Why Is Rainwater Leaking In Around My uPVC Window Frame?
For rainwater appearing on an internal window sill, reveal or frame during wet weather even though the sealed glass unit itself is intact.
What this usually means
Rainwater around a uPVC window frame commonly comes from blocked frame drainage holes, failed weather seals or failed sealant between the frame and the wall opening. Everest explains that uPVC windows are designed to collect incidental water in the frame and drain it back outside through built-in drainage holes; if those holes block, water can spill indoors. Anglian also identifies failed weather seals and failed perimeter sealant as common routes for window leaks.
What may be causing this
The frame drainage holes are blocked
Clues: Water collects in the lower frame channel and does not emerge from the designed external drain slots.
What to do: Clear only the intended drainage holes and retest with a small amount of clean water.
The weather gasket or seal is damaged
Clues: Rubber seals are split, flattened, displaced or fail to contact the opening sash evenly.
What to do: Refit or replace the correct gasket and check sash alignment.
Perimeter sealant between the frame and wall has failed
Clues: Water appears at the edge of the reveal/frame and external sealant is cracked, missing or separated.
What to do: Remove failed sealant and reseal the installation correctly using an appropriate external glazing sealant system.
The opening sash is misaligned
Clues: The window also whistles, catches or needs force to close, and seal compression is uneven.
What to do: Realign the hinges/locking mechanism so the sash seals consistently.
How to check what is happening
Blocked drainage holes are a strong possibility.
The internal drainage path is obstructed.
Wind-driven rain may be passing the weather seal.
Rain can travel around the installation rather than through the frame itself.
Can you fix this yourself?
Cleaning designed drainage slots and reseating a loose user-accessible gasket are reasonable checks. Failed perimeter sealing, misalignment and persistent leaks are better handled by a window fitter, particularly on upper floors.
- Soft brush
- Thin non-damaging cleaning tool
- Mug of clean water
- Dry cloth
- Phone camera
What you can safely try
- 1
Dry the frame before testing
Start with a dry sill and channel so you can see exactly where new water appears.
- 2
Check the designed drainage slots
Remove debris without drilling or enlarging holes in the frame.
- 3
Test the drainage path
Use a small amount of clean water and confirm it exits externally.
- 4
Inspect gaskets and perimeter sealant
Look for splits, gaps, shrinkage or sealant separation.
- 5
Escalate persistent rain ingress
A window fitter can distinguish blocked drainage from gasket, alignment or installation-seal failures.
Call a window fitter if…
- Water is reaching electrical equipment.
- The window frame is loose or visibly distorted.
- The leak comes from inaccessible upper-floor external joints.
- Water persists despite clear drainage holes.
- The surrounding wall/reveal is becoming saturated or mouldy.
Get the right window fitter.
We already know this problem maps primarily to a window fitter. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- Why Is My uPVC Door or Window Frame Leaking?Everest
- How to Tell if Your Double Glazing Has FailedAnglian
Last reviewed: 1 September 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.