Who this guide is for
- Homeowners noticing soft, stained or crumbling window sills
- People preparing to brief a window installer, glazier or joiner
- Anyone worried that sill rot signals ongoing water ingress
- Owners wanting a clear record before a professional assessment
What sill and frame rot looks like
Look for softness underfoot of a finger's light press, dark or grey staining, flaking or blistering paint, splits opening at corners, and a powdery or spongy texture in timber. On other materials, watch for swelling or separation.
Record these signs plainly; you are noting condition, not deciding whether it is surface weathering or deeper decay.
- Soft or spongy feel to the timber
- Dark staining or paint blistering
- Splits or gaps opening at corners
- Crumbling or powdery surface
Where water collects and runs
Note whether the sill slopes water away or holds puddles, whether the worst decay is at the corners or under the glazing, and which elevation faces the weather.
How water reaches and leaves the sill is something a professional will want to understand, so describe what you see during and after rain.
Checking for related signs inside
Look at the wall and reveal beneath and beside the window indoors for staining, peeling or damp, which can indicate water travelling past a failing sill.
Record any internal signs alongside the external ones rather than treating them separately — the connection is informative.
Photographing without deep probing
Photograph the sill and frame from several angles, including corners and any splits, and the internal reveal. A light press to note softness is fine; do not gouge, drill or dig into the timber to test depth.
Date the photos and keep a record so you can show whether the decay is spreading.
- Capture corners, splits and staining
- Photograph the internal reveal too
- Do not gouge or drill to test depth
Briefing a window or joinery professional
Bring your external and internal photos, the frame material, and notes on water behaviour before contacting a window installer, glazier or joiner.
Let them establish how far the rot extends and whether water is still entering; your record gives them a head start.
Documentation checklist
- 1Record where the sill is soft, stained, split or crumbling
- 2Note the frame material and which elevation it faces
- 3Observe whether water drains off or pools on the sill
- 4Check the internal reveal and wall for related damp or staining
- 5Photograph the sill, corners and splits from several angles
- 6Photograph the inside of the window reveal
- 7Press lightly to note softness but avoid gouging or drilling
- 8Date photos to track whether decay is spreading
Common mistakes to avoid
- Filling or painting over soft timber, which hides decay and any active leak
- Probing deeply with a screwdriver and worsening the damage
- Recording the outside only and missing internal water signs
- Assuming the rot is shallow when it may extend behind the surface
- Delaying a professional look while water continues to enter
When to involve a professional
- A window installer, glazier or joiner can assess how far rot extends and whether water is still entering
- If moisture has reached the wall or interior, a building professional may need to check the path
- Do not gouge or fill the sill before assessment, as that can hide the extent and the source
- What is involved in repairing a rotten sill varies by frame material, exposure and location
Frequently asked questions
Questions readers ask about this topic
How do I know if my sill is rotten or just weathered?
Softness, crumbling and dark staining suggest decay rather than surface weathering, but confirming the extent means assessment by a professional. Record the signs and a light-press observation, and avoid deep probing.
Does a rotten sill mean water is still getting in?
It can, which is why checking the internal reveal and wall for damp matters. Note any internal signs alongside the external decay and let a professional establish whether ingress is ongoing.
Can I just fill and repaint it?
Filling soft timber hides the decay and any active leak, and rot can extend behind the surface. It is more useful to document it and have a professional assess the true extent first.
Which professional should I call?
A window installer, glazier or joiner can assess sill and frame rot, and if moisture has reached the wall a building professional may be involved. Your photos and notes help whoever attends arrive prepared.
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