Signs of dry rot on exterior trim, sills, and siding
The paint on my window trim is bubbling and one sill feels soft. Is that dry rot, and how bad is it?
Probably, yes. Dry rot is wood decay caused by a fungus that needs moisture, and bubbling or cracked paint over a sill that gives when pressed is its most common first sign. Press the wood with a screwdriver: sound wood resists, rotted wood is soft, spongy, or crumbles. Rot travels along the grain behind the paint, so the visible spot is usually smaller than the damage, and the fix is two parts: replace the decayed wood, and stop the water that fed it.
What dry rot is, and why the name is misleading
Dry rot is decay caused by fungi that digest the cellulose in wood. Despite the name, it needs moisture to start and to keep going; the "dry" refers to how the wood looks after the fungus has consumed it: dull, brittle, and cracked into cube-shaped blocks. On a house it begins wherever water sits against wood or gets behind a finish and cannot dry out: window sills, the bottom ends of trim and corner boards, fascia behind a gutter that leaks or has pulled away, deck ledgers, door thresholds, and siding that touches soil or concrete.
The signs, from earliest to latest
- Paint that bubbles, cracks, or lifts along a joint or the bottom edge of a board while the paint next to it is fine. Water behind the finish is the cause, and rot is often already started.
- Wood that gives when pressed with a thumb or a screwdriver. This is the definitive test: sound wood resists; rotted wood is soft, spongy, or crumbles.
- Staining, dark streaks, or a musty smell at a sill or trim bottom, or on the wall or drywall below a window.
- Cracks across the grain and a dull, dried-out surface, sometimes with cube-like checking, on wood that has lost its finish.
- Sills or trim that look sunken or shrunken compared with the piece next to them, or trim that has separated from the wall.
- Fungal growth: a white or gray cottony film, or in advanced cases mushroom-like fruiting bodies, on or near the wood.
Where it starts on Sacramento-area homes
The pattern here is seasonal and directional. Rot starts in the wet months, November through March, wherever wood stays damp: north walls under tree canopy that dry slowly, fascia behind gutters that overflow, sills under windows with failed caulk, and the bottom course of siding near the ground. It then advances through the dry months behind paint that looks fine from the street, because the wood inside never fully dries. South- and west-facing walls fail differently, through sun-driven paint breakdown, which then lets water in and starts the same cycle a year or two later.
On the prewar and postwar wood-sided homes in central Sacramento, Land Park, East Sacramento, Fair Oaks, and the older parts of Roseville and Folsom, the first places to check are the window sills, the bottom of every corner board, and the fascia under any gutter that has ever overflowed. On stucco homes the wood is hidden, and rot shows up at door and window trim, at the fascia and eaves, and at any wood that meets the stucco at the ground.
Rot, peeling paint, or termites?
| What you see | Likely cause | What to do |
|---|---|---|
| Paint peeling; wood underneath hard and dry | Paint failure (sun, age, poor prep) | Scrape, prime, repaint; fix caulk |
| Paint bubbling or cracked; wood soft when pressed | Dry rot | Find the water source, replace the decayed wood, repaint |
| Wood crumbles along the grain in blocks; no tunnels | Dry rot | Same as above |
| Galleries or tunnels in the wood, sawdust-like frass, mud tubes | Termites or carpenter ants | Licensed pest inspection before any repair |
| Both soft wood and tunnels | Rot and insects together (both need moisture) | Pest inspection, then repair and water-source fix |
What to do first
- Probe the spot and the wood around it. Note how far the softness extends; rot is usually wider than the paint failure shows.
- Find the water. Look up: a gutter that overflows, a missing drip cap above the window, a failed caulk joint, a downspout that dumps at the base of the wall, or siding that touches soil. The repair fails if the water path is not fixed.
- Do not paint over it. Paint seals moisture in and hides the spread; a repainted rotted sill fails within a season.
- If there is any sign of insects, get a wood-destroying organism inspection from an inspector licensed by the California Structural Pest Control Board before the repair. Rot repair and pest treatment are separate trades and separate licenses.
- Get a written repair scope that maps each damaged piece, names the replacement material, states the flashing or sealing change that stops the water, and gives a unit rate for damage found behind the visible piece.
When it becomes a bigger repair
A rotted sill or trim board is a small job. Rot that has reached the sheathing or framing behind it, rot that recurs in the same place, or rot across several trim pieces on one wall is a signal that the flashing or siding on that wall has failed and needs a larger opening to fix. That is the point where a dry rot repair turns into a siding or window scope, and it is better to know before the estimate is written than after the first board comes off. A site review probes the usual spots and writes the repairs in stages so the scope can grow in writing rather than as a surprise.
The materials matter too. Primed finger-joint wood matches most trim and takes paint; fiber cement trim resists rot on the most exposed pieces; cellular PVC is the choice where wood touches the ground or stays wet. The dry rot service page compares them.
Next step
If you have found a soft sill or bubbling trim, request an estimate and describe where it is, which direction the wall faces, and what is above it (gutter, window, roof edge). That is enough to plan a site review. The estimate comparison guide explains how a per-unit rate for hidden damage should appear in the scope.