Timber decay develops when vulnerable wood remains damp for long enough to support fungal growth. On sash and case windows the underlying cause may be failed paint, defective putty or perimeter mastic, open joints, poor water shedding or another building defect. Repairing the timber without dealing with the moisture source risks the problem returning.
Where does decay commonly appear?
Lower parts of a window are particularly exposed because rainwater runs down the glass and joinery. Sills, lower sash rails, joints and the lower sections of the case deserve careful inspection. Decay can also occur beside failed glazing putty or where cracked perimeter seals allow water to reach concealed timber.
Does rotten timber mean the whole window needs replaced?
Not necessarily. The condition of each component should be assessed separately. Where decay is localised and surrounding timber remains sound, a joinery repair may be possible. This can involve removing defective material and splicing in compatible new timber while retaining the serviceable original window. More extensive structural failure may require replacement of a larger component or, in some cases, the complete window.
Why is paint condition important?
External coatings help protect timber from repeated wetting. Cracking, peeling or open joints should therefore be treated as maintenance warnings rather than purely cosmetic defects. Simply painting over soft or wet timber does not repair decay. The cause and extent of deterioration should be established first.
What about glazing putty and sand mastic?
Putty protects the junction between glass and sash; perimeter mastic helps weather the joint between the window case and surrounding masonry. Failure in either location can contribute to moisture reaching vulnerable timber. They perform different jobs, so the survey should identify which detail has failed rather than applying one sealant everywhere.
How can repeat decay be reduced?
Keep external coatings, putty and perimeter joints under observation and address small defects before prolonged water entry develops. Check that water can shed from the sill and that surrounding masonry or rainwater goods are not directing moisture onto the window. After a repair, retain details of the timber work and coating system so future maintenance can be planned.
Related advice and services
- Rotten timber sash window repairs
- Can a rotten sash window sill be repaired?
- Annual sash window maintenance checklist
- Sand mastic replacement
Back to all sash window resources
Discuss a timber defect
For a property in Stirling or Central Scotland, send photographs and a description of the affected window. An inspection can establish whether the decay is suitable for a local repair and what associated weatherproofing work is needed.