Dry rot is wood decay caused by fungi, and the wood has to be wet for it to happen. Most rot in houses is brown rot, which leaves wood dark, cracked into small cubes and crumbly. That dry, crumbly look after the water is gone is where the name comes from, and it is why the USDA Forest Products Laboratory calls the term incorrect. Its Wood Handbook puts the line at the fiber saturation point, about 30 percent moisture content: serious decay happens only above it, and air-dried wood at 20 percent or less has a reasonable margin of safety.

That makes rot in western Oregon a water problem more than a weather problem. Damp air alone does not wet wood enough to rot it. Rain, condensation or wet ground does, and on a house, rain usually reaches the wood at a detail: a window, a roof edge, a deck, the bottom of a wall. This article covers what the fungus needs, what the Willamette Valley’s climate does to wood, whether rot spreads, and what each kind of repair can do. If you need the rot on your own walls found and fixed, see dry rot repair in Salem.

Where dry rot shows up on siding and trim

Rot on the outside of a house starts where water reaches wood and stays. On a wall, that is a short list of places, and they are worth checking before the rest of the house:

  • The bottom of the wall. The lowest course of siding, the bottoms of corner boards and the sill plate behind them, especially where soil, bark mulch, a patio or steps sit close to the siding.
  • Under windows and doors. Window sills, the ends of the trim below them, and the siding just under each corner of the opening.
  • Below roof-to-wall corners. Where a lower roof ends against a wall, water runs down the face of the siding unless a kickout flashing turns it away. Rot or staining in a stripe below one of those corners points to a missing or failed kickout.
  • Behind downspouts and under overflowing gutters, and anywhere a sprinkler hits the wall.
  • Where a deck or porch meets the house, at the ledger board and the siding just above it.
  • Around holes and failed joints, such as woodpecker holes, cracked caulk at butt joints and open miters on trim.

What rot looks like depends on the siding:

  • Cedar and other solid wood siding and trim. Brown rot, the common kind in houses, leaves wood darker, cracked across the grain into small cubes, and crumbly; probed with an awl, it comes up in short pieces rather than long splinters. Paint can hide it until the wood is probed.
  • Hardboard and OSB lap siding. The first sign of water is usually swelling and flaring along the lower edge of each board, not the cubed look of rot. The cause is the same, water getting in at edges and nail holes, and swelling hardboard or OSB siding covers it.
  • T1-11 plywood panels. The layers of veneer separate at the bottom edge and at horizontal seams, and the plies go soft from the edge in. See T1-11 siding damage.
  • Fiber cement and vinyl. The siding itself is not wood, but the trim, sheathing and framing behind it are. Rot shows as soft wood trim, as siding that feels loose or bulges when pressed, or as stains and soft drywall on the inside of the same wall.

A soft spot in one board is a repair. Soft sheathing behind it is wall work. Either way, check Salem’s permit rules first: the city requires a building permit to repair sheathing, and its permit page lists dry rot repair among work that always needs one. Dry rot in siding and trim covers the checks that tell the two apart and what a repair includes. The rest of this article explains why the wood rotted in the first place.

What is dry rot?

Decay fungi spread by spores. When a spore lands on wood that is wet enough, it germinates, and threadlike strands of the fungus grow into the wood and digest it. The Wood Handbook describes three kinds of decay:

  • Brown rot removes mainly the cellulose. The wood turns browner, cracks across the grain into cube-like pieces, shrinks, and can be crushed to powder. Brown-rot fungi commonly attack softwoods, and because softwoods have long been the main building woods, a 1999 Forest Products Laboratory review calls them the most common agents of decay in buildings.
  • White rot removes both lignin and cellulose. The wood looks bleached, keeps its shape and often feels spongy. It is more common in hardwoods.
  • Soft rot is shallow, and the Handbook calls it generally less important.

Rot also does its damage before it shows. In laboratory tests reported in the Wood Handbook, by the time decay had taken 10 percent of a piece’s weight, most strength losses could be expected to exceed 50 percent, and the decay was still detectable only under a microscope. Wood with visible rot, the Handbook says, can be assumed to have lost a great deal of its strength.

How wet, and how warm, does wood have to be?

