What Wood Rot Actually Is
Wood rot is fungal decay. Specific species of fungi consume the cellulose and lignin in wood, breaking down the fibers that give it strength. There are two main types you'll see in inspection reports: brown rot and white rot.
Brown rot, sometimes called dry rot (a misleading name), is more common in residential settings. It breaks down cellulose while leaving lignin behind, which turns the wood brownish and causes it to crack into small cubes. White rot breaks down both cellulose and lignin, leaving the wood pale, spongy, and stringy.
Both types require moisture to survive and spread. Fungal spores are everywhere, but they can't do damage without sustained moisture above about 20% wood moisture content, according to the USDA Forest Products Laboratory. That's why fixing the moisture source is always step one in addressing wood rot, not the rot itself.
How Inspectors Find and Evaluate Rot
Inspectors use a few methods to identify rot, and not all of them are visible to you in the photos.
Visual Inspection
Obvious signs include discoloration (gray, brown, or black staining), surface cracking, paint peeling, or wood that looks different in texture from the surrounding material. Soft spots, sagging, or visible deterioration around joints are red flags inspectors note immediately.
The Probe Test
Inspectors use a screwdriver or awl to probe suspect wood. Healthy wood resists penetration. Rotted wood gives way easily or even crumbles. This test is especially useful for painted surfaces where rot hides under an intact coat of paint but has already compromised the wood beneath. If you see probe test marks in your inspection photos, that's what happened.
Moisture Meter Readings
Many inspectors carry moisture meters that measure the water content in wood. Readings above 20% indicate conditions favorable for fungal growth. This helps inspectors flag areas that may not show visible rot yet but are at high risk. Some reports will note elevated moisture readings in specific areas alongside the rot findings.
Where Rot Is Most Commonly Found
Some locations almost always get flagged when rot is present. These are places where moisture predictably accumulates or where water management fails.
Sill plates. The sill plate is the horizontal wood member sitting on top of the foundation. It's the lowest piece of framing in the house and is often the first to rot when groundwater, condensation, or sprinkler overspray gets into the basement or crawl space. Sill plate rot shows up constantly in crawl space findings.
Window and door frames. Failed caulking, improper flashing, or missing drip caps allow water to enter around windows and doors. The sill, jamb, and framing around the rough opening are common rot locations. This is usually caught during the exterior inspection.
Deck ledger boards. The ledger is the board that connects a deck to the house. Water gets behind it, sits against the house framing, and creates ideal rot conditions. This is a structural concern when the ledger is significantly deteriorated because the deck's load depends on that connection holding.
Fascia and soffit boards. These trim boards under the roofline are exposed to weather and often get damaged by clogged gutters that overflow and saturate them. Fascia rot is usually cosmetic but can spread into the rafter tails if left long enough.
Exterior trim and siding bottom courses. Areas near ground contact, especially where soil grades toward the house, routinely develop rot. The HUD Rehabilitation Guidelines note that proper clearance between wood and soil is a basic durability requirement for exterior finishes.
Surface Rot vs. Structural Rot
This is the distinction that determines how much you should care.
Surface rot (also called incipient decay) affects the outer layer of wood but hasn't compromised the structural integrity. Paint peeling off a window sill with shallow rot is an example. The wood still does its job. The repair is straightforward: remove the deteriorated material, apply a wood hardener and filler, prime, and paint. Cost: typically $100-$400 per window.
Structural rot has progressed deep into load-bearing members. A rotted sill plate, compromised floor joist, or decayed ledger board needs to be partially or fully replaced. The repair is more involved because you may need to temporarily support the structure during replacement. A single rotted sill plate section might cost $800-$2,500 to address, depending on length and access. Multiple joists can run $3,000-$8,000 or more.
Your report will usually indicate whether the inspector probed the rot and what they found. Phrases like "surface decay noted" or "cosmetic" signal minor findings. "Compromised," "significant deterioration," or "recommend further evaluation by a contractor" indicate more serious concerns.
What to Do With Rot Findings
A few rot items in an exterior inspection are not unusual and not necessarily a dealbreaker. Most houses have some degree of wood rot, especially those over 20 years old. The questions to answer are: Is it structural? Is the moisture source still active? How extensive is it?
For any rot findings flagged in your report, get a quote from a licensed contractor. This gives you a number to work with in negotiations and tells you how serious the issue actually is. The inspection report can describe what was observed, but only someone who physically opens walls or accesses the framing can tell you the full scope.
Don't confuse rot repair with rot prevention. If the house has an active roof leak, a sprinkler that hits the foundation, or gutters that overflow onto the fascia every rain, fixing the damaged wood without fixing those things means you'll be back in the same situation in five years. Moisture source first. Repairs second.