Symptoms
- Soft or spongy spot on the interior or exterior sill
- Paint peeling or bubbling on the sill
- Discoloration or dark staining
- Gaps opening up between the sill and frame
Likely causes
- CommonWater pooling on a sill without enough outward slopeA flat or inward-sloped sill holds rainwater against the wood instead of shedding it.
- CommonCracked caulk letting water behind the sillOnce water gets behind the sill, it stays wet longer and rots from the inside out.
- PossibleFailed exterior paint exposing bare woodUnprotected wood absorbs water directly from rain and dew.
- PossibleCondensation dripping onto the interior sill for yearsPersistent indoor moisture can rot the interior sill even without an exterior leak.
What to do
- Probe the sill with a screwdriver to map how far the soft, rotted wood extends.
- For a small area, dig out the rotted wood with a chisel down to solid material.
- Let the area dry fully, then apply a wood hardener/consolidant to the exposed wood.
- Fill the cavity with an exterior-grade epoxy wood filler, shaping it to match the sill's original profile.
- Sand smooth once cured, then prime and paint, sealing all edges including the underside if accessible.
- Reseal the joint between the sill and the wall with fresh exterior caulk to stop future water intrusion.
- For rot that's deep or extends into the frame, the entire sill piece typically needs to be cut out and replaced by a carpenter.
Tools and parts
- Chisel
- Screwdriver
- Wood hardener
- Epoxy wood filler
- Sandpaper
- Caulk gun
Exterior wood hardener and filler kitSearch: “exterior wood rot repair epoxy kit”
When to call a pro
Call a carpenter if the rot extends beyond a small patch or into the structural frame, since sill replacement is a bigger job.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.