  • Wetter than fiber saturation. Serious decay happens only above about 30 percent moisture content. Research summarized in the 1999 review found fungal growth greatly slowed below fiber saturation and stopped below 20 percent.
  • Wetter than damp air can make it. The Handbook says the water vapor in humid air will not wet wood enough for significant decay, though it can support mold. Wood reaches fiber saturation only from rain, condensation or contact with wet ground.
  • Mild temperatures. Most decay fungi grow best at about 77°F. Decay is relatively slow below 50°F and above 95°F, and essentially stops at 35°F or 100°F.
  • Long enough. In laboratory tests set up to favor decay, small pieces of untreated, nondurable wood can decay substantially in a few weeks. The 1999 review also says that wood wetted above fiber saturation only briefly and now and then, or for a few months while it is cold, does not decay.

What does western Oregon’s climate do to wood?

Less than you might expect, as long as the wood stays out of the rain. A 1998 Forest Products Laboratory note lists, month by month, the moisture content that wood settles at outdoors when it is protected from direct rain and sun. For Salem, it runs from 11.6 percent in July and August to 17.8 percent in November and 18.0 percent in December. Portland peaks at 17.4 percent and Eugene at 20.2 percent, both in December. In Salem, the monthly average for sheltered wood stays below 20 percent all year, and nowhere near 30.

So wood that rots here has been wetted by liquid water, and research from a similar climate shows where it gets in. In the 1990s, newer low-rise wood-frame housing in coastal British Columbia, where winters are wet and mild much like western Oregon’s, began leaking and rotting on a large scale. A 1996 survey for the Canada Mortgage and Housing Corporation looked at 46 of those buildings in the Lower Mainland around Vancouver, 37 with problems and 9 without. It found:

  • About 90 percent of the problems were at interface details and penetrations, chiefly windows and the edges of decks, balconies and walkways. About 10 percent traced to the basic wall itself, and problems with the cladding, sheathing paper and sheathing made up less than 10 percent.
  • Water that got in at those details stayed long enough to start rot in siding, framing and sheathing, and the damage spread into the wall around each leak.
  • Walls under roof overhangs had fewer problems, and the share of walls with problems fell as overhangs got wider.

Those were mostly stucco-clad multi-unit buildings, not Salem houses with lap siding, but the places to look translate directly: window heads and sills, the corners where a lower roof meets a wall, deck ledgers and railings, and the bottom of the wall.

The bottom of the wall is where Oregon’s residential code expects wood to get wet. Section R317.1 of the 2023 Oregon Residential Specialty Code calls for wood siding, sheathing and wall framing to be naturally durable or preservative-treated where it sits less than 6 inches from the ground, or less than 2 inches above concrete steps, porch slabs or patio slabs. Soil or bark mulch piled against a wall eats into that clearance. A 2026 edition of the code took effect on October 1, 2026, with a six-month phase-in, to April 1, 2027, during which either edition may be used, according to the Building Codes Division’s adoption timeline.

Which months carry the most decay risk?

In 1971, T.C. Scheffer published a climate index for the decay hazard of wood above ground, built from two figures for each month: the average temperature and the number of days with at least 0.01 inch of precipitation. The Forest Products Laboratory recalculated the index in 2009 from 1971-2000 weather data and maps it in three bands: under 35, 35 to 65, and over 65, with higher numbers meaning greater hazard. Salem sits in the middle band with its Northwest neighbors:

  • Willamette Valley and Puget Sound: Eugene 40.8, Olympia 43.7, Salem 43.9, Seattle-Tacoma 49.9, Portland 52.4.
  • Elsewhere in the region: Astoria, on the coast, 69.2; Medford 24.7.
  • For scale: Phoenix 9.0, Atlanta 70.7, Houston 77.2, Miami 145.9.

The formula also shows when the hazard builds. Each month counts the degrees its average temperature sits above 35°F, multiplied by its wet days beyond three, and the twelve months are added up and divided by 30. Run NOAA’s 1991-2020 normals for Salem through it, which is our arithmetic and not a published figure, and the annual total comes to about 50, still in the middle band. April, May, October and March carry the most weight, each close to twice as much as December. December and January bring the most rain, but they are cold enough to slow decay. July and August are warm but nearly dry. Spring and fall are both wet and mild.

