it, insurers may deny claims after a failure. Document findings and actions.

Severity Score “Calculator Tool” (No app needed)

Assign points; then follow the recommendation.

A. % Cross-Section Affected
0–10% = 0 pts • 10–25% = 2 pts • 25–50% = 5 pts • >50% = 8 pts

B. Screwdriver Penetration (deepest)
≤1/8″ = 0 • 1/8–1/4″ = 1 • 1/4–1/2″ = 3 • >1/2″ = 6

C. Location of Rot
Top/mid only = 0 • Base but above bracket = 2 • At base connection = 4 • Embedded in concrete = 6

D. Post Size & Load
6×6 light load = 0 • 6×6 heavy/4×4 light = 1 • 4×4 heavy = 3

E. Moisture Content (highest reading)
≤18% = 0 • 18–22% = 1 • >22% = 3

Total & Action
0–3 = Monitor quarterly
4–7 = Repair (consolidate + harden + seal + sleeve)
8–10 = Replace soon (shore first)
≥11 = Replace now (immediate shoring)

Repair Methods for Minor to Moderate Soft Rot

Excavating Rotted Material Completely

Open to sound wood; undercut pockets to expose all compromised fibers. Dry to ≤15% before consolidation.

Wood Hardener Application: Consolidation Technique

Saturate with a penetrating consolidant; multiple wet-on-wet coats until refusal. Let cure fully.

Epoxy Filler Systems for Structural Repair

Use a two-part epoxy designed for load-sharing. Rebuild geometry; predrill and reconnect hardware after full cure.

For <20% section loss, I’ve seen a good epoxy repair last 1–3 years—but only when the moisture source is fixed. Otherwise, it buys time, not safety.

Sister Post Installation for Partial Replacement

Bolt 2×6 sisters tight to both sides of the post spanning well above the decay zone. Treat this as temporary or non-critical support.

I’ve used the sister method dozens of times as a band-aid—always with a clear disclosure that it’s not a permanent fix.

Post Sleeve Retrofit Solutions

Add non-wicking sleeves/bases to keep the new repair isolated from splash and soil, maintaining airflow at the base.

Full Post Replacement: Step-by-Step Process

Supporting the Deck During Replacement

Install temporary shoring with at least two hydraulic jacks and 2×6 bearing plates. Lift to touch—not to re-level the deck.

Common mistake I see: over-jacking a beam, transferring stress to other connections. Gentle contact is the target.

Removing Old Post: Embedded vs. Bracketed

Bracketed: Unbolt hardware; cut free and remove.
Embedded: Cut above concrete, core-drill or chisel remaining stub without disturbing footing.

Concrete Footing Considerations

Verify footing size/depth for local loads and frost line. Replace embedded detail with an above-grade post base.

Proper Post Installation to Prevent Future Rot

End-seal cuts with preservative.
Use elevated, code-listed post bases to separate wood from concrete.
Add post caps to stop top-end absorption.
Maintain clearance for airflow and drainage.

Special Cases: Embedded Posts in Concrete

Why Concrete-Embedded Posts Rot Faster

Concrete is porous; it keeps the end grain wet. Capillary action = 24/7 moisture exposure.

Extraction Techniques Without Disturbing Footing

Core around the stub, break out, and install a surface-mounted bracket with anchors per manufacturer specs.

Above-Ground Post Bracket Conversion

Upgrade to hot-dip galvanized or stainless brackets; maintain ≥1″ standoff from slab/footing.

In the Pacific Northwest I’ve seen embedded PT posts fail in 8–18 years—far earlier than owners expect.

Preventing Future Soft Rot in Posts

Post Base Protection Systems

Use ventilated bases and non-absorptive shims; never trap water under the post.

Proper Flashing Installation

Flash ledger-to-wall and any intersections above posts to divert water.

Moisture Barriers and Drainage

Gravel around footings, downspout extensions, and grade slope away from posts.

Using Rot-Resistant Wood Species

Specify pressure-treated UC categories suitable for ground contact or choose naturally durable species where code allows.

Post Caps and Top Protection

Cap posts to stop vertical water absorption and check annually for seal failures.

