Repairing Rotten Sash Window Sills
Soft or cracked timber at a sill can indicate prolonged wetting. Find the moisture source before making cosmetic repairs.
Assess the extent
Inspect failed paint, open joints, trapped water and nearby masonry. Localised decay may be suitable for a timber splice; extensive deterioration may affect the case and require specialist joinery.
Do not conceal decay
Filler alone is not a structural repair. Sound timber, appropriate detailing and weather protection are essential for lasting work.
Why sills decay
Sills are exposed to rain, failed coatings, open joints and moisture held against masonry. Decay often starts at end grain or where water cannot drain away.
Recognise the signs
Soft timber, persistent damp patches, blistering paint, dark staining and joints that open repeatedly can indicate moisture problems. A painted surface may conceal more decay than is visible.
Assess the extent
Localised damage in a non-critical section may be repairable with a carefully detailed timber splice. Decay extending into the case, pulley stile or surrounding structure needs a more thorough assessment.
Why filler is not enough
Filler can improve appearance but does not restore lost structural timber. Trapping moisture beneath a surface repair can accelerate deterioration. Correct the source of water before deciding on a repair.
Repair principles
A durable timber repair needs sound surrounding wood, compatible replacement timber, sound joints, drainage details and a suitable protective finish. Avoid cutting away sound historic material unnecessarily.
Professional assessment
Seek a joiner experienced in traditional windows if the sill supports other decayed components, access is difficult or the building is listed.
Further reading
Use our sash window fault finder, maintenance checklist and tools and materials guide for related advice. If a repair is beyond safe DIY maintenance, contact The Sash & Case Window Company.