Where it starts
Dry rot is a misnomer — it's a fungal decay that needs sustained moisture. The fungus needs wood at 20%+ moisture content for weeks at a time to take hold. So the question is always "where does water sit?"
The five places we find it most often: window sills (especially north-facing), exterior door thresholds, the sub-trim under fascia boards, deck-to-house attachments, and the bottom plate of any wall sitting on a slab where rainwater can wick. Add irrigation overspray and you can usually predict the rot map of a house from the landscaping.
The Bay Area climate angle
South Bay homes face a specific moisture profile. Five months of effectively zero rain (May–September) lull homeowners into ignoring the exterior, and then atmospheric river events in December–February dump 6–15 inches in days. Caulk that cracked over the summer doesn't seal, paint that thinned over the summer doesn't shed, and water finds the weakest joint.
The other regional factor: coastal fog and marine layer. In Los Gatos foothills and Saratoga, north-facing walls stay damp for hours each morning year-round. North-facing window sills and trim there rot 2–3x faster than the same details on the south side of the house.
How to spot it early
Walk the exterior in February when the wood is at peak moisture. Feel the trim — healthy wood is firm, rotting wood feels soft or spongy. Press a screwdriver into any suspect spot; if it sinks more than a quarter-inch with light pressure, you've found rot.
Pin moisture meters are cheap ($30–$80) and tell you whether the wood is currently wet behind paint or trim. Anything reading above 20% in summer is a problem. We carry both pin and pinless meters on every walkthrough.
Visual cues most homeowners miss
Beyond the screwdriver test, the early-warning signs are subtle:
- Paint blistering or peeling on a single window sill or trim board: The moisture is venting through the paint film. Spot-painting won't fix it; the rot beneath will keep working.
- Fine cracks running along grain on south-facing trim: UV breakdown of paint that no longer sheds water. Caulking the cracks is the wrong fix; replacing the trim or re-priming is the right one.
- Moss or algae growth on shaded fascia or siding: Indicates surfaces staying wet for extended periods. Treat the cause (gutter overflow, no kick-out flashing) not just the symptom.
- Window frames out of square or sticking: The sill underneath has compressed from rot or termite damage. Replace the sill, not just the window operator.
- A musty smell near a wall after rain: Active fungal growth inside the wall cavity. The smell stops when conditions dry out; it returns next rain. The rot doesn't stop.
Termites vs dry rot
People conflate the two, and the damage looks similar from the outside. The difference matters because the treatment is different:
- Dry rot leaves wood crumbly, brown, cube-shaped (cubical fracture), with no visible insect debris.
- Subterranean termites leave mud tubes on foundation walls or piers, and the damaged wood has galleries that run with the grain, often with dirt or mud inside.
- Drywood termites leave small piles of granular pellets (frass) below infested wood, with smooth-walled galleries inside.
Many homes have both. A WDO (Wood Destroying Organism) inspection from a licensed structural pest control operator costs $200–$400 and clears up the question. We coordinate with two inspectors we've worked with for years.
The repair scope
Cosmetic patching of dry rot is the most common bad fix we see. Filling soft wood with epoxy and painting over it doesn't address the source — the moisture is still getting in, and the rot will spread under the paint.
The right fix: cut back the affected wood until you reach sound material (usually 6–12 inches past the visible damage), identify the moisture source, fix the source, then sister or replace the structural members. Re-flash any window or door pan, then trim and paint. Lasts 30+ years if done right.
When epoxy consolidation is acceptable
One nuance: epoxy wood consolidant has a legitimate place on historic homes where original trim profiles can't be replicated easily, or on decorative elements that don't carry structural load. The process is intensive (drying, drilling, injecting two-part consolidant, sanding, priming) and only works if the moisture source is fixed first.
What we never do: epoxy-fill a structural sill, jamb, or framing member. That's a hidden defect that gets worse. Structural rot gets cut out and replaced. Period.
Source identification
Most dry rot sources are simple: missing flashing above a window, a roof diverter that sends water into a wall, irrigation that hits the foundation, a deck ledger without flashing, a clogged downspout that overflows. The fix is usually $50–$500 of materials.
The framing repair to undo the rot is usually $2–$15K. Catching it early can save 10x on the back end.
The annual exterior walk
The cheapest insurance policy any South Bay homeowner can run: a one-hour exterior walk in late February or early March, after the wet season but before paint and caulk start cracking from summer heat. What we recommend looking at:
- Every window sill and frame: tap with a screwdriver handle, feel for sponginess at the corners.
- Every exterior door threshold: same test.
- Fascia boards along the roof line: binoculars from the ground are fine for the visual; pull a ladder out for any softness.
- Deck attachments to the house, plus deck post bases at grade.
- Bottom plate of any wall sitting on a slab, especially behind landscaping.
- Gutters, downspouts, and the splash zones beneath them.
Fixing a $200 flashing issue in March prevents a $5K framing repair in November. The math is hard to argue with.
Frequently asked questions
How can I tell dry rot from termite damage?
Dry rot leaves wood crumbly and brown with a cubical fracture pattern. Subterranean termites leave mud tubes on foundation walls and dirt-filled galleries. Drywood termites leave small piles of granular pellets (frass) below infested wood. A WDO inspection ($200–$400) confirms which.
When is the best time to inspect for dry rot?
Late February to early March, after the wet season but before summer heat cracks paint and caulk. Wood is at peak moisture then, so soft spots are easiest to identify with a screwdriver test.
Can dry rot be repaired without removing the wood?
Only in specific cases — historic trim profiles that can't be replicated and decorative non-structural elements. Epoxy consolidation buys another 5–10 years on non-structural pieces. Structural framing with rot must be cut out and replaced.
What causes dry rot to spread?
Sustained moisture in the wood at 20%+ moisture content for weeks. Common sources: missing flashing above windows, roof diverters aimed at walls, irrigation overspray at the foundation, deck ledgers without flashing, clogged downspouts, and clogged gutters.