The 2009 report is clear that the index is not meant to predict how fast decay will progress, or when any particular building will fail. The pattern is still useful. A leak that shows up in the winter rain is still wetting the wood through the mild, wet weeks of spring and fall that the formula weights most heavily, and a wall repaired in the dry months, June through September, goes into the next wet season dry.

Does dry rot spread?

Not the way it is often described online. Much of that advice comes from Britain, where dry rot means one fungus, Serpula lacrymans, which can pass through cracks in masonry and plaster. For the brown rot in Oregon siding and trim, the research says three things:

  • Ordinary rot does not move into dry wood. The Wood Handbook says wood kept air-dry will not decay, and decay already present will not progress. When rot seems to spread along a wall, it is following the water. In the British Columbia survey, leaks entered at details and the damage reached into the wall areas around them, which could not dry out fast enough.
  • Water-conducting fungi are the exception, and they are rare. A few brown-rot fungi grow root-like strands that carry water, usually from damp soil, into wood that would otherwise be too dry to decay. A 1968 USDA Forest Service bulletin said the water-conducting fungus then called Poria incrassata, now Meruliporia incrassata, causes “the most spectacular decay of wood in buildings in the United States,” but also that attacks are not frequent. The 1999 review called decay by water-conducting fungi in houses rare, usually starting in crawlspaces or basements, and knew of no documented case in a wall above grade that had not spread from a porch, sill or crawlspace. Oregon State University Extension’s wood preservation manual says these fungi are rare in the western U.S. They also depend on their water: in the 1968 bulletin, the fungus in naturally infected wood could not survive 32 days of air-drying.
  • Dried out is not the same as dead. Decay fungi can go dormant in dry wood and revive when it is wet again, and the 1999 review reports that common decayers of exterior woodwork have survived as long as a decade that way. A soft sill that has dried over the summer is not cured. If water still reaches it, decay picks up again when the rain does.

That is also why cutting far into sound wood is not the standard fix. The old advice to remove all apparently sound wood within 2 to 3 feet of visible infection was aimed at the water-conducting fungus, and the 1968 bulletin called it very expensive. It recommended cutting off the fungus’s water and replacing only wood too weak to carry its load. For ordinary rot, the Wood Handbook’s rule is to correct the moisture condition that caused the damage. Once the source of the water is gone, only the weakened wood needs replacing, with dry lumber. Because strength loss runs ahead of visible decay, finding where the weakened wood ends is a job for probing, not a fixed distance.

Active rot or old damage?

No simple test labels rot active or inactive; moisture decides it. The Forest Products Laboratory’s condition assessment manual also says plainly that visual inspection cannot detect early decay, which is why a contractor may open up a wall before quoting. These are the signs that decay is, or recently was, active:

  • Wet wood. Decay needs wood above fiber saturation, so a moisture meter tells you whether the conditions are there: below 20 percent, decay is not progressing; toward 30 percent and above, it can. A reading taken after a week of rain says more than one taken in August.
  • Fresh fungus. The Wood Handbook describes fresh growth of decay fungi as fan-shaped patches, strands or root-like structures, usually white or brown and sometimes yellow or orange. Fruiting bodies look like mushrooms, brackets or crusts. The condition assessment manual calls fruiting bodies a positive sign of fungal attack, though not of how far it has gone.
  • Stains. The same manual reads staining or discoloration as a sign the wood has been wet enough to support decay, and rust stains around nails and hardware point the same way.
  • The pick test. National Park Service Preservation Brief 9 describes jabbing an ice pick or awl into a wetted wood surface at an angle and prying up a small piece. Sound wood separates in long fibrous splinters. Decayed wood lifts in short, irregular pieces. The condition assessment manual adds two cautions: wood softened by water may still be sound, and soft woods such as western redcedar are harder to judge.

The insects that follow rot in the Northwest

Three insects of the Pacific Northwest are tied to wet or decaying wood. Finding any of them in siding, trim or sills is a reason to look for water.

