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Why Is Rainwater Appearing on the Inside Sill of My uPVC Window?

For rainwater that appears at the bottom of a uPVC window frame or internal sill during wet weather, rather than condensation forming over the whole pane.

By Home ManualReviewed 4 September 2026Safety level S1UK guidance
Quick answer

What this usually means

uPVC frames are designed to collect some water in their drainage channels and discharge it outside. Everest notes that blocked drainage holes can make a sound frame appear to leak. First distinguish rain ingress from condensation, then inspect only the visible drainage slots and remove loose debris without damaging the frame. If drains are clear but water enters around the sash, glazing beads or frame-to-wall joint, a window specialist should check seals, alignment and external installation joints.

Likely causes

What may be causing this

1

Frame drainage slots are blocked

Clues: Water pools in the lower frame and fails to drain outside.

What to do: Clear visible loose debris carefully using the manufacturer's maintenance approach.

2

The sash weather seal is not compressing correctly

Clues: Ingress appears at one corner in wind-driven rain and the sash gaps are uneven.

What to do: Have the sash alignment and seals checked.

3

The external frame-to-wall or glazing seal has failed

Clues: Water appears despite clear frame drainage and may track from the top or side reveal.

What to do: A glazing specialist should trace and repair the external weatherproofing path.

Diagnose the pattern

How to check what is happening

Confirm the moisture appears during rain rather than normal indoor humidity
It is weather-dependent and localised at the frame

Rain ingress is more likely than condensation.

Inspect visible lower drainage slots
They contain dirt or standing water does not drain

Blocked frame drainage is a strong possibility.

Check whether the sash gaps and gasket contact are even
One corner is visibly loose or misaligned

Weather-seal compression may be failing.

DIY decision

Can you fix this yourself?

Clear only visible drainage slots and debris. Do not drill extra holes or inject sealant blindly because that can trap water inside the frame.

You'll usually need:
  • Soft brush
  • Torch
  • Cloth
Step by step

What you can safely try

  1. 1

    Confirm it is rain ingress

    Note whether the water appears specifically during rain and where it first becomes visible.

  2. 2

    Check visible drainage paths

    Remove loose debris without enlarging or drilling the frame.

  3. 3

    Inspect sash alignment and seals

    Uneven gaps can allow wind-driven rain past the weather seal.

  4. 4

    Book a glazing inspection if it persists

    External seals, beads and installation joints may need targeted repair.

Stop conditions

Call a glazier if…

  • Water is reaching electrical sockets or wiring.
  • The frame or surrounding wall is visibly rotten, loose or structurally damaged.
  • The leak persists despite clear drainage and a properly closed sash.
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Evidence

Sources used for this guide

We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.

  1. Why Is My uPVC Door or Window Frame Leaking?Everest
  2. Windows - repair and maintenanceGlass and Glazing Federation

Last reviewed: 4 September 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.

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