Exterior wood trim rots when water keeps it wet above about 30 percent moisture content, the point the USDA Forest Products Laboratory gives for decay to start. That happens at flat tops with no cap flashing, at bottom ends and sills where water soaks into end grain, where trim touches a roof, deck or soil, and below leaking windows. The fix is to replace or repair the rotted wood and fix the water source, usually by adding head flashing, sealing ends and restoring clearances. This page is general information.
On this page: Signs · Causes · Probe check · Repairs · Basis · When to call
Signs of trim rot
- Paint peeling or cracking at the ends of boards, the bottom of corner boards and the corners of sills.
- Soft spots that dent with a fingernail; dark staining or fungal growth.
- Gaps opening where trim meets siding or the window frame.
- Sills that dip, crack or hold water.
- Swollen or split head trim over windows and doors.
Why trim rots
| Location | Usual water source |
|---|---|
| Window and door head trim | No cap flashing; water sits on the flat top |
| Window sills | Not sloped enough; water soaks into end grain; leaking window above |
| Corner boards | Bottom end in contact with a roof, deck or soil; unsealed cut ends |
| Band and frieze boards | Flat top with no flashing; water running down the siding |
| Door brickmold | Splash from steps and landings; snow piled against it |
Painting over weathered or bare wood speeds the process. Forest Products Lab research found that wood left exposed for even a week or two before painting holds paint poorly, and failing paint lets water in.
A safe probe check
- From the ground, look at the ends and bottoms of trim pieces first. That is where rot starts.
- Push an awl or thin screwdriver into suspect spots you can reach without a ladder. Sound wood resists; rotted wood is soft or crumbles.
- Look above each rotted piece for the water source: missing cap flashing, a leaking window, a gutter spilling over.
- Check upper trim with binoculars for peeling paint and gaps. Leave probing up high to the contractor.
Repairs, and the wrong fixes
| Repair | When it fits |
|---|---|
| Cut out small soft areas, consolidate and fill, prime and paint | Rot is shallow and limited to a small spot. |
| Replace individual pieces with stock primed on all sides and sealed at the ends | Rot runs through the board or along its length. |
| Add cap flashing over head trim and band boards, tucked under the weather barrier | Always when the top of the trim is flat and unflashed. |
| Replace trim with a water-tolerant material such as cellular PVC where allowed | Spots that will always see splash or snow. |
| Open the wall and repair sheathing | Rot has spread behind the trim. |
Wrong fixes: painting or caulking over soft wood, filling a large void with caulk, or replacing a board without fixing the leak above it.
Code and research basis
- Code. The 2021 IRC, section R703.4, requires flashing continuously above projecting wood trim, and at window and door openings and under and at the ends of sills.
- USDA Forest Products Laboratory. Decay starts when wood is above the fiber saturation point (about 30 percent moisture); keeping wood dry prevents it. FPL weathering research shows bare wood degrades within one to two weeks outdoors and holds paint poorly afterward.
- Purdue and FPL (NCR-133). Removing the moisture source is the cure for moisture-driven paint failure; repainting alone does not stop it.
Rot along the roof edge behind the gutters is covered in rotted fascia and missing drip edge. If the trim rots below a window, see window leaks at the install flashing.
When to call
Call when probing finds soft wood, or when paint keeps peeling from the same trim after repainting. Integrity replaces trim, flashes it and paints it in one job. The exterior problems guide and our Colorado Springs painting page explain how prep and repair fit together.
Have your trim checked
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We probe and photograph trim during an exterior inspection. Colorado Springs area: call (719) 488-2800 or book a free inspection. Denver metro: call (303) 209-0152 or book a free Denver inspection. Austin: call (512) 610-5897 or book a free Austin inspection. We reply the same business day.
Sources: 2021 IRC Chapter 7, section R703.4 (UpCodes); USDA Forest Products Laboratory, Home Wreckers in Search of Moisture; R. Sam Williams, Weathering of Wood (USDA FPL, 2005); Cassens and Feist, NCR-133 Paint Failure Problems and Their Cure.