Cost Analysis: Repair vs. Replace

Material Costs Comparison

Epoxy/hardener kit: ~$60–$150 per post
New 6×6 PT post + hardware: ~$80–$180 (region-dependent)

Labor Time and Complexity

Minor repair: 1–2 hrs hands-on (plus cure time)
Full replacement: 3–6 hrs with shoring and hardware

Long-Term Value Assessment

Repair can buy time if decay is <25% and moisture is solved; replacement resets the clock and improves liability posture.

When DIY Makes Sense vs. Hiring Pros

DIY for low-severity repairs and straightforward replacements; hire pros when multiple posts, heavy loads, or access issues exist.

The $200 vs. $2,000 decision is real: a small repair or timely replacement beats an emergency call and potential legal exposure.

When to Call a Structural Engineer

Signs of Widespread Structural Issues

Multiple rotted posts, beam checking, ledger separation, uneven settlement.

Multi-Post Failure Scenarios

If two or more supports are compromised, get an engineer or qualified contractor to design shoring and sequencing.

Legal and Permitting Requirements

Local code may require permits, engineered details, and inspections for structural repairs.

Temporary Shoring and Safety During Repairs

Shore before touching hardware.
Use proper bearing plates to spread jack loads.
Keep bystanders away; post temporary support warnings.
Re-check shoring after any adjustments or wind events.

ASCII Decision Flowchart (Repair vs. Replace)

Start
  │
  ├─ Moisture > 22%? ── Yes ──► Increase risk score (+3)
  │                         │
  │                         No
  │
  ├─ Screwdriver penetration > 1/2"? ── Yes ──► REPLACE NOW + SHORE
  │                                           ▲
  │                                           │
  ├─ % cross-section ≥ 25%? ─────────── Yes ──┘
  │
  ├─ Rot at base connection or embedded in concrete?
  │          │
  │          ├─ Yes → Replace soon (shore first)
  │          └─ No  → Continue
  │
  ├─ Severity Score (from calculator) ≥ 8?
  │          │
  │          ├─ Yes → Replace soon
  │          └─ No  → Repair and control moisture
  │
  └─ Schedule re-inspection (3–6 months) → Monitor

FAQs (targeting long-tails)

How much rot before a deck post needs replacement?
If ≥25% of the cross-section is compromised anywhere, replace the post. Strength drops nonlinearly; don’t gamble.

Can you repair a rotted 6×6 deck post without replacement?
Sometimes—if rot is localized, <25%, and moisture is fixed. Consolidate, fill, and consider a sister post as a temporary measure.

Deck post rotted at concrete junction—what’s the fix?
Extract the embedded stub, then convert to an above-grade bracket with a proper standoff. End-seal, cap, and ensure drainage.

Screwdriver test: how deep is bad?
At >1/4″, plan a serious repair; at ≥1/2″, treat it as a failure and replace.

Cost to replace rotted deck posts (DIY vs. professional)?
DIY materials may be under $200 per post; pro replacement commonly $600–$1,200 depending on access, shoring, and hardware.

Can you repair a deck post with epoxy filler?
Yes, for minor to moderate decay after complete excavation and consolidation. It’s not a cure-all and won’t restore a badly reduced section.

Sister post method for rotted deck support repair—when?
Use for localized base rot and light loads as a band-aid or temporary stabilization—not a permanent fix for heavy loads.

Deck post embedded in concrete rotted—how to remove?
Cut above slab, core or chisel out the stump, clean the footing, and retrofit a surface-mounted bracket.

What percentage of deck post rot is acceptable for repair?
Generally <25%, away from critical connections, with moisture under control.

When is a structural engineer needed for deck post assessment?
Heavy loads, multiple compromised posts, or any visible settlement/deflection. When in doubt, get the engineer.

How to temporarily support a deck during post replacement?
Use two hydraulic jacks minimum, with 2×6 bearing plates under the beam. Lift to contact, not to re-level.

How do I prevent rot in ground-contact deck posts?
Use the right UC-rated PT lumber, elevated brackets, proper caps, drainage, and periodic inspections.

Inspection and moisture control align with Deck Care Encyclopedia, while product selection for stabilization is covered in Wood Rot Consolidants.