Updated
Clogged Window Tracks, Sills And Trickle Vents
Water pooling in the bottom of the frame, a sliding door that grinds, and vents choked with dust.
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What's happening?
The bottom of a window frame is a drainage system, not a shelf. Small slots in the outer face of the sill let water that gets past the gasket run out; the channel behind them fills with dust, pollen, dead insects, moss and grit. Once the slots block, the channel holds water permanently — which rots timber, corrodes hardware, feeds mould in the gasket and, on a sealed unit, keeps the edge seal wet until it fails. Sliding patio and bi-fold tracks do the same thing and then grind their rollers. Trickle vents are the third part: the small controllable vents in the head of the frame are what Building Regulations rely on for background ventilation, and a blocked or painted-over vent is a direct cause of the condensation and mould people blame on the glass.
What usually causes it?
- Dust, pollen and dead insects silting up the drainage channel behind the sill slots
- Moss and grit blocking the external drainage slots so water cannot escape
- Grit and debris in a sliding or bi-fold track wearing the rollers and the bottom guide
- Trickle vents blocked with dust, cobwebs or insect debris, or painted over entirely
- Builders' dust and plaster left in the frame after glazing or decorating work
- Leaves collecting in a recessed reveal or behind a fitted secondary sill
- Blocked slots leaving standing water against the sealed unit edge seal until it fails
Is it dangerous?
Not immediately, but this is where cleaning neglect turns into a repair bill. Standing water in the frame rots timber, corrodes hinges and locks, and keeps the sealed unit edge seal wet, which is a main cause of misted double glazing. A blocked trickle vent removes background ventilation the house was designed around, and persistent condensation and mould carry real health risks, particularly for children and anyone with a lung condition.
Can I fix it myself?
Yes, and it is worth doing. Vacuum the track and the bottom channel with a crevice tool, then wipe out what is left with a damp cloth. Clear the external drainage slots gently with a soft brush or a cocktail stick — never a screwdriver or a drill, which damages the profile. Open and clean the trickle vents, and check the flap moves. Do not lubricate a track with oil or grease, which holds grit; a dry silicone spray on the rollers is what manufacturers specify. Do not pressure wash into the drainage slots.
When should you call a window cleaner?
Call a window cleaner when the frames need doing properly across the house, when water is still sitting in the channel after you have cleared what you can see, or when the vents and tracks are on upper floors. This is routine work on a professional frame clean and it is the part homeowners skip. Call a window or door installer instead if a track is worn, a roller has failed, or a unit has already misted — those are repairs, not cleaning.
What will the window cleaner actually do?
- Vacuum and flush the bottom channel and sliding tracks rather than wiping only the visible face
- Clear the external drainage slots so water can leave the frame again
- Open, clean and free the trickle vents and check the flaps operate
- Clean the gasket line and rebate where the dirt that re-silts the channel actually sits
- Wash the sill, end caps and reveal so debris is not washed straight back in
- Report standing water that will not clear, a worn track, corroded hardware or a misted unit as a repair
Frequently asked questions
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