A rotten sash window sill does not automatically mean the complete window needs to be replaced. Decay is often concentrated in exposed lower sections where water has entered through failed paint, open joints, defective putty or poor external sealing.
How is the extent of rot assessed?
The visible surface can be misleading. Paint may conceal softened timber, while a weathered-looking sill can still have substantial sound material beneath it. A joiner should identify the cause of moisture, determine how far the decay extends and check adjacent jambs, pulley stiles, lower sash rails and joints.
Localised splice repairs
Where decay is limited, defective timber can often be cut back to sound wood and a matching timber repair section spliced in. The repair should restore the original profile and shed water correctly. This retains more of the original window than wholesale replacement.
When is a larger section needed?
If decay extends across most of the sill or into structural joints, a full sill or larger section may be required. The correct decision depends on access, window construction and the condition of adjoining components. Our timber sash window repair service covers these types of repairs.
Fix the cause, not just the damaged timber
A repair will last longer when the source of water is corrected. External paint, glazing putty, joints and perimeter seals should be checked. Traditional sand mastic may also form part of the weathering detail around appropriate window openings.
Repair versus replacement
Keeping sound original timber is often preferable, especially on traditional and historic buildings. If decay is extensive throughout the sashes and case, replacement may eventually become more economical. Read our repair or replace guide before deciding.
Have a soft or damaged sill? Send photographs or request a survey and we can assess the repair options.