Wood rot is fungus eating wet wood, and every sentence about it follows from those four words: no moisture, no rot; find the rot, find the moisture; fix the rot without the moisture and buy it again. Wood rot repair therefore runs in pairs, the wood fixed and the water path closed, and this reference covers identification, the exterior geography where rot starts, the epoxy-versus-replacement decision, and the pairing rule.
Identification: the kinds and the probe
The decay fungi that matter split by what they eat. Brown rot consumes the cellulose and leaves the brown lignin behind: wood darkens, shrinks, and cracks into the signature cubical checkerboard, eventually crumbling dry, which is where the misleading folk name dry rot comes from, the wood is dry after the fungus finished, but the growing was done wet, always. White rot eats the whole menu, leaving pale, stringy, spongy fiber. Soft rot works the chronically saturated surfaces. The practical identification tools ignore the taxonomy: the probe test, an awl or screwdriver pressed along suspect wood, sound wood refusing the point and rot taking it with soft, punky acceptance, exactly the sill-plate discipline applied to the exterior; the moisture meter, wood persistently above roughly 20 percent being fungus’s operating range, and readings that high in “dry” weather revealing the chronic path; and paint as informant, coatings peeling and bubbling in one patch over sound paint elsewhere marking moisture pushing from behind, per the paint-failure reference. The look-alike to rule out is insect work, carpenter ants and termites hollowing wood along the grain, frass and mud tubes as signatures, often side by side with rot in the same wet real estate.
The geography: where exterior rot starts
Rot maps the water map, and the exterior’s chronic addresses repeat house to house. Horizontal surfaces and their joints: window sills and stool ends, door thresholds and the jamb bottoms beside them, deck surfaces, and handrail tops, everything that holds water faces the sky. End grain everywhere: wood drinks through its cut ends at many times the rate of its faces, so trim bottoms, post bases on concrete, mitered corner joints, and the cut ends of siding at windows and doors rot first, which is why priming end grain is the cheapest durability upgrade in carpentry. Splash and grade zones: siding bottoms and trim within splash-back range of hard surfaces, wood within inches of soil or mulch per the grading rules, and the gutter-failure drip lines that hose one band of wall all storm. The flashing failures: headerless windows, unflashed deck ledgers, and every joint where caulk substituted for laps until it retired. The concealed cases: rot behind intact paint and cladding, found by probe and meter at the usual addresses, and the reason trim that looks fine gets pressed anyway.
Repair: epoxy versus replacement
The decision runs on structure and extent. Epoxy systems, a liquid consolidant soaking and hardening the punky margins, then a structural filler sculpted to profile, earn their keep on localized rot in members that are non-structural and hard to replicate: historic window sills and sash, molded trim profiles, column bases and porch details, door frames, with the discipline that all loose rot is removed first, the wood is dry when treated, and the repair gets primed and painted like the wood it saved. Replacement in kind answers structural members (per the sill and framing references), extensive rot where filler would outweigh wood, and the repeat offenders where modern material upgrades close the case: rot-prone trim replaced in cellular PVC or fiber cement, post bases re-set on standoff pedestals, and ground-contact locations given ground-contact-rated treated lumber. Either path, the carpentry ends the same way: primed cuts and end grain, caulked joints that should be caulked and flashed joints that should be flashed, and paint maintained as the sacrificial layer it is.
The pairing rule
The half of every rot repair that decides its lifespan happens away from the wood: the gutter fixed, the flashing rebuilt, the grade and mulch pulled back, the sprinkler redirected, the end grain sealed, the splash zone given a drip edge or a kickout flashing. A rot repair with the moisture path closed is permanent; the same repair without it is a subscription with a renewal date the meter could have predicted, and the inspection habit that beats both is the annual probe walk, sills, thresholds, post bases, ledger, trim bottoms, five minutes with a screwdriver finding trouble while it is still a paint-and-primer errand.
The bottom line
Rot is wet wood being eaten: probe the usual addresses yearly, trust the point over the paint, meter anything suspicious, epoxy the precious and non-structural, replace the structural and extensive, and never leave a repair without closing the water path that ordered it. The fungus only ever loses to dryness.