Symptoms
- Wood feels soft or spongy when pressed
- Paint is peeling or bubbling on the frame
- Dark staining or discoloration on the sill
- A screwdriver sinks in easily where it shouldn't
Likely causes
- CommonLong-term water intrusion from failed caulk or flashingGaps let rain reach the wood repeatedly over months or years.
- CommonPoor drainage at the sillA sill without a slight outward slope holds water against the wood.
- PossibleFailed exterior paint exposing bare woodOnce the protective coating cracks, wood absorbs water directly.
- PossibleCondensation dripping onto the sill for yearsChronic indoor moisture can rot the interior sill over time.
- Less commonWood-destroying insects in already-damp woodTermites or carpenter ants sometimes move into softened wood.
What to do
- Press a screwdriver into the suspect wood to map out how far the soft, rotted area extends.
- Photograph the damage and check nearby areas (siding, trim) for related water staining.
- For small, shallow rot, dig out the soft wood with a chisel down to solid material.
- Treat the exposed wood with a wood hardener/consolidant product, then fill with an exterior-grade epoxy wood filler once dry.
- Sand the patch smooth, prime, and paint to match, sealing every edge.
- For rot that's extensive or structural (affects the frame's strength), the damaged section or the whole sill/jamb needs to be cut out and replaced.
- Fix the water source, whether that's caulk, flashing, or a sill slope, before repairing the wood, or the rot will return.
Tools and parts
- Chisel
- Screwdriver
- Sandpaper
- Wood hardener
- Epoxy wood filler
Exterior wood hardener and filler kitSearch: “exterior wood rot repair epoxy kit”
When to call a pro
Call a carpenter or window company if rot has spread into the structural frame, affects more than a small patch, or you find it near multiple windows.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.