  • Pacific dampwood termites. Oregon State University’s pest pages say they eat and nest in moist, water-damaged wood. Washington State University Extension’s termite bulletin adds that they need a high moisture level to get established, usually in wood soaked by leaky pipes, poor gutters or contact with soil, and that once established they can extend into sound, even fairly dry wood. Their droppings are small, slightly hexagonal pellets about 1/25 inch long. The bulletin says correcting the conditions that let them in is likely to eliminate them.
  • Carpenter ants. The Wood Handbook says carpenter ants use wood for shelter rather than food, usually preferring wood that is naturally soft or softened by decay, so ants nesting in a house can point to a moisture problem. Ants that are only foraging, or that came in with firewood, are a different matter. Species that prefer partly decayed wood, such as Camponotus modoc, can be especially damaging, and the Handbook lists porch columns, porch roofs and window sills among the parts of houses they frequent most. An OSU Extension Master Gardener note says C. modoc is more common in western Oregon than C. vicinus.
  • Moisture ants. Washington State University Extension calls them ecological indicators of a moisture problem. Their colonies set up in wood already in an advanced stage of decay, and long-term control means removing the rotted wood and the moisture.

Borates, epoxy or new wood: what each repair can do

Once the water is dealt with, the choice depends on how much sound wood is left and whether it can be kept dry.

  • New wood. Dry lumber is enough where the water source is gone. Where wood cannot be kept dry, the Handbook recommends preservative-treated wood or all-heartwood lumber of a decay-resistant species. Its groupings put the heartwood of western redcedar, incense-cedar, Port-Orford-cedar and yellow-cedar in the resistant class, Douglas-fir in the moderately resistant class, and true firs, hemlocks and most pines in the slightly or nonresistant class. Two cautions from the same chapter: untreated sapwood of essentially all species has low resistance to decay, and decay-resistant species from old-growth forests often resist decay better than wood from second-growth and younger trees. Durable wood buys time. It does not replace flashing.
  • Borates. Borate preservatives, such as disodium octaborate tetrahydrate, are EPA-registered wood preservatives. They are sprayed, brushed or injected into wood in existing buildings because they diffuse into wood that is wet. The Handbook’s preservatives chapter also says they are readily soluble in water and highly leachable, and should be used only above ground where the wood is protected from wetting. Little of the chemical moves once wood is below 20 percent moisture, and in one study it diffused less than about 5 inches along the grain and 2 inches across it. They are applied according to the product label. That makes them a treatment for framing and sheathing that will be dry and covered once the repair is done, not a substitute for replacing rotted wood.
  • Epoxy. On older houses, partly decayed trim and sills can often be kept. Preservation Brief 9 describes consolidating partly decayed wood with semi-rigid epoxies that soak into the porous wood and harden, then filling, sanding and painting it. Preservation Brief 45 limits epoxies to areas that will be painted, because they do not take stain and deteriorate in sunlight. A Park Service technical note on log repairs adds that patching decay works only where enough sound wood remains to bond to; in that project, the wood was dried below 20 percent moisture content first.
  • Dutchman repair. Preservation Brief 45 also describes chiseling out the decayed section and fitting a piece of seasoned wood of the same type, called a dutchman, after applying a fungicide to the surrounding old wood. Done with skill and care, the brief calls it a permanent repair.

On a historic house in Salem, repairs like these keep the original material. Ordinary repair does not need the city’s historic design review, while replacing siding with a different type, style or material does; the Salem page explains how that works.

What stops dry rot

  1. Find where the water gets in: window and door heads and sills, roof-to-wall corners, deck ledgers and railings, the bottom of the wall near soil, mulch or paving, and anywhere gutters overflow or sprinklers hit the wall. A wall that grows mildew or moss again soon after cleaning is staying damp too.
  2. Fix the detail that let it in, whether that is flashing, clearance from the ground or a failed joint.
  3. Replace the weakened wood with dry lumber, or with treated or durable wood where it cannot be kept dry.
  4. Look again after the first heavy rains of the season, when a leak that was missed shows itself.

Do not count on insurance for it. Oregon’s Division of Financial Regulation says policies generally exclude dry rot as a maintenance problem; storm damage and insurance covers what policies usually do pay for.

If you have found soft wood, dry rot in siding and trim covers how to tell whether it is the boards or the wall behind them, and when a Salem repair needs a building permit. Request a siding quote, say where you found the soft wood, and we will follow up.

This page is general information, not professional advice about your specific situation. Have it assessed by a qualified professional before making decisions. Last reviewed October 4, 2